Download a practical guide to identification and control watermelon diseases

2014
A PRACTICAL GUIDE TO IDENTIFICATION AND
CONTROL WATERMELON DISEASES
Authors
Ravi Shankar, Seema Harsha, Raj Bhandary
R & D department
TROPICA SEEDS PVT LTD | No 54,
South End Road, 1st Floor, Nama
Aurore Building, Basavangudi,
Bangalore 560004 INDIA
A PRACTICAL GUIDE TO IDENTIFICATION AND CONTROL WATERMELON DISEASES
Key aspects covered
 Disease Diagnosis and
Identification
 Cultural aspects of
disease control
 Biological aspects of
disease control
 Chemical aspects of
disease control
Tropica seeds pvt ltd
Page
I
Global Research Local Roots
A PRACTICAL GUIDE TO IDENTIFICATION AND CONTROL WATERMELON DISEASES
A PRACTICAL GUIDE TO IDENTIFICATION AND CONTROL OF WATERMELON DISEASES
PURPOSE OF THIS GUIDE
This guide outlines the key issues that should be considered in relation to the Diagnosis and Identification of Watermelon diseases and also Integrated Disease
Management (IDM) for Watermelon crop. The guide addresses the key control methods and their rating in an IDM system for the major disease from Fungi,
Bacteria, virus, and nematode, and it also covers non-infectious diseases.
The guide is useful for research scientists, field assistants, marketing personnel and farmers.
CONTACT US
Phone: +9180267660/79
Email: [email protected]
Web: www.tropicaseeds.com
Disclaimer
This publication may be of assistance to you but Tropica seeds and its employees
do not guarantee that the publication is without flaw of any kind or is wholly appropriate
for your particular purposes and therefore disclaims all liability for any error, loss or other
Consequence which may arise from you relying on any information in this publication.
TROPICA SEEDS PVT LTD
No 54, South End Road, 1st Floor,
Nama Aurore Building,
Basavangudi, Bangalore 560004
INDIA
Page
II
Global Research Local Roots
A PRACTICAL GUIDE TO IDENTIFICATION AND CONTROL WATERMELON DISEASES
Contents
Page no
Southern blight
Sclerotium rolfsii
21
Plant disease diagnosis
1-2
Sudden wilt
Pythium aphanidermatum
22
Plant disease management
3
Verticillium wilt
Bacterial diseases
23
Verticillium dahliae
Nematodes, parasitic
4
Lesion
Pratylenchus spp.
24
Root-knot
Meloidogyne spp.
25
Zucchini yellow mosaic virus
ZYMV
26
6
Watermelon silver mottle virus
WSMoV
27
Anthracnose (stem, leaf and fruit) Colletotrichum orbiculare
7
Melon yellow spot virus
MYSV
28
Black root rot
Thielaviopsis basicola
8
Squash leaf curl virus
SqLCV
29
Cercospora leaf spot
Cercospora citrullina
9
Cucumber mosaic virus
CMV
30
Papaya ring spot virus
PRSV
31
Watermelon bud necrosis virus
WBNV
32
Bacterial fruit blotch
Acidovorax avenae subsp. citrulli
5
Fungal diseases
Alternaria leaf blight
Alternaria cucumerina
10
Charcoal rot Vine decline and
fruit rot
Macrophomina phaseolina
Collapse of watermelon
Monosporascus cannonballus
11
Crater rot
Myrothecium roridum
12
Crown and foot rot
Fusarium solani
13
Pseudoperonospora cubensis
14
Fusarium wilt
Gummy stem blight (vine
decline)
Phytophthora root rot
Fusarium oxysporum f.sp. niveum
15
Phytophthora capsici
17
Plectosporium blight
Plectosporium tabacinum
18
Powdery mildew
Sphaerotheca fuliginea
Erysiphe cichoracearum
Pythium fruit rot (cottony leak)
Pythium spp.
Downy mildew
Didymella bryoniae
16
19
Viral Diseases
Miscellaneous diseases and disorders
Blossom end rot
calcium deficiency
33
Sunscald (fruit)
Excessive or intense direct heat
34
Air pollution injury
Ozone,
35
Mature watermelon
Vine decline
36
Mn, Zn, Cu Toxicity
Table 1: Relative Effectiveness of various chemicals for cucurbit diseases
control
References
37
38
20
III
Angular leaf spot
Page
Pseudomonas amygdali pv.
lachrymans
A PRACTICAL GUIDE TO IDENTIFICATION AND CONTROL WATERMELON DISEASES
Plant Disease Diagnosis
This article presents the various steps/activities which are associated with
accurate plant disease diagnosis. The process may vary with different diseases and
conditions but the overall process is relatively consistent. The steps all require
careful observations and questions. The steps include:
Know what Normal is
Proper plant identification. Identification of affected plants is one of the first
steps in diagnosing a plant disease. Both scientific and common names of the plant
should be noted.
Recognize healthy plant appearance. It is important to know the normal
appearance of the plant species you are investigating. Each plant species has
Check for Symptoms and Signs
Identify characteristic symptoms. Describing the characteristic symptoms
exhibited by a specimen can be very difficult to do accurately. Because of this, it is
often difficult, if not impossible, to determine what is wrong with a plant when a
person is describing symptoms over the phone.
 Underdevelopment of tissues or organs. Examples include such
symptoms as stunting of plants, shortened internodes, and inadequate
development of roots, malformation of leaves, inadequate production of
chlorophyll and other pigments, and failure of fruits and flowers to develop.
 Overdevelopment of tissues or organs. Examples include: galls on roots,
stems, or leaves, witches' brooms, and profuse flowering.
 Necrosis or death of plant parts. These may be some of the most
noticeable symptoms, especially when they affect the entire plant, such as
wilts or diebacks. Other examples include shoot or leaf blights, leaf spots,
and fruit rots.
 Alteration of normal appearance. Examples include mosaic patterns of
light and dark green on leaves, and altered coloration in leaves and flowers.
Identify symptom variability. Variations in symptoms expressed by diseased
plants may lead to an improper diagnosis. These variations can result from a
couple of factors. It is possible that there is more than one problem present, and in
some cases there may be more than one pathogen infecting a plant. Symptoms
associated with these infected plants may be significantly different from the
symptoms expressed in response to each of the different pathogens acting
separately
Look for signs of biotic causal agents. Signs of plant disease agents are the
observable evidence of the actual disease-causing agent. Signs may include the
mycelia of a fungal agent, fungal spores, and spore-producing bodies. Indications of
insects causing problems may include the actual insect, insect frass, mite webbing,
and insect eggs. Signs are much more specific to disease-causing agents than are
symptoms and are extremely useful in the diagnosis of a disease and identification
of the agent causing the disease. The use of a hand lens and a knife can be valuable
for a diagnostician in the field.
1
Often, plant pathologists have to rely on symptoms for the identification of a
disease problem. Because similar symptoms can be produced in response to
different causal agents, the use of symptoms alone is often an inadequate method
for disease identification. The identification of the disease-causing agent may take
a week or more. One needs to ask many questions related, in order to eliminate or
identify possible causes of the problem. He also needs to consider various
environmental and cultural factors. As a result of his questions and observations he
may:
 Be able to identify a disease and disease-causing agent,
 Be able to narrow the problem down to several possibilities which will
require further study in the laboratory before he can make a final diagnosis,
or
 Be completely baffled by the problem.
special growth habits, colours and growth rates. If you do not know what to expect
of the plant you cannot recognize when something is wrong.
Page
The diagnostician must have very good observation skills, and he or she also
needs to be a good detective. It is important to keep an open mind until all of
the facts related to the problem can be collected. The possibility of multiple
causal factors must also be considered. Control measures depend on proper
identification of diseases and of the causal agents. Therefore, diagnosis is one
of the most important aspects of a plant pathologist's training. Without
proper identification of the disease and the disease-causing agent, disease
control measures can be a waste of time and money and can lead to further
plant losses. Proper disease diagnosis is therefore vital.
A PRACTICAL GUIDE TO IDENTIFICATION AND CONTROL WATERMELON DISEASES
Check for host specificity. Is the problem occurring in only one plant species or
are different plant species affected? If different plant species are affected, this
suggests the possibility of a non-infectious problem which could be related to
cultural or environmental problems. However, Phytophthora and Pythium root
rots can cause problems on many different plant species; therefore, the fact that
more than one plant species is affected does not completely eliminate infectious
agents. If there is more than one species of plant involved, are these plants closely
related and can they be infected by a common pathogen?
Laboratory Tests
Sometimes neither symptoms nor signs provide enough specific or characteristic
information to decide the cause of an infectious plant disease. In such cases, it may
be necessary to bring a sample back to the laboratory for further tests to isolate
and identify the causal agent.
Incubation of plant material. One of the first steps when getting back to the
laboratory may be to place a sample of the diseased tissue under conditions that
will allow an infectious agent to grow and possibly induce sporulation. This can be
accomplished by placing a leaf in a moist chamber. A moist chamber can be a
sterile petri dish containing a wet filter paper in the bottom of the dish and a
Isolation and identification of biotic plant disease causal agents. Isolation of
fungi usually requires that pieces of infected plant tissue be placed on various
nutrient media. The organism that grows out of this tissue is then isolated in pure
culture. Bacteria are often isolated by chopping up infected tissue in a small
amount of sterile water. This water: bacteria suspension is then streaked onto a
bacteriological medium such as nutrient agar. Several problems can occur when
trying to isolate the plant pathogenic agent. The infected plant tissue may contain
one or more saprophytes which have moved into the infected tissue. These
saprophytes may outgrow the plant pathogen on the nutrient medium, obstructing
accurate identification of the pathogen. In some cases where a specific plant
pathogen is suspected, a medium selective for the suspected pathogen may be
utilized. It is also beneficial to attempt to isolate the plant pathogen from the
margins of the diseased tissue where the pathogen is more numerous or more
active than saprophytes that quickly colonize the recently killed tissue.
Diagnostic tests for identification of biotic causal agents. A major problem in
identification of biotic causal agents is the inability of some infectious pathogens to
grow on artificial media. Viruses, as well as some fungi (e.g. powdery and downy
mildew causing agents) and some prokaryotes (e.g. phytoplasmas), require a living
host in order to grow. In cases where the plant pathogen is difficult or impossible
to grow on artificial media, other methods may be used for their detection, such as
the use of serological tests for viruses. Viral identification is often accomplished
utilizing ELISA (enzyme-linked immunosorbent assay) which is based on the
binding of an antibody produced to a specific virus with the virus in the infected
plant material1. More tests are currently being developed using the polymerase
chain reaction (PCR) for detection of specific organisms. These types of reactions
take specialized equipment and reagents, and the tests are not commonly done
outside diagnostic and research laboratories. Other techniques used for the
identification of viruses include negative staining and electron microscopy to view
the viral particles in plant tissue or suspensions.
2
Check distribution of symptoms. One of the first things that a diagnostician
should note is how the diseased plants are distributed over the affected area. Are
they distributed uniformly across an area or are they localized? Is there a definite
pattern to the distribution? For example, does it occur only along the edges of a
greenhouse near open windows, next to roadways or driveways, in low spots of a
field, along a planted row, or is it affecting plants at random in a field? This
distribution can be especially important in looking at the possibility of noninfectious problems, such as improper herbicide use or various soil factors. A
uniform pattern on an individual plant and uniform damage patterns over a large
area are generally not associated with biotic agents, but are usually due to abiotic
agents.
triangle of glass tubing on which the sample is placed so that the sample is not
directly on the wet filter paper but is exposed to humid conditions. This type of
moist chamber will work for small and relatively flat specimens such as leaves.
Plastic bags or boxes may be necessary for larger specimens. Saprophytes that are
present on the specimen can also be encouraged to grow in a moist chamber and a
brief surface swab with 70% isopropanol or 0.1-1% sodium hypochlorite may be
useful in reducing these saprophytes. Moist chambers are generally incubated at
room temperature.
Page
Identify Plant Part Affected - Are symptoms associated with specific plant
parts?
It is important to note if the symptoms observed are associated with specific plant
parts. For example, is a wilt observed correlated with a disruption of the vascular
system which may be indicated by browning of the vascular system or are the roots
of the plants abnormal including rots, decreased feeder roots, etc.; are necrotic
lesions observed strictly on younger leaves? The symptoms of some diseases are
most commonly seen on specific plant parts and this observation can be important
in diagnosis.
A PRACTICAL GUIDE TO IDENTIFICATION AND CONTROL WATERMELON DISEASES
EXCLUSION
This principle is defined as any measure that prevents the introduction of a
disease-causing agent (pathogen) into a region, farm, or planting. The basic
strategy assumes that most pathogens can travel only short distances without the
aid of some other agent such as humans or other vector, and that natural barriers
like oceans, deserts, and mountains create obstacles to their natural spread.
RESISTANCE
Use of disease-resistant plants is the ideal method to manage plant diseases, if
plants of satisfactory quality and adapted to the growing region with adequate
levels of durable resistance are available. The use of disease-resistant plants
eliminates the need for additional efforts to reduce disease losses unless other
diseases are additionally present.
ERADICATION
This principle aims at eliminating a pathogen after it is introduced into an area but
before it has become well established or widely spread. It can be applied to
individual plants, seed lots, fields or regions but generally is not effective over large
geographic areas.
INTEGRATED DISEASE MANAGEMENT
Integrated Disease Management (IDM) is a concept derived from the successful
Integrated Pest Management (IPM) systems developed by entomologists for insect
and mite control. In most cases IDM consists of scouting with timely application of
a combination of strategies and tactics. These may include site selection and
preparation, utilizing resistant cultivars, altering planting practices, modifying the
environment by drainage, irrigation, pruning, thinning, shading, etc., and applying
pesticides, if necessary. But in addition to these traditional measures, monitoring
environmental factors (temperature, moisture, soil pH, nutrients, etc.), disease
forecasting, and establishing economic thresholds are important to the
management scheme.
The goal of plant disease management is to reduce the economic and aesthetic
damage caused by plant diseases. Specific management programs for specific
diseases are not intended since these will often vary depending on circumstances
of the crop, its location, disease severity, regulations and other factors. Plant
disease management practices rely on anticipating occurrence of disease and
attacking vulnerable points in the disease cycle (i.e., weak links in the infection
chain). Therefore, correct diagnosis of a disease is necessary to identify the
pathogen, which is the real target of any disease management program.
PROTECTION
This principle depends on establishing a barrier between the pathogen and the
host plant or the susceptible part of the host plant. It is usually thought of as a
chemical barrier, e.g., a fungicide, bactericide or nematicide, but it can also be a
physical, spatial, or temporal barrier. The specific strategies employed assume that
Page
The many strategies, tactics and techniques used in disease management can be
grouped under one or more very broad principles of action. Included four general
disease control principles, exclusion,eradication, protection&Immunization(the
latter principle is more appropriately called resistance since plants do not have an
immune system)
pathogens are present and that infection will occur without the intervention of
protective measures.
Many cultural practices can be modified to manage the occurrence, intensity or
severity of plant diseases. These include selection of suitable growing sites for the
crop, adequate tillage to bury pathogen-infested plant residues, rotation to nonsusceptible crops, selecting pathogen-free planting stocks, orientation of plantings
to improve exposure to sun and air currents, pruning and thinning to eliminate
sources of infection and improve aeration in and around susceptible plants, water
management on both plants and in soil, adequate nutrition, proper cultivation to
improve root growth and avoid plant injury, and sanitation procedures to eliminate
sources of inoculum.
Biological control involves the use of one living organism to control another, and
this management technology has received much attention in recent times. However,
the number of biological agents registered for use is relatively small, success has
been limited, and application has been largely restricted to intensively managed,
high value crops such as greenhouse plants.
3
Plant Disease Management
A PRACTICAL GUIDE TO IDENTIFICATION AND CONTROL WATERMELON DISEASES
Angular leaf spot of
watermelon
Symptoms of angular leaf spot
are a chlorotic halo and may
appear “shiny” (due to bacteria
on the lesion surface). Small
irregular lesions expand and
become angular. On watermelons
the borders are chlorotic. Older
lesions may turn brown, dry and
tear to produce a tattered
appearance.
Small, round to irregular, watersoaked spots appear on infected
leaves. The spots expand until
they are limited by larger veins,
which give the spots an angular
appearance. The spots on the
upper leaf surfaces turn whitish
gray to brown and die. On the
lower leaf surfaces, the lesions
are gummy and shiny.
Conditions for disease development
Conditions after transplanting to the
field, which are typically hot and dry,
usually do not favour angular leaf spot
development. However conditions this
year differ. The current cool
wet weather is highly favourable for
angular leaf spot.
The disease may be seed borne. In
addition it has a wide host range and
can also survive as an epiphyte on
several weeds. The pathogen spreads
from plant to plant in splashing rain,
irrigation, or mechanically (such as on
hands,
windblown
sand,
or
equipment).Pseudomonas syringae pv.
lachrymans, is widespread and
particularly damaging after extended
and frequent summer rains, especially
when temperatures are between 75
and 82 F (23 to 28 C).
Angular tan appearance
and shine on cotyledons
Cultural and Biological control
measures if any
Chemical control
Chemical name
Treatment of seed for 20 minutes
with water at 122 F (50 C)
containing various acidic organic
chemicals reduces the number of
bacteria in the seed but does not
entirely eliminate them.
Do not grow cucurbits in the same
field or garden area more than
once every 3 or 4 years. Rotate
with other vegetables, flowers, or
small fruits. Avoid fields that
receive run-off water from nearby
cucurbit fields.
Avoid cultivating, harvesting, or
otherwise handling plants when
they are wet. Limit the use of
overhead irrigation and excessive
levels of nitrogen fertilization.
Dosage(g
PHI
or ml/litre)
Remarks
Streptomycin sulphate+
tetracycline hydrochloride
(Agrimycin)
6g/10 litre
15
Should be used on
younger stage of the
crop growth, do not use
during fruiting stage
Copper oxychloride
(Blitox)
3g/litre
4
Used as soil drenching
at the root zone of the
plant
Copper hydroxide
(kocide)
2g/litre
0
7-10 day interval
application
Cuprous oxide
(Nordox)
2g/litre
7
7-10 day interval
Copper sulphate
(cuproxat)
2g/litre
1
5-10 day interval; a
tank mix with
mancozeb will give
added control
Note: Copper sprays should not be continued in dry weather,
especially hot, dry weather, because the sprays may cause plant injury.
4
Symptoms
Lower surface
Page
Upper surface
Causal agent: Pseudomonas
syringae pv. lachrymans
A PRACTICAL GUIDE TO IDENTIFICATION AND CONTROL WATERMELON DISEASES
Bacterial fruit blotch of watermelon
Causal agent: Acidovorax avenae subsp. citrulli,
Water-soaked, "oily" areas on the
underside of the cotyledons or seedling
leaves often paralleling the veins with a
yellow halo are characteristic symptoms.
The infected areas dry up and become
elongated, angled, black, necrotic patches.
Some seedlings will collapse and die
immediately from infection. Others can
retain the bacterial infection and will not
exhibit symptoms until fruit set. The
lesions on leaves tend to be small, dark,
and angled. Leaf lesions are significant
reservoirs of bacteria for fruit infection.
Leaf lesions will usually be observed at
temperatures
above
90
degrees
Fahrenheit. Stems, petioles and roots are
not infected, and thus do not show
symptoms.. The diagnostic symptom of
BFB is a dark green stain or blotch on
the upper surface of the developing fruit.
The blotch may be 0.5 inch in diameter at
first, but will rapidly expand to cover the
entire fruit surface within a week if
environmental conditions are favourable
The greenhouse is an excellent
environment for spread of
BFB. High temperatures and
humidity are requisites for
development of BFB. Many
transplant production
greenhouses
top-water
transplants, which aids in
splash dispersal of the
pathogen to other plants.
Bottom
watering
of
transplants will help control
BFB. Movement of infected
plants from the greenhouse to
the field can have serious
consequences. Once in the
field,
high
temperature,
humidity and wet weather
favour spread of the disease.
Diseased fruit decays and
infected seeds and cucurbit
weeds (e.g., citron) are hosts
for the BFB pathogen
Cultural and chemical control
1. Purchase watermelon seed that has been tested negative for bacterial fruit blotch in a seedling growout test.
2. Monitor transplants for symptoms.
3. Try to segregate triploid seedless varieties and seed lots from diploid seeded varieties in the
greenhouse.
4. Reduce physical contact with the seedlings.
5. If suspicious symptoms are observed, remove infected plants and have them diagnosed immediately
for bacterial fruit blotch. If transplants are confirmed to have BFB, destroy all transplants from that
seed lot. Transplants from other varieties in the same house should not be planted in the field
Trade
name
Common name
Rate per
acre
Comments
Kocide 2000
Copper hydroxide (54 percent)
700 g
applied immediately after planting on a 7- to 10-day
schedule
Kocide 4.5 LF
Copper hydroxide (38 percent)
600 ml
applied immediately after planting on a 7- to 10-day
schedule
Kocide DF
Copper hydroxide (61 percent)
900 g
Apply at 14-day intervals.
ManKocide
DF
Copper hydroxide (46 percent) +
Mancozeb
1200 g
Apply at 14-day intervals. pH of spray solution
must be above 6.5.
5
Conditions for disease
development
Page
Symptoms
A PRACTICAL GUIDE TO IDENTIFICATION AND CONTROL WATERMELON DISEASES
Alternaria leaf blight of
watermelon
Causal agent: Alternaria
cucumerina
Cultural and Biological
control measures if any
Chemical control
Chemical name
Conidia produced on diseased
plants or crop refuse may be blown
by the wind for long distances.
Clothing, tools and other equipment,
running and splashing water are
other means of spread. The
germinating
spores
penetrate
susceptible tissue directly or
through wounds and soon produce a
new crop of conidia that are further
spread by wind, splashing rain,
tools, or workers.
At least 18 hours of high relative
humidity, producing leaf wetness, is
required before infection can occur.
The period between infection and
the appearance of symptoms varies
from 3 to 12 days. Young plants less
than a month old and plants that are
bearing fruit and 70 to 75 days old
appear to be more susceptible than
plants 45 to 60 days of age.
Grow cucurbit crops under the
best possible conditions of soil
preparation, texture, fertility,
moisture, and pH. Keep plants
growing vigorously throughout
the season.
Plant cucurbits in the same field
or garden area only once in 3 or
4 years. Rotate with other
vegetables, flowers, or small
fruits.
Where feasible, cleanly plow
under or burn crop debris after
harvest, and control weeds.
Follow suggestions outlined in
the
above
mentioned
publication
Dosage(g or
ml/litre)
PHI
Remarks
(days)
Mancozeb
(Dithane M-45)
2g/litre
5
Chlorothalonil
(kavach)
2g/litre
0
Pyraclostrobin
(cabrio)
1g/litre
0
Azoxystrobin
(amistar)
0.5ml/litre
1
Copper hydroxide
(kocide)
2g/litre
0
7-10 day interval
application
Difenoconazole
(Score)
0.5ml/litre
14
2-3 application per
season
Used as a protective
application; 5-7 days
interval
Can be used as a
protective
application;
5-7 days interval
7-14 day interval;
no more than two
sequential
application;
No more than two
application per
season, can be tank
mixed with
protectant fungicide
Note:
For mixing chemical always see the label of the product.
6
Plants usually develop circular spots or
lesions on the oldest leaves. The number of
spots increases rapidly in warm, humid
weather, later spreading to the younger
leaves toward the tips of the vines. At first
the lesions are small, circular, and
somewhat water-soaked or transparent.
They enlarge rapidly until they are 1/2
inch or more in diameter, turning light
brown dark brown or black on watermelon
when mature. Definite concentric rings
may often be seen in the older, round to
irregular spots, giving them a target like
appearance. Spots may merge, blighting
large areas of the leaf.
More or less circular sunken spots develop
on watermelon fruits. Often the spots later
becomecovered with a dark olive green to
black mold, the mycelium and spores of the
Alternaria fungus.
Conditions for disease
development
Page
Symptoms
A PRACTICAL GUIDE TO IDENTIFICATION AND CONTROL WATERMELON DISEASES
Anthracnose of watermelon
Causal agent: Colletotrichum orbiculare
Cultural and Biological control
measures if any
Chemical control
Chemical name
Warm,
wet
weather
is
favourable for anthracnose
infection and spread. Moisture
is needed for infection to take
place and rain helps disperse
the fungal spores from plant to
plant. Symptoms often become
severe
when the plant canopy has
developed
sufficiently
to
provide
a
favourable
environment for the fungus to
infect.
Fall tillage and crop rotations of at
least three years without a cucurbit
crop will help reduce crop residue
and thus help manage anthracnose.
Host resistance exists in some
Watermelon
varieties
to
anthracnose.
Anthracnose may be transmitted on
seed. Growers should inspect
transplant seedlings carefully for
this
disease
upon
delivery.
Greenhouse-grown
transplants
should be inspected regularly for
anthracnose
symptoms.
Poor
sanitation can lead to the survival
of the anthracnose fungus from
year to year in a greenhouse.
Dosage(g
or ml/litre)
PHI
Remarks
Mancozeb
(Dithane M-45)
2g/litre
5
Chlorothalonil (kavach)
2g/litre
0
Used as a protective
application; 5-7 days
interval
Can be used as a
protective application;
5-7 days interval
Pyraclostrobin
(cabrio)
1g/litre
0
7-14 day interval; no
more than two
sequential application;
Azoxystrobin
(amistar)
0.5ml/litre
1
No more than two
application per season,
can be tank mixed with
protectant fungicide
Copper hydroxide
(kocide)
2g/litre
0
7-10 day interval
application
Note:
For mixing chemical always see the label of the product.
7
Symptoms typically are much more
common on watermelon than on
cucumber or muskmelon. On leaves,
the lesions are typically irregular and
jagged in appearance. The centres of
larger, older leaf lesions may fall out,
which gives the leaf a
“shot-hole” appearance.
Stem lesions are light brown and
appear spindle-shaped. Muskmelon
and watermelon fruit also may have
anthracnose lesions that appear
sunken and round, and may be orange
or salmon-colored. Such lesions often
start on the lower surface of the fruit
where moisture accumulates.
Conditions for disease
development
Page
Symptoms
A PRACTICAL GUIDE TO IDENTIFICATION AND CONTROL WATERMELON DISEASES
Black root rot of watermelon
Causal agent: Thielaviopsis basicola
Cultural and Biological control
measures if any
Chemical control
Chemical name
The fungus can be spread
through contaminated soil and
via infested (but not necessarily
diseased) plant material. Spores
can also be easily moved in
water. Black root rot is favoured
by:
• Wet soils
• Soil temperatures between
55o and 65o F
• High soil pH (over 5.8)
• Cultural conditions which
induce stress (e.g. high soluble
salts,
excessive
nitrogen
fertilizer, low organic matter,
etc.)
Poor
drainage
and
soil
temperatures of 17-25°C favour
the disease.
It affects a wide range of crops
and survives in the soil for long
periods. Common sow thistle is
a weed host.
Avoid planting susceptible plants in Azoxystrobin+
soils known to be infested with the Chlorothalonil
fungus.
Plant only disease-free plants in the
landscape.
Examine
planting
material carefully prior to planting
to ensure that roots appear healthy Flusilazole(Nustar)
and white in colour.
Plants with blackened roots should
not be used.
Dosage(g
or ml/litre)
PHI
Remarks
0.5ml/litre+
2g/litre
1
2ml/10litre
14
Make two applications,
(Drenching)the first at
the 1 to 3 leaf stage and
the second just prior to
vine tip over or 10-14
days after the first,
whichever comes first.
Apply at early stages of
crop growth, not more
than 2 applications per
season.
Note:
For mixing chemical always see the label of the product.
8
Foliar symptoms include yellowing,
wilting, and necrosis (death) of foliage.
Herbaceous plants may collapse, while
dieback is often observed on woody
plants. Severely infected plants
eventually die. Above-ground foliar
symptoms are the result of root decay;
as a result, the reduced root system is
unable to take up sufficient water and
nutrients to support foliage and stems.
Root symptoms begin as dark brown to
black lesions that contrast sharply to
otherwise healthy white portions of
roots. Black root rot lesions often begin
in the middles of roots and expand in
both directions. As the disease
progresses,larger portions of roots are
affected until the entire root system
appears black and decayed
Conditions for disease
development
Page
Symptoms
A PRACTICAL GUIDE TO IDENTIFICATION AND CONTROL WATERMELON DISEASES
Cercospora leaf spot of
watermelon
Causal agent; Cercospora citrullina
Cultural and Biological control
measures if any
Chemical control
Chemical name
The fungus overwinters in
crop debris and on weeds
in the cucurbit family. The
spores can be wind-blown
or carried in splashing
water. Free water on leaf
surfaces is required for
infection,
which
is
favoured by temperatures
of 79 - 90 degrees F. The
disease progresses rapidly
at these temperatures and
infections of new leaves
can occur every 7 - 10
days.
To manage this disease, avoid overhead
irrigation if possible, or water in the early
morning hours when leaves are already
wet with dew. Remove and destroy old
cucurbit vines and residues, since this is
where the fungus survives the winter. Keep
the garden well weeded to prevent a
buildup of humidity in the canopy and take
special precautions to destroy weeds in the
cucurbit family, such as bur cucumber and
coyote gourd. Rotate away from cucurbits
(melons, squash and cucumber) for 2 - 3
years, if possible.
Dosage(g
PHI
or ml/litre)
Remarks
Mancozeb
(Dithane M-45)
2g/litre
5
Chlorothalonil (kavach)
2g/litre
0
Used as a protective
application; 5-7 days
interval
Can be used as a
protective application;
5-7 days interval
Pyraclostrobin
(cabrio)
1g/litre
0
Azoxystrobin
(amistar)
0.5ml/litre
1
Copper hydroxide
(kocide)
2g/litre
0
7-10 day interval
application
Difenoconazole
(Score)
0.5ml/litre
14
2-3 application per
season
7-14 day interval; no
more than two
sequential application;
No more than two
application per season,
can be tank mixed with
protectant fungicide
Note:
For mixing chemical always see the label of the product.
9
Cercospora leaf spot is generally
limited to the leaves but can affect
petioles and stems in a favourable
environment. Dark spots are usually
first seen on older leaves and are
circular to irregularly shaped, with
lighter colored centres and darker
margins. Tissue surrounding the
spots often yellows. Centres of the
spots may drop out, leaving holes in
the leaves. As lesions expand they
often merge, blighting entire leaves.
Lesions don't form on the fruit, but
if disease is severe enough for
defoliation to occur, fruit may be
smaller and of a lesser quality.
Conditions for disease
development
Page
Symptoms
A PRACTICAL GUIDE TO IDENTIFICATION AND CONTROL WATERMELON DISEASES
Charcoal rot Vine decline and fruit rot
of watermelon
Conditions for disease development
Cultural and Biological control measures if any
The charcoal rot fungus attacks
roots, stems and fruits. Stems
develop basal cankers that girdle
the stem, resulting in yellowing of
foliage and eventual wilting and
collapse of the entire plant.
Initially the lesions are brown and
may have amber colored droplets
on them (resembling gummy stem
blight), but they later become light
tan in colour and are dotted with
small, spherical dark colored
fruiting bodies (sclerotia). Fruit
develop large sunken areas that
are dark grey to black in colour.
Macrophomina phaseolina is a soil borne
fungus occurring in most soils in
California. The fungus persists in soil as
microsclerotia for 3 to 12 years and can
infect 500 plant species. The pathogen
most commonly infects melon stems at
the soil line within 1 to 2 weeks after
planting, but the first disease symptoms
occur late in the growing season, usually
within 1 to 2 weeks of harvest.
The fungus is a stress pathogen and
disease
incidence
increases
with
increases in water stress, a heavy fruit
load, and high temperatures. Although
severe charcoal rot is relatively
uncommon in furrow- irrigated fields;
Start looking for charcoal rot during the vegetative growth stage, and note infections to make
management decisions for the next crop. Rotation to a nonhost crop for 2 to 3 years can be an
effective disease management strategy in some crop production systems. However, avoidance of
drought stress throughout the growing season is paramount to disease management. Leaching soil to
reduce salinity levels, particularly at soil surface layers, may help reduce the incidence of disease in
drip-irrigated fields. Further, destruction of infected plant tissue before the pathogen reproduces at
the end of the growing season will prevent a build-up of soil inoculum. The use of grafted transplants
(i.e., susceptible scions grafted onto resistant cucurbit rootstock) has been proposed as an effective
management strategy for the control of charcoal rot as well as many other soil borne root-infecting
pathogens where the use of chemicals is not feasible. No preplant or post plant chemical control
measures have been reported. Solarization is not promising for diseases favoured by heat like
charcoal rot.
Page
Symptoms
10
Causal agent: Macrophomina phaseolina
A PRACTICAL GUIDE TO IDENTIFICATION AND CONTROL WATERMELON DISEASES
Collapse of watermelon
Causal agent: Monosporascus cannonballus
Cultural and Biological control
measures if any
Chemical control
Chemical name
Infection is favored by warm soil
temperatures (25 to 35 °C). In
climates where double cropping of
melons occurs (two crops per
year), disease typically is more
severe in the late-summer or
autumn-planted crop than the
spring or winter-planted crop.
Perithecia are formed on the roots
most abundantly late in the season
but may form throughout the life
cycle. When mature, the Perithecia
rupture,
discharging
the
ascospores into the soil. External
stresses, both biotic and abiotic,
may exacerbate disease symptoms
and speed up wilting and vine
collapse
At present, there is no one
method available that is both cost
effective and long lasting that
provides adequate control of the
disease.
Destruction of infected roots
immediately after the final
harvest is critical to maintenance
of low soil populations of the
pathogen (not only in fumigated
soils but also in fields that have
little or no history of the disease).
An
immediate
postharvest
application of metam sodium
(applied via the drip irrigation
system) or cultivation (which lifts
the roots onto the soil surface for
rapid desiccation) will inhibit
pathogen
multiplication
in
infected roots and prevent a
buildup of inoculum (ascospores)
in soil.
Dosage(g
PHI
or ml/litre)
Remarks
Methyl bromide
See the label
-----
Preplant, tarped, ormulched for
24-48hours. Application 10-14
days before planting.
Metam
sodium(vapam)
See the label
-------
Preplant, tarped, ormulched for
24-48hours. Application 10-14
days before planting.
Chloropicrin
See the label
-------- Preplant, tarped, ormulched for
24-48hours. Application 10-14
days before planting.
Fumigate only as a last resort when other management strategies have
not been successful or are not available. Tarp seal is mandatory for
application of this product in cucurbits. Fumigation may temporarily
raise the level of ammonia nitrogen and soluble salts in the soil. This is
most likely to occur when heavy rates of fertilizer and fumigant are
applied to soils that are cold, wet, acid, or have high inorganic matter. To
avoid injury to plant roots, fertilize as indicated by soil tests made after
fumigation. To avoid ammonia injury and/or nitrate starvation to crops,
avoid using fertilizers containing ammonia salts and use only fertilizers
containing nitrates until after the crop is well established and the soil
temperature is about 65° F. Liming highly acid soils before fumigation
stimulates nitrification and reduces the possibility of ammonia toxicity.
11
Monosporascus cannonballus infects
the roots of plants, primarily the
secondary and tertiary roots.
Infection can occur very early in the
growing season, but above ground
symptoms are not usually apparent
until much later in the season. The
primary symptoms are necrosis of
the small roots and lesions on the
larger roots, typically around the root
junctions (Figure1), however the
most dramatic symptom is the
collapse of the vine late in the season.
Root colonization and necrosis
continues throughout the life of the
plant causing increasingly more
damage to the root system. In severe
cases, larger roots, including the
taproot, become infected. The above
ground symptoms displayed as a
result of root damage are early
season stunting of the plant and a
gradual death of the leaves, beginning
with the crown leaves.
Conditions for disease
development
Page
Symptoms
A PRACTICAL GUIDE TO IDENTIFICATION AND CONTROL WATERMELON DISEASES
Myrothecium leaf spot and
stem canker
Causal agent: Myrothecium roridum
Closeup image of leaf
spot having sporodochia
Cultural and Biological control
measures if any
Chemical control
Chemical name
These lesions can coalesce. The
disease is favored by wet
weather conditions that favor
the development of other foliar
pathogens
Average daily humidity between
76 ± 9%, and average high and
low temperatures 32.3 ± 2.7°C
and 22.5 ± 1.3°C, respectively
favored
the
disease
development.
To manage this disease, avoid overhead
irrigation if possible, or water in the
early morning hours when leaves are
already wet with dew. Remove and
destroy old cucurbit vines and residues,
since this is where the fungus survives
the winter. Keep the garden well
weeded to prevent a build-up of
humidity in the canopy and take special
precautions to destroy weeds in the
cucurbit family, such as bur cucumber
and coyote gourd. Rotate away from
cucurbits
(melons,
squash
and
cucumber) for 2 - 3 years, if possible.
Dosage(g
PHI
or ml/litre)
Remarks
Mancozeb
(Dithane M-45)
2g/litre
5
Chlorothalonil
(kavach)
2g/litre
0
Used as a protective
application; 5-7 days
interval
Can be used as a
protective application;
5-7 days interval
Pyraclostrobin
(cabrio)
1g/litre
0
Azoxystrobin
(amistar)
0.5ml/litre
1
Copper hydroxide
(kocide)
2g/litre
0
7-10 day interval
application
Difenoconazole
(Score)
0.5ml/litre
14
2-3 application per
season
7-14 day interval; no
more than two
sequential
application;
No more than two
application per
season, can be tank
mixed with
protectant fungicide
Note:
For mixing chemical always see the label of the product.
12
In the early stage of the leaf spot phase,
lesions appeared round, oblong, and/or
irregular in shape, were dark brown to
black in color with a light interior, and
possessed a yellow halo around the
perimeter of the lesion. Frequently, the
necrotic leaf tissue broke away leaving
the appearance of shot holes in the leaf.
On some leaves, a majority of lesions
were at the leaf margin suggesting
fungal entry through the hydathodes or
that guttation provided a more
favorable environment for infection.
Older leaf spots frequently exhibited
concentric rings along with sporodochia
arranged in a somewhat concentric
pattern
Conditions for disease
development
Page
Symptoms
A PRACTICAL GUIDE TO IDENTIFICATION AND CONTROL WATERMELON DISEASES
Crown and foot rot of watermelon
The vascular system in crown rot
infested plants is not discolored
Cultural and Biological control
measures if any
Chemical control
Chemical name
The ascospore germination
occurs during prolonged periods
of high humidity (ie. greater than
95%).
A slow temperature increase
early in the morning of 1°C per
hour ensures that fruit and stem
temperatures reach daytime
targets before sunrise. Also, if the
greenhouse
has
restricted
ventilation and poor drainage,
this may create a "wet" climate
that N. haematococca can exploit
for
ascospore
germination.
Perithecia present on rockwool
blocks and on fruit lesions
provide further aerial inoculum
which, when accompanied by a
"wet" greenhouse climate, result
in numerous fruit and stem
infections.
Do not allow rockwool blocks to
dry out at the top because
damaging levels of evaporated
fertilizer salts may accumulate
around the stem base and thus
favour infection.
Avoid dripping fertilizer solution
at the stem base by positioning the
dripper away from the stem base.
Avoid excessively high fertilizer
concentrations that contribute to
salt damage.
Avoid overlap of crop production
since the airborne spores of
Nectria haematococca could be
spread from the old crop to the
early seeded peppers.
Seedling root dip treatment of bio
control agents like Trichoderma
viridae or Trichoderma harzianum
during transplanting(10g/1litre of
water)
Dosage(g
PHI
or ml/litre)
Remarks
Copper oxycloride
(Blitox)
3g/litre
3
Carbendazim
(Bavistin)
1g/litre
3
Used as drenching in the
soil; for large scale
application drenching is
not feasible.
Used as drenching in the
soil: for large scale
application drenching is
not feasible
Note
1) For tank mixing of different chemicals see the label
2) Soil fumigation with effective materials is the only chemical control
available for reducing soil borne populations of the pathogen.
3) Various chemical (e.g. chloropicrin, dazomet, formaldehyde, metam
sodium) or non-chemical (e.g. steaming, solarization, bio fumigation)
methods can be used on infested soil. None are 100% effective and
they will only penetrate to a limited depth. Plants can still become
infected if the wilt pathogen is re-introduced into the treated area by
drainage / run-off water or capillary action, or by the roots growing
down beyond the treated soil.
4) Fungicide treatment against wilt diseases gives variable and often
poor results. For this reason there are few specific recommendations
13
The first sign of an affected plant is
leaf wilt, which is eventually followed
by vine wilt and plant death.
However, unlike Fusarium wilt
caused by F. oxysporum f. sp. niveum,
the vascular system in crown rot
infected plants is not discoloured.
Examination of the stems reveals
dark reddish to brown surface
discoloration and a restriction of
growth at the soil line.At the neck,
damaged tissues are wet and shows a
dark brownish tint.This damage
eventually surround the rod several
centimetres. The roots located in the
same layer of the soil are also
affected. Their cortex yellow, brown
and decomposes. The shank portion
near the surface is covered
by mycelium white to pinkish which
form sporodochia and macroconidia.
The fruits are also attacked a moist
and soft rot develops on the part in
contact with the ground.
Conditions for disease
development
Page
Symptoms
Causal agent: Fusarium solani
A PRACTICAL GUIDE TO IDENTIFICATION AND CONTROL WATERMELON DISEASES
Downy mildew of watermelon
Causal agent: Pseudoperonospora cubense
Downy mildew symptoms first appear
as small yellow spots or water-soaked
lesions on the topside of older leaves.
The centre of the lesions eventually
turns tan or brown and dies. The
yellow spots sometimes take on a
“greasy” appearance and do not have a
distinct border. During prolonged wet
periods, the disease may move onto
the upper crop canopy.Under humid
conditions, the lesion often develops a
downy growth on the underside of the
light yellow lesions observed on the
top of the leaf. This downy growth is
particularly noticeable in the mornings
after a period of wet weather or when
conditions favour dew formation. The
downy growth on the underside of the
lesions is frequently speckled with
dark purple to black sporangia (spore
sacks) that can be observedwith a hand
lens. The presence of the downy
growth on the underside of the lesion
is a key to diagnosing this disease.
Conditions for disease
development
Cultural and Biological control
measures if any
Chemical control
Chemical name
Downy mildew is favoured by cool,
wet and humid conditions. The
pathogen
produces
microscopic
saclike structures called sporangia
over a wide range of temperatures
(5°C–30°C). Optimum sporangia
production occurs between 15°C–
20°C and requires at least 6 hours of
high humidity. The sporangia act
similar to spores. They are easily
transferred to healthy plant tissue by
air currents or splashing rain. Once
they land on a susceptible host, they
germinate and can directly infect the
leaf within one hour. During
prolonged cool wet periods, the
sporangia can also burst open and
release
many
zoospores.
The
zoospores swim through the film of
water along the leaf surfaces towards
the stomates. These natural pores are
a primary point of entry for the
pathogen, resulting in multiple
infections on the leaf.
Manage the crop to promote air
movement and reduce humidity
levels inside the crop canopy.Avoid
excess
overhead
irrigation.
Consider irrigating during the late
morning to facilitate rapid leaf
drying.
Maintain good weed control in the
field. Control alternate weed hosts
(wild cucumber, golden creeper and
volunteer
cucumbers)
in
neighbouring fence rows and field
edges.Ensure field workers wash
their hands before moving from one
field to another and, if possible,
wear freshly laundered clothing
each day.If possible, work in
diseased fields at the end of the day.
Mancozeb
(Dithane M-45)
2g/litre
5
Used as a protective
application; 5-7 days
interval
Chlorothalonil
(kavach)
2g/litre
0
Can be used as a
protective
application;
5-7 days interval
Fosetyl aluminium
(Aliette)
3g/litre
14
Azoxystrobin
(amistar)
0.5ml/litre
1
Preventive
treatment prior to
disease onset foliar
application
No more than two
application per
season, can be tank
mixed with
protectant fungicide
Dimethomorph
(Acrobat)
1g/litre
2
Dosage(g
PHI
Remarks
or ml/litre) (days)
5-10 days interval,
not more than five
application per
season
Note:
For mixing chemical always see the label of the product.
14
Symptoms
later stages of downy
mildew on watermelon
Page
Initial symptoms of downy
mildew on watermelon
A PRACTICAL GUIDE TO IDENTIFICATION AND CONTROL WATERMELON DISEASES
Fusarium wilt of watermelon
Causal agent: Fusarium oxysporum f.sp niveum
Cultural and Biological control measures if any
Fungal infection may occur at any age of
the plant. For young seedlings damping
off may occur and rot in the soil, where
the hypocotyls are surrounded by a
watery and soft rot causing the plants to
become stunted. Later wilting occurs in
more mature plants causing the plant to
die. A one sided wilting and others
remaining healthy, flaccid, withered and
brown leaves, as well as vascular
discoloration are common disease
symptoms. The roots of infected plants
may be healthy, but the vascular tissue is
brown and discolored. Thus rotten roots
are not necessarily associated with
Fusarium wilt disease.
Symptoms of disease incidence are expected to be There are no fungicides available for this disease management can be achieved
most severe at temperatures between 25-27 C. This through crop rotation, use of resistant varieties and other culturalpractices.
disease is not spread above ground from plant to
1. Crop rotation: Controlling this disease is very difficult once the soil is infected.
plant, but rather from fungal spores in the soil. The
Crop rotation for 4-10 years might reduce the density of the fungus in the soil
fungus can be introduced to the soil through
and with it disease incidence. Crop rotation should be done with non-cucurbit
contaminated seeds, in compost, from farming tools
vegetation. Avoid planting watermelon in the same field for minimum of 5-7
and vehicles, and with drainage water, in the field
years.
this symptoms often occurs in patches. When the
2. Use of disease free transplants and seeds when transplanting in the green
plants are dead, the fungus remains on dead vines or
house, the soil used must be pathogen free or disinfected by steam or soil
in the soil in the form of white mycelia, macroconidia
fumigation.
and chlamydospores until a new susceptible host is
3. Bio control strategies for this disease are not commercially available this time
available. This disease can be quite severe in fields
but may become a management option in the future. Green house studies of
with light sandy soils with pH 5.5-6.5 and less than
organic fertilizers in combination with different antagonistic strains of
25% soil moisture and high nitrogen content.
nonpathogenic Fusarium oxysporum and other microorganisms have shown
promise, but are insufficient for recommendations at this time.
15
Conditions for disease development
Page
Symptoms
A PRACTICAL GUIDE TO IDENTIFICATION AND CONTROL WATERMELON DISEASES
Gummy stem blight of watermelon
Causal agent: Didymella bryoniae
Cultural and Biological control
measures if any
Chemical control
Chemical name
The fungus that causes
gummy
stem
blight
(Didymella
bryoniae)
favourswarm, rainy weather.
The fungus requires rain to
disperse, and wet leaves
provide
favourable
conditions
where
new
infections can occur. The
spores of the gummy stem
blight fungus survive on crop
residue.
Fall tillage and crop rotations of at
least three years without a cucurbit
crop will help reduce crop residue
and help manage gummy stem blight.
Gummy stem blight may be seed
borne. Upon delivery, growers should
inspect transplant seedlings for
disease.
Regularly
inspect
greenhouses for gummy stem blight
symptoms. Poor sanitation can lead
to the survival of the gummy stem
blight fungus from year to year.
Dosage(g
PHI
or ml/litre)
Remarks
Azoxystrobin+
Chlorothalonil
0.5ml/litre+
2g/litre
1
Chlorothalonil (kavach)
2g/litre
0
Make two applications,
the first at the 1 to 3 leaf
stage and the second just
prior to vine tip over or
10-14 days after the first,
whichever comes first.
Can be used as a
protective application;
5-7 days interval
Copper hydroxide
(kocide)
2g/litre
0
7-10 day interval
application
Pyraclostrobin
(cabrio)
1g/litre
0
7-14 day interval; no
more than two
sequential application;
Tebuconozole
(Folicur)
1ml/litre
7
For suppression only;
10-14 day interval
Note:
For mixing chemical always see the label of the product.
16
The earliest symptom of gummy stem
blight is often an indefinite shaped
lesion on the leaf or stem. Often, these
lesions are first observed on the vines
or on leaf parts that are shaded or that
accumulate
moisture
for
long
periods.Lesions on watermelon leaves
tend to be a chocolate brown, while
lesions on muskmelon are a lighter
brown. Gummy stem blight lesions
may be surrounded by areas of
chlorosis. The lesions also may have
minute ridges that appear as a result of
the lesion’s growth over time.In the
final stage of infection, the plant tissue
may appear woody. Closely inspecting
the lesions at this time with a 10X hand
lens may reveal the dark fungal
structures
(pycnidia)
that
are
characteristic of this disease
Conditions for disease
development
Page
Symptoms
A PRACTICAL GUIDE TO IDENTIFICATION AND CONTROL WATERMELON DISEASES
Phytophthora root and fruit rot of watermelon
Causal agent: Phytophthora capsici
Cultural and Biological control
measures if any
Chemical control
Chemical name
Phytophthora parasitica and P.
capsici occur in most soils.
Infection of plants occurs at
any stage of growth when
there is free water in the soil.
Damage is greatest in poorly
drained,
compacted,
or
overirrigated soils.
Provide
good drainage and
prevent
flooding. Avoid wide fluctuations in soil
moisture, which predisposes plants to
infection. Keep tops of bed dry to
avoid buckeye rot of the fruit. Planting
cereals as a rotation crop may reduce the
level of infestation in the soil. Resistant
varieties are not yet commercially
available.
Dosage(g
PHI
or ml/litre)
Remarks
Mancozeb
(Dithane M-45)
2g/litre
5
Used as a protective
application; 5-7 days
interval
Fosetyl aluminium
(Aliette)
3g/litre
14
Preventive treatment
prior to disease onset
foliar application
Azoxystrobin
(amistar)
0.5ml/litre
1
No more than two
application per season,
can be tank mixed with
protectant fungicide
Dimethomorph
(Acrobat)
1g/litre
2
5-10 days interval, not
more than five
application per season
Note:
For mixing chemical always see the label of the product.
17
The most distinctive symptoms of
Phytophthora root rot are the
brown lesions on roots of all
sizes. The xylem of the roots
above the lesions often turns
yellowish or brown in colour. In
severe cases, nearly all roots may
be girdled or rotted off.
Aboveground, infected plants are
slow growing and may wilt or die
in hot weather. When fruit in
contact with the ground are
infected,
the disease is
calledbuckeye rot. Symptoms
include tan or brown spots with
concentric
rings. Phytophthora
capsici also
causes
greasy,
purple-brown stem lesions.
Conditions for disease
development
Page
Symptoms
A PRACTICAL GUIDE TO IDENTIFICATION AND CONTROL WATERMELON DISEASES
Plectosporium blight of
watermelon
Causal agent: Plectosporium tabacinum
Cultural and Biological
control measures if any
Chemical control
Chemical name
Dosage(g or
ml/litre)
PHI
Remarks
Plectosporium
tabacinum
occurs in soil and decaying
plant material. Little is known
about the disease cycle, but
spores are most likely spread
by wind and rain.
Disease
management
recommendations include a
three-year
crop
rotation,
planting in sites with good air
circulation to encourage rapid
drying of the foliage, switching
to trickle irrigation, scouting
fields to confirm the presence
of Plectosporium,
Azoxystrobin+
Chlorothalonil
0.5ml/litre+
2g/litre
1
Chlorothalonil (kavach)
2g/litre
0
Copper hydroxide
(kocide)
2g/litre
0
Make two applications,
the first at the 1 to 3 leaf
stage and the second just
prior to vine tip over or
10-14 days after the first,
whichever comes first.
Can be used as a
protective application;
5-7 days interval
7-10 day interval
application
Pyraclostrobin
(cabrio)
1g/litre
0
7-14 day interval; no
more than two
sequential application;
Tebuconozole
(Folicur)
1ml/litre
7
For suppression only; 1014 day interval
Note:
For mixing chemical always see the label of the product.
18
Plectosporium tabacinum infects stems,
leaf veins, and fruit. Symptoms of
Plectosporium blight are very distinctive
and easily distinguished from other
cucurbit diseases. Initially, lesions on stems
and leaf veins are small, white, and
diamond-shaped. Lesions quickly coalesce,
causing the entire surface of the vine or leaf
vein to turn white. Because leaf lesions are
restricted to the veins and do not spread to
the interveinal tissue, they may be
overlooked in the early stages of disease
development. Leaves on severelyaffected
vines die and complete defoliation may
occur in severe cases.
Conditions for disease
development
Page
Symptoms
A PRACTICAL GUIDE TO IDENTIFICATION AND CONTROL WATERMELON DISEASES
Powdery mildew of watermelon
Causal agent: Sphaerotheca fuliginea
Erysiphe cichoracearum
Powdery
mildew
likes
warm
temperature and moisture, and often
forms within dense foliage in the field.
Once mildew colonies form on the leaf,
temperature and moisture are less
important to the pathogens ability to
spread. Rain can wash off powdery
mildew spores when they have just
landed on the leaf surface, but once
infection begins the colonies will not
wash off. The microclimate has a great
effect on powdery mildews ability to
infect. Leaves within a canopy can be
warmer and surrounded by more
moisture than outer leaves. Low lying
areas of a field where there is less air
flow favor infection. Powdery mildew
also likes shade, one of the reasons for
its ability to develop quickly in dense
canopies.
Cultural and Biological control
measures if any
Chemical control
Chemical name
Crop rotation and many other
cultural practices have limited
effect on the incidence and
development of powdery mildew.
Selecting a site with good air
circulation and low humidity may
help reduce infections. New
plantings should be separate from
old plantings to avoid the spread of
inoculum. Control of cucurbit
weeds and other weeds may also
be helpful.
In general, healthy, vigorous leaves
and stems are less prone to
infection. Plants under nutritional
stress in most cases will develop
powdery mildew much sooner
than plants the same age grown
under a good nutritional program.
Azoxystrobin
(amistar)
Dosage(g
PHI
or ml/litre)
0.5ml/litre 1
Remarks
Chlorothalonil (kavach)
2g/litre
0
Can be used as a
protective application;
5-7 days interval
Myclobutanil
(Systhane)
1g/litre
0
7-10 day interval;30 day
plant back restriction
Pyraclostrobin
(cabrio)
1g/litre
0
7-14 day interval; no
more than two
sequential application;
Tebuconozole
(Folicur)
1ml/litre
7
For suppression only; 1014 day interval
No more than two
application per season,
can be tank mixed with
protectant fungicide
Note:
For mixing chemical always see the label of the product.
19
The
first
symptoms
of
powdery
mildew
usually
appear in mid to late summer.
Once the spores land on a
susceptible leaf, they infect the
outer layer of cells, and then
produce new spores which are
further carried by air currents.
Powdery Mildew is readily
diagnosed in the field as white
powdery colonies on the
leaves surface. The pathogen
can also infect the underside
of the leaves causing yellow
spots to form on the leaf upper
surface. Typically in the field
older
leaves
are
more
susceptible than new tissue.
Conditions for disease development
Page
Symptoms
A PRACTICAL GUIDE TO IDENTIFICATION AND CONTROL WATERMELON DISEASES
Pythium fruit rot (cottony leak)
Causal agent: Pythium spp
Cultural and Biological
control measures if any
Chemical control
Chemical name
Several species of Pythium, a funguslike organism, have been implicated
in this disease. These soil-borne
pathogens can overwinter as dormant
spore structures in the residue of
many different crops and weeds.
Infection occurs through wounds or
where the fruit touches the wet
ground.
Pythiumspp. is
easily
disseminated via water and soil
particles. Wet conditions promote
infection and decay.
Manage excess soil moisture by
Providing good drainage and
monitoring irrigation practices.
Use plastic mulch.
Fungicides may provide some
disease
Suppression.
Dosage(g
PHI
or ml/litre)
Remarks
Metalaxyl(Ridomil MZ
gold)
2g/litre
21
days
use preventatively; one
application per crop
cycle; apply as drench
immediately after
transplanting
Propomocarb
(Previcure)
1g/litre
2
use preventatively;
maximum 2 applications
per crop cycle after
transplanting, thereafter 710 days interval
Rotate registered fungicides with different chemical groups and strictly follow
label directions to avoid resistance development in Pythium.
20
This disease generally appears first
on portions of fruit in contact with
soil. Small, water-soaked spots
expand rapidly until large portions
of the fruit are necrotic and soft.
Profuse, white fungal growth
resembling tufts of cotton can be
found on rotted areas when the
humidity is high.
Conditions for disease
development
Page
Symptoms
A PRACTICAL GUIDE TO IDENTIFICATION AND CONTROL WATERMELON DISEASES
Southern blight of watermelon
Causal agent: Sclerotium rolfsii
Southern blight is caused by the soilborne fungus Sclerotium rolfsii. This
fungus survives as mycelium and sclerotia
in the soil and in decomposing plant
residue. The fungus is moved by running
water, on infested soil particles clinging
to cultivating tools, on infected plant
material, and as sclerotia mixed with
seeds. Disease development is enhanced
by high temperatures and humidity.
Southern blight is also more severe where
undecomposed organic matter is left on
and in the soil. Sclerotia enable the fungus
to survive adverse conditions and can
persist in the upper layers of soil for
many years.
Cultural and Biological control
measures if any
Remove infected plants and fruit
Whenever practical.
Deep plow plantings early to bury
sclerotia and to allow for the
complete decomposition of plant
residues.
Practice crop rotation using less
susceptible plants such as corn,
sorghum, small grains, and grasses.
Chemical control
Chemical name
Dosage
PHI
Remarks
Carbendazim
( Bavistin)
1g/litre
3
Soil drenching after
planting at the root
zone to avoid the
infection of the
Sclerotium once
Pyraclostrobin
(cabrio)
1g/litre
0
7-14 day interval;
no more than two
sequential
application;
Tebuconozole
(Folicur)
1ml/litre
7
For suppression only;
10-14 day interval
Note:
For mixing chemical always see the label of the product.
21
Symptoms begin where the fruit comes
in contact with the soil surface. Affected
areas are soft and water-soaked, later
becoming covered with a dense mat of
white, fan-like fungal growth. As the
disease progresses, numerous small,
round fungal survival structures
(sclerotia) develop in the fungal mat.
Initially the sclerotia are white; later
becoming light brown, reddish brown,
or golden brown in colour. Each
Sclerotium is roughly the size of a
mustard seed. The pathogen also attacks
stems and crowns, resulting in sudden
wilting of the foliage.
Conditions for disease development
Page
Symptoms
A PRACTICAL GUIDE TO IDENTIFICATION AND CONTROL WATERMELON DISEASES
Sudden wilt of watermelon
Causal agent: Pythium aphanidermatum
Cultural and Biological control
measures if any
Chemical control
Chemical name
Pythium can be introduced
into a greenhouse in plug
transplants, soil, growing
media, and plant refuse and
irrigation water. Greenhouse
insects such as fungus gnats
and shore flies can also
carry Pythium. Pythium sprea
ds by forming sporangia,
sack-like structures, each
releasing
hundreds
of
swimming zoospores
Wet areas in the greenhouse
where Pythium is more likely
to be present.
Low light levels, low pH, high
salts and warm growing
conditions above28°C which
favour Pythium.
Follow a strict greenhouse sanitation
program throughout the year and a
thorough year-end clean up. Clean and
disinfest all interior greenhouse surfaces
and equipment including tools, hoses,
walkways, carts, totes, troughs, tanks
and water supply lines. Use sterile
propagating media. Remove dying
plants by placing them directly into
plastic bags for disposal away from the
greenhouse.
Avoid low light levels, low pH, high salts
and warm growing conditions (above
28°C)
which
favour Pythium.
In
greenhouse cucumbers, the nutrient
solution should be delivered at pH 5.0
for approximately 5 weeks followed by
adjusting the pH to a 5.8-6.2 regime for
one week.
Target rockwool block wetness at 7075% between watering.
Use T. harzianum preventatively; apply
to growing medium soon after
transplanting, repeat thereafter once in
20 days
Dosage(g
PHI
or ml/litre)
Remarks
Metalaxyl(Ridomil MZ
gold)
2g/litre
21
days
use preventatively; one
application per crop
cycle; apply as drench
immediately after
transplanting
Propomocarb
(Previcure)
1g/litre
2
use preventatively;
maximum 2 applications
per crop cycle after
transplanting, thereafter 710 days interval
Rotate registered fungicides with different chemical groups and strictly follow
label directions to avoid resistance development in Pythium.
22
In mature plants, Pythium causes
crown and root rot, where plants
suddenly wilt when weather turns
warm and sunny and when plants
have their first heavy fruit load.
Often, upper leaves of infected
plants wilt in the day and recover
overnight but plants eventually die.
In the root system, initial symptoms
appear as brown to dark-brown
lesions on root tips and feeder roots
and, as the disease progresses,
symptoms of soft, brown stubby
roots, lacking feeder roots, become
visible. In larger roots, the outer
root tissue or cortex peels away
leaving the string-like vascular
bundles underneath. Pythium rot
also occurs in the crown tissue at
the stem base. In Watermelon,
diseased crown turns orange-brown
in colour, often with a soft rot at the
base; brownish lesions extending 10
cm up the stem base may be seen.
Conditions for disease
development
Page
Symptoms
A PRACTICAL GUIDE TO IDENTIFICATION AND CONTROL WATERMELON DISEASES
Complete wilt associated with later
stages of verticillium wilt
Yellow, V-shaped lesions form
at the margins of older
leaves.
As
the
disease
progresses, these lesions will
enlarge and occupy a good
proportion of the leaf surface,
maintaining
their
characteristic V-shape.
The vascular tissue of infected
plants becomes brown and
discoloured, causing the plant
to wilt and eventually die.
Premature foliar chlorosis and
necrosis and a tan to brown
colored
discoloration
or
streaking of the vascular
system,
however,
are
characteristic of all hosts.
Microsclerotia formed in the
dying issue are frequently
visible with a hand lens.
Symptoms of wilting are most
evident on warm, sunny days
Micro
sclerotia of
verticillium
dahliae
Foliar chlorasis and early
stages of necrosis associated
with verticillium wilt
Conditions for disease
development
Cultural and Biological control
measures if any
Chemical control
Chemical name
The fungus penetrates the root of a
susceptible plant in the region of
elongation and the cortex is
colonized. From cortex, the hyphae
penetrate the endodermis and invade
the xylem vessels where conidia are
formed. Vascular colonization occurs
as conidia are drowning up into the
plant along with water. As the
diseased plant senesces, the fungus
ramifies throughout cortical tissue
then produces Microsclerotia which
are released into the soil with the
decomposition of plant material.
Temperatures not exceeding an
average
of
21-25°C
facilitate
infections.
Long
distance
dissemination of the pathogen occurs
through infected seed tubers and
planting stock. Once established in
the field spread of the pathogen
occurs by irrigation water, diseased
plant debris or soil particles removed
by agricultural tools and machinery.
2.5g/litre
3 days
Foliar application
Fungicides
are
generally
not Fosetyl 80% WP
economical for control of Verticillium
wilt.
The disease may be controlled Carbendazim 50% WP
1g/litre
14 days Soil drenching in
through the use of resistant cultivars
bedseeds
and pathogen-free plants.Among the
recommended cultural measures for
preventing the disease are: removal Note
1) For tank mixing of different chemicals see the label
of diseased plants or plant debris and
2) Soil fumigation with effective materials is the only chemical control
the careful use of nitrogen fertilizers.
available for reducing soil borne populations of the pathogen.
Soil solarisation (by covering the soil
3) Various chemical (e.g. chloropicrin, dazomet, formaldehyde, metam
with transparent polythene sheets
sodium) or non-chemical (e.g. steaming, solarization, bio fumigation)
during summer) in sunny climates
methods can be used on infested soil. None are 100% effective and
can be also helpful. The method,
they will only penetrate to a limited depth. Plants can still become
which could be applied with low
infected if the wilt pathogen is re-introduced into the treated area by
doses of soil fumigants, is especially
drainage / run-off water or capillary action, or by the roots growing
down beyond the treated soil.
applicable to countries around the
4)
Fungicide treatment against wilt diseases gives variable and often
Mediterranean basin.
Dosage(g
PHI
or ml/litre)
Remarks
poor results. For this reason there are few specific recommendations
23
Symptoms
Causal agent: Verticillium dahliae
Page
Partial foliar wilt associated with
early stages of verticillium wilt
Verticillium wilt of watermelon
A PRACTICAL GUIDE TO IDENTIFICATION AND CONTROL WATERMELON DISEASES
Root lesion of watermelon
Causal agent: Pratylenchus spp
Lesion nematodes overwinter as eggs,
larvae, or adults in host roots or soil.
The length of the life cycle depends on
the species and the soil temperature.
For
example,
the
optimum
temperature
for
population
development on soybeans for P. alleni,
P. neglectus, and P. scribneri is 86°F
(30°C) while that for P.penetrans is
77°F (25°C). The optimum for P.
hexincisus on corn is 86°F (30 °C).
Lesion nematodes remain inactive
when soil temperatures are below
59°F (15°C); except for P. penetrans,
there
is
little
activity
until
temperatures rise above 68°F (20°C).
P. penetrans completes its life cycle in
30 days at 86°F (30°C), 35 days at
76°F(24°C) and 86 days at 59°F (15°C).
Although the other species have not
been thoroughly studied, their
developmental biology is probably
similar to that of P. penetrans
Cultural and Biological control
measures if any
Chemical control
Chemical name
Dosage(g
or
ml/litre)
In fields infested with root knot
nematodes, crop rotation may not be
feasible because of their extensive
host range; care is needed in the
selection of rotation crops because
some may be good alternate hosts.
New resistant varieties of peppers
may prove useful. Soil solarization
may help to lower the nematodes in
the top layers of the soil and avoid an
early infestation of the plants. Roots
are likely to become infested as the
season progresses by nematodes
that survived in the deeper soil
layers.
Application of 2 kg of MULTIPLEX
Niyantran (Poaecilomyces) in 100 kg
FYM and broadcast to 1 acre
uniformly.
Application of 250-400 kg of neem
cake/hac
Methyl bromide
PHI
Remarks
As a
fumigant
15
Oxamyl
(vydate)
2ml/litre
14
Carbofuron
(Furadan)
4kg/hac
30
Preplant, tarped, or
mulched for 24-48
hours. Application 10=14
days before planting.
Foliar applications are not
effective for moderate and
high populations of
nematodes.
Soil application,
application into soil
before planting.
Fenamiphos
(nemacure)
30L/hac or
1L/100
litre of
water
7 days
Soil application Apply
prior to anytime from 7 days
planting before up to the time of
planting.
Note
For tank mixing of different chemicals see the label
24
The
common
above
ground
symptoms of damage induced by P.
brachyurus are very similar to those
caused by other root parasitic
nematodes, such as stunted growth,
reduced plant vigour, leaves
exhibiting chlorotic symptoms,
defoliation and gradual decline in
yield.The type, colour and size of the
lesions vary according to the level of
infestation, age of the lesion and the
host species. Lesions are formed
during feeding. Initially they are
small but as the nematodes feed, the
lesions enlarge, often girdling and
later severing the root. Feeder roots
thus get destroyed and the entire
root system is reduced. When the
lesion breaks open, the cortex
sloughs off like a sleeve, leaving only
the vascular cylinder. Secondary
pathogens often enter these lesions
causing rot.
Conditions for disease development
Page
Symptoms
A PRACTICAL GUIDE TO IDENTIFICATION AND CONTROL WATERMELON DISEASES
Root knot disease of watermelon
Causal agent: Meloidogyne spp
Cultural and Biological control measures
if any
Chemical control
Chemical name
Dosage(g
or
ml/litre)
The host range of these three
nematode species is very wide
and includes many agricultural
crops and weeds.
Disease is most severe in warm
areas with long growing
seasons. In general, lighter,
sandy soils favour nematode
infection and result in more
severe damage to roots.
In fields infested with root knot nematodes,
crop rotation may not be feasible because of
their extensive host range; care is needed in
the selection of rotation crops because some
may be good alternate hosts. New resistant
varieties of peppers may prove useful. Soil
solarization may help to lower the
nematodes in the top layers of the soil and
avoid an early infestation of the plants.
Roots are likely to become infested as the
season progresses by nematodes that
survived in the deeper soil layers.
Application of 2 kg of MULTIPLEX Niyantran
(Poaecilomyces) in 100 kg FYM and
broadcast to 1 acre uniformly.
Application of 250-400 kg of neem cake/hac
Methyl bromide
PHI
Remarks
As a
fumigant
15
Oxamyl
(vydate)
2ml/litre
14
Carbofuron
(Furadan)
4kg/hac
30
Preplant, tarped, or
mulched for 24-48
hours. Application 10=14
days before planting.
Foliar applications are not
effective for moderate and
high populations of
nematodes.
Soil application,
application into soil before
planting.
Fenamiphos
(nemacure)
30L/hac or
1L/100
litre of
water
7 days
Soil application Apply
prior to anytime from 7 days
planting before up to the time of
planting.
Note
For tank mixing of different chemicals see the label
25
Plants affected by plant-parasitic
nematodes show foliar symptoms
similar to those caused by inadequate
moisture, poor nutrition or root rot.
Affected plants may be stunted and
have pale or yellow-green foliage.
They may wilt readily under warm,
breezy conditions even when soil
moisture
is
adequate.
Plant
symptoms may range from the
undetectable
to
plant
death,
depending on the susceptibility of the
plant, environmental conditions and
the number and species of nematodes
attacking the plant. Established
plants
usually
can
withstand
nematode feeding on the roots, but
young plants are particularly
vulnerable.
Conditions for disease
development
Page
Symptoms
A PRACTICAL GUIDE TO IDENTIFICATION AND CONTROL WATERMELON DISEASES
Zucchini yellow mosaic virus on watermelon
Causal agent: ZYMV
Transmission: Aphids
Symptoms
Conditions for disease development
Control measures
Symptoms may resemble those caused by PRSV-W
depending on the strain involved.
Leaves of affected plants show yellow
mosaic(dark green bubbles on the leaf contrast
with lighter green of rest of the leaves). Severe
malformation, blisters serrations and extreme
reduction in the size of leaf lamina
The virus is spread in a non-persistent manner
by a number of aphid species including Myzus
persicae. The virus is also easily transmitted
mechanically through contaminated tools
implements. There is circumstantial evidence
of seed transmission but it has been very
difficult to prove.
The virus is found in infect wild cucurbits
which may be important reservoirs.
Use of resistant varieties, Use virus free seedlings.
Remove infected plants as early as virus symptoms are observed to prevent/minimize
spread of the virus by aphids.
Remove weeds that serve as alternate host/virus reservoirs.
Disinfect tools, farm implements with chlorine solution before moving from diseased
areas to healthy areas.
Chemical control
1. Spray seedlings with Acephate (0.15%) or Monocrotophos (0.1%) prior to
transplanting.
2. Spray insecticides like Monocrotophos (0.15%), Acephate (0.15%), at fortnightly
intervals after transplanting till flowering stage.
3. Chemical spray followed by neem seed kernel extract (2%) is also effective in
rotation with insecticides.
4. Spraying imidacloprid(confidor) at the rate of 1ml/litre of water to control of
sucking insects
26
Bumpy areas on watermelon fruit
Page
Severe stunting, extreme reduction in lamina
A PRACTICAL GUIDE TO IDENTIFICATION AND CONTROL WATERMELON DISEASES
Watermelon silver mottle on watermelon
Causal agent: WSMoV
Transmission: Thrips
Symptoms
Conditions for disease development
control measures
Watermelon plants showing shorter internodes and
erect shoots. At an early stages oily spots can be
clearly seen on the upper leaf surface. Old infected
plant become pale to whitish yellow with green
mottle. Disease leaves showed malformed with
narrow or curled leaf blades and appeared moldy
because of the presence of many hairs. The surface
of infected fruits have ring spots which later turn to
tan necrotic or scab like lesions.
Diseases occur throughout the entire year but the
disease incidence is generally higher during dry and
hot periods when thrips populations increase rapidly.
The virus is transmitted by thrips species, thrips
palmi in a persistent (Propagative) manner (vector
can acquire and transmit the virus after feeding for
several minutes to hours: virus replicates inside the
vector) young thrips (larval stage) acquire the virus
and adult thrips spread the virus from plant to plant
during feeding. The virus is not seed transmitted.
Use virus free seedlings, remove infected plants as early as virus
symptoms are observed to prevent/minimize spread of the virus by
thrips. Remove crop debris, weeds and other sources of thrips after
the cropping season. Plowing and harrowing may help reduce vector
population in the soil. Control thrips population by using plastic
mulch, blue sticky traps.
Chemical control
1. Spray seedlings with Acephate (0.15%)
or Monocrotophos (0.1%) prior to transplanting.
2. Spray insecticides like Monocrotophos (0.15%), Acephate
(0.15%), at fortnightly intervals after transplanting till
flowering stage.
3. Chemical spray followed by neem seed kernel extract (2%) is
also effective in rotation with insecticides.
4. Spraying imidacloprid(confidor) at the rate of 1ml/litre of
water to control of sucking insects
27
Severe stunting of leaf lamina
Water soaked chlorotic spots
watermelon
Page
Initial symptoms with dark and
light green mosaic
A PRACTICAL GUIDE TO IDENTIFICATION AND CONTROL WATERMELON DISEASES
Melon yellow spot on watermelon
Causal agent: Melon yellow spot virus (MYSV)
Transmission: Thrips
Conditions for disease development
control measures
Initially chlorotic and stunted growth, leaf margin
become yellowish and severe stunting of leaves
can be observed, yellow spots is seen in later
stages and may turn in to small dotted spots all
the areas of leaf.
Diseases occur throughout the entire year but the disease
incidence is generally higher during dry and hot periods
when thrips populations increase rapidly.
The virus is transmitted by thrips species, thrips palmi in
a persistent (Propagative) manner (vector can acquire
and transmit the virus after feeding for several minutes
to hours: virus replicates inside the vector) young
thrips(larval stage) acquire the virus and adult thrips
spread the virus from plant to plant during feeding. The
virus is not seed transmitted.
Use virus free seedlings, remove infected plants as early as virus
symptoms are observed to prevent/minimize spread of the virus by
thrips. Remove crop debris, weeds and other sources of thrips after the
cropping season. Plowing and harrowing may help reduce vector
population in the soil. Control thrips population by using plastic mulch,
blue sticky traps.
Chemical control
1. Spray seedlings with Acephate (0.15%) or Monocrotophos (0.1%)
prior to transplanting.
2. Spray insecticides like Monocrotophos (0.15%), Acephate
(0.15%), at fortnightly intervals after transplanting till flowering
stage.
3. Chemical spray followed by neem seed kernel extract (2%) is also
effective in rotation with insecticides.
4. Spraying imidacloprid(confidor) at the rate of 1ml/litre of water to
control of sucking insects
Page
Symptoms
28
Yellow spots, later may turn into necrotic areas
A PRACTICAL GUIDE TO IDENTIFICATION AND CONTROL WATERMELON DISEASES
Squash leaf curl virus on watermelon
Causal agent: Squash leaf curl virus (SqLCV)
Conditions for disease development
control measures
This virus is transmitted by whiteflies. Symptoms of
the disease are crumpled leaves with yellowed,
mottled areas Image. Leaves have shortened petioles
that cluster around the vines. Squash is also
susceptible to this virus. Severe yield losses are
associated with infection of young seedlings, usually
when whitefly populations are high. Infections of older
plants do not affect yield.
The squash leaf curl virus is economically important
only in fall-grown watermelons. To alleviate the impact
on yield, transplants should be used, rather than
direct-seeded plants. Fields should be ploughed under
after harvest to minimize carryover of the virus to
subsequent crops. The virus can persist in whiteflies
for 3 weeks after feeding on infected plants, so there
should be a 4 week break between melon crops.
The virus is transmitted in a persistent manner
by Bemisia tabaci, especially biotype B. It is not
transmitted by mechanical inoculation.The
vegetables as such would not be likely to carry
B. tabaci. So the main risk of movement is in B.
tabaci on other host plants (e.g. ornamentals),
given the fact that the vector moves readily
from one host to another and that the virus can
persist in the vector for several weeks after
acquisition.
In the field, disease control is mainly aimed at eliminating or excluding the whitefly
vector, B. tabaci. Insecticides have been used in an attempt to reduce whitefly
density. Protecting rows of seedlings with spun-bonded polyester as a floating
cover has also been effective .
Chemical control
1. Spray seedlings with Acephate (0.15%) or Monocrotophos (0.1%) prior
to transplanting.
2. Spray insecticides like Monocrotophos (0.15%), Acephate (0.15%), at
fortnightly intervals after transplanting till flowering stage.
3. Chemical spray followed by neem seed kernel extract (2%) is also
effective in rotation with insecticides.
4. Spraying imidacloprid(confidor) at the rate of 1ml/litre of water to
control of sucking insects
Page
Symptoms
29
Transmission: Whitefly
A PRACTICAL GUIDE TO IDENTIFICATION AND CONTROL WATERMELON DISEASES
Cucumber mosaic virus on watermelon
Causal agent: Cucumber mosaic virus (CMV)
Conditions for disease development
control measures
Leaves of new growth are crinkled and
deformed with a slight yellowing to them.
CMV produces a systemic infection in most
host plants with the older plant tissues that
developed before infection rarely being
affected by the virus.
CMV is transmitted primarily by aphids, but also by
cucumber beetles, mechanically and to a lesser extent in
seed. Many species of aphid can vector the virus in a nonpersistent manner – the most common species in
Maryland are: Melon aphid Aphis gossypii and Green
peach aphid Myzus persicae. The virus is acquired by
aphids within 10 seconds after they begin to probe an
infected plant. The virus can be transmitted to other
plants by aphids in less than one minute. This is why
insecticides do not stop initial infections. Aphids lose the
ability to transmit CMV after about 2 minutes and
completely lose the ability after 2 hours.
CMV is easily transferable through sap carried on the
hands, clothes and tools of people harvesting fruit,
weeding or turning vines in a watermelon field.
Using reflective mulch reduces the early season infection from aphids and gives
an additional 2-4 weeks of a virus-free cucurbit field. Once the plants cover the
plastic the reflective mulch ceases to be an effective deterrent. Pesticides only
work to reduce the in-field spread of aphids and therefore, CMV and other
viruses.
Chemical control
1. Spray seedlings with Acephate (0.15%) or Monocrotophos (0.1%)
prior to transplanting.
2. Spray insecticides like Monocrotophos (0.15%), Acephate (0.15%), at
fortnightly intervals after transplanting till flowering stage.
3. Chemical spray followed by neem seed kernel extract (2%) is also
effective in rotation with insecticides.
4. Spraying imidacloprid(confidor) at the rate of 1ml/litre of water to
control of sucking insects
Page
Symptoms
30
Transmission: Aphids
A PRACTICAL GUIDE TO IDENTIFICATION AND CONTROL WATERMELON DISEASES
Papaya ring spot on watermelon
Causal agent: Papaya ring spot virus (PRSV)
Conditions for disease development
control measures
The first symptom is a clearing of veins, followed by
development ofmosaic patterns or mottling consisting
of irregularly shaped, dark green areas alternating
with light green or yellow areas. Leaves on some
species and cultivars are drastically reduced in size
and growth is often retarded.Mosaic virus tends to
cause raised, blisterlike areas on leaves and to reduce
leaf size severely. Zucchini yellow mosaic virus typically
causes the leave lobes to become long and
narrow.Malformations of the fruit can occur with all
three of the viruses.The fruit will exhibit bumps and
the classic "ringspot". A severe isolate of PRSV has also
been shown to cause tissue necrosis.
PRSV is transmitted primarily by aphids, but
also by cucumber beetles, mechanically and to
a lesser extent in seed. Many species of aphid
can vector the virus in a non-persistent
manner – the most common species in
Maryland are: Melon aphid Aphis gossypii and
Green peach aphid Myzus persicae. The virus is
acquired by aphids within 10 seconds after
they begin to probe an infected plant. The
virus can be transmitted to other plants by
aphids in less than one minute. This is why
insecticides do not stop initial infections.
Aphids lose the ability to transmit PRSV after
about 2 minutes and completely lose the
ability after 2 hours.
PRSV is easily transferable through sap carried
on the hands, clothes and tools of people
harvesting fruit, weeding or turning vines in a
watermelon field.
Using reflective mulch reduces the early season infection from aphids and
gives an additional 2-4 weeks of a virus-free cucurbit field. Once the
plants cover the plastic the reflective mulch ceases to be an effective
deterrent. Pesticides only work to reduce the in-field spread of aphids.
Chemical control
1. Spray seedlings with Acephate (0.15%)
or Monocrotophos (0.1%) prior to transplanting.
2. Spray insecticides like Monocrotophos (0.15%), Acephate
(0.15%), at fortnightly intervals after transplanting till
flowering stage.
3. Chemical spray followed by neem seed kernel extract (2%) is
also effective in rotation with insecticides.
4. Spraying imidacloprid(confidor) at the rate of 1ml/litre of
water to control of sucking insects
Page
Symptoms
31
Transmission: Aphids
A PRACTICAL GUIDE TO IDENTIFICATION AND CONTROL WATERMELON DISEASES
Watermelon bud necrosis
Causal agent: Watermelon bud necrosis virus
(WBNV)
Transmission: Thrips
Symptoms
Conditions for disease development
control measures
An unusual disease of watermelon (Citrullus lanatus) in
India, with symptoms of leaf crinkling, mottling,
yellowing, necrotic streaks on vines, shortened
internodes, upright branches and necrosis and dieback
of the buds was caused by the watermelon strain of
tomato spotted wilt tospovirus (TSWV-W).
Diseases occur throughout the entire year but
the disease incidence is generally higher
during dry and hot periods when thrips
populations increase rapidly.
The virus is transmitted by thrips species,
thrips flavus in a persistent (Propagative)
manner (vector can acquire and transmit the
virus after feeding for several minutes to
hours: virus replicates inside the vector)
young thrips(larval stage) acquire the virus
and adult thrips spread the virus from plant to
plant during feeding. The virus is not seed
transmitted.
Use virus free seedlings, remove infected plants as early as virus
symptoms are observed to prevent/minimize spread of the virus by
thrips. Remove crop debris, weeds and other sources of thrips after
the cropping season. Plowing and harrowing may help reduce vector
population in the soil. Control thrips population by using plastic mulch,
blue sticky traps.
Chemical control
1. Spray seedlings with Acephate (0.15%)
or Monocrotophos (0.1%) prior to transplanting.
2. Spray insecticides like Monocrotophos (0.15%), Acephate
(0.15%), at fortnightly intervals after transplanting till
flowering stage.
3. Chemical spray followed by neem seed kernel extract (2%) is
also effective in rotation with insecticides.
4. Spraying imidacloprid(confidor) at the rate of 1ml/litre of
water to control of sucking insects
32
Infected with WBNV
Page
Healthy
A PRACTICAL GUIDE TO IDENTIFICATION AND CONTROL WATERMELON DISEASES
Blossom end rot of
watermelon
Causal agent: Calcium deficiency
control measures
The first symptom of the disorder is a slight,
water-soaked discoloration on the blossom
end (opposite of the stem) of the fruit. As the
lesions enlarge, turning leathery and dark
brown or black, they often become sunken
into the fruit. Although the affected tissue is
normally dry, bacteria and fungi may invade
the lesion, producing a soft, watery rot.
Blossom-end rot is a physiological disorder due to a short-age of calcium in the To avoid blossom-end rot, the grower should use
young fruit. Moisture imbalances or water stress aggravate the problem by cultural practices that allow for proper uptake of
interfering with calcium uptake in the plant.
calcium by the plant.
 Avoid water stress of fluctuating soil moisture
During the day, the pores on the leaves are open and water transpires from
by using infrequent, deep irrigation to keep the
them drawing sap into the leaves. Since the fruit do not lose much water by
soil uniformly moist. Do not allow plants to be
transpiration, they receive little of the calcium-containing sap. At night, the leaf
water stressed at night. The best way to
pores close, root pressure forces sap into the plant, and the developing fruit get
maintain even the soil moisture is by using
their share of calcium and other nutrients. If the plant is water stressed at
organic or plastic mulch to prevent wide
night, the system fails and the fruit receive very little calcium, causing blossomfluctuations in soil moisture.
end rot.
 Avoid over-fertilizing; do not use ammoniumbased nitrogen fertilizers.
Excessive salts in the soil, including ammonium, potassium, and magnesium
 To avoid injuring roots, do not hoe or cultivate
interfere with calcium uptake by plants and can result in blossom-end rot. Root
near plants. Pull weeds next to plants or use a
damage caused by improper cultivation practices or excess soil water may also
plastic mulch.
lead to blossom-end rot. The incidence of blossom-end rot varies with the
 Do not overwater, especially in heavy clay soils.
variety and is greatly influenced by environmental conditions.
33
Conditions for disease development
Page
Symptoms
A PRACTICAL GUIDE TO IDENTIFICATION AND CONTROL WATERMELON DISEASES
Sun scald of watermelon
Conditions for disease development
control measures
Sunscald occurs on the side of fruit exposed to direct
sunlight. It first appears as a wrinkled area that can
be soft and lighter in colour than surrounding tissue.
In peppers, this area later collapses and turns white
and paper-like. The affected area often turns black
due to colonization by saprophytic fungi. Sunscald
primarily affects fruit, but leaves and stems also can
be injured.
Fruit near maturity are more sensitive to sunscald
injury than immature fruit. Symptoms are similar in
appearance to those of blossom-end rot, but they are
consistently associated with exposure to direct
sunlight.
Fruit suddenly exposed to direct sunlight due to defoliation
from disease, pruning or stem breakage are most likely to
develop sunscald. Sunscald occurs when internal fruit
temperature increases and tissue is damaged
To prevent sunscald, the internal temperature of thefruit
should not rise above 35° C (95° F). Encourage abundant,
healthy foliage with proper fertilization and irrigation. In
greenhouse operations, shade plants during summer to help
reduce the incidence of this disorder. Use disease-resistant
varieties, and follow an effective disease and pest management
program to help reduce losses due to sunscald.
Page
Symptoms
34
Causal agent: Environmental
A PRACTICAL GUIDE TO IDENTIFICATION AND CONTROL WATERMELON DISEASES
Ozone injury of watermelon
Conditions for disease development
control measures
Ozone injury generally appears first on older leaves
as chlorosis or yellowing followed by a mottling and
then a bleaching of small areas of the leaves. Severe
ozone injury will result in the necrosis of the older
leaves first, before reaching newer growth.
Ozone production at the ground level is the result of a
photochemical degradation of nitric oxides or other related air
pollutants. Although automobile exhaust and power plants are
generally the primary sources of nitric oxides in the
atmosphere, ozone injury is often observed in areas far
removed from automobile traffic. The production of ozone at
ground level is a light driven process. Therefore, high levels of
ozone pollution are often seen during the hot days of summer,
typically in July, when light intensity is highest
Unfortunately when ozone injury occurs there is little the
grower can do. If ground ozone levels decline below the
threshold for damage most plants will continue to grow, and
the effects will be minimal. If ozone levels remain high for an
extended period of time, significant foliage can be lost, which
may result in an increased chance for sunburned melons. It is
important to recognize ozone injury; however, so that time and
money are not wasted on unnecessary pesticide sprays.
Page
Symptoms
35
Causal agent: Environmental
A PRACTICAL GUIDE TO IDENTIFICATION AND CONTROL WATERMELON DISEASES
Mature watermelon
vine decline (MWVD)
Conditions for disease development
control measures
Initial MWVD symptoms include necrosis and wilting
of leaves, followed by the wilt and collapse of the
vines. Vine collapse reduces fruit quantity, size, and
quality; prevents normal ripening; and exposes fruit
to sunburn. On symptomatic plants, the root systems
are generally sparse — the primary roots are
necrotic (dead tissue) and the plant has few
secondary roots.
Symptoms often appear on mature plants in low,
poorly drained areas. Under the right conditions,
MWVD incidence will increase through the summer,
often resulting in the collapse and decline of large
portions of affected fields. Plants with MWVD often
yield no marketable fruit.
Mature watermelon vine decline can be serious effect on
watermelon crops. Efforts to identify a pathogen responsible
for MWVD has been unsuccessful. Indicating cultural
condition that might contribute to the syndrome. Many
factors like transplanting, less organic matter content of the
soil that decreases soil aeration, nutrition and water
retention. Watermelon crop grown in black plastic mulch.
Excess uptake of the manganese, zinc, copper uptake in the
plant system due to soil acidic condition could contribute to
inhibition of watermelon root growth. Acid pH increases the
Mn+2 solubility and phytoavailability which results in toxicity.
Mn+2 toxicity is characterized by competition with Mg uptake
and utilization; Mn+2 can compete with cation uptake,
displace Mg+2 enzyme cofactors, displace Ca+2 in cell
membrane and enhance axillary shoot growth.
Use of recommended agronomic practices, that enhance the
proper root growth and deep root growth, and avoid growing in
the acidic soils and also avoiding using too much fertilizers that
make the root zone acidic, such as ammonium fertilizers and
DAP.
Page
Symptoms
36
Causal agent: Mn, Zn, Cu toxicity
A PRACTICAL GUIDE TO IDENTIFICATION AND CONTROL WATERMELON DISEASES
Table 1:Relative Effectiveness of various chemicals for cucurbit diseases control
Anthracnos
e
Bacterial
fruit blotch
Bacterial
wilt
Belly rot
Cercospora
leaf spot
Cottony
leak
Damping
off(Pythiu
m)
Downy
mildew
Gummy
stem blight
++++
++++
+++
+
+++
++++
++
+++
+
+++
++
+++
-
++++
++++
++++
++
?
_
?
_
++R
++
++++
+
+++
+
++++R
++++
+++
+
4+M
3
33
28
11
3
5
0
1
2
0
7
++
++++
?
-
++
+++++
?
-
-
?
-
++
?
-
+
?
-
?
-
+++R
++
+++
++++R
-
++
-
Target spot
Angular
leaf spot
1
0
3
0
5
1
Powdery
mildew
Alternaria
leaf spot
11
M
27
15
M
M
Phytophtho
ra blight
Preharvest
interval(days)
Azoxystrobin
Chlorothalonil
Cymoxanil
Dimethomorph
Mancozeb
Copper hydroxide
(fixed copper)
Mefenoxam+mancozeb
Myclobutanil
Fosetyle-al
Propamocarb
Pyraclostrobin
Tebuconazole
Relative control rating (- =ineffective: +++++= very effective; ? = lacking efficacy
Fungicide
group
Fungicides
++
+
+
?
+++R
++
+
+
++++
++++
++++
+
+++++R
+++R
++
+
++++
-
+++R
+
++++R +
++
-
Key to fungicide groups:
11=quinone outside inhibitors; 27=cyanoacetamide oximes; 28=carbamates; 33=phosphonates; M=Multisite activity; 3=demethylation inhibitors; 15=cinnamic
acids
Page
37
R= Pathogen resistance to this fungicide has been reported, greatly reducing its efficacy. Combine with a protectant fungicide like Chlorothalonil to extend the
usefulness of the product
A PRACTICAL GUIDE TO IDENTIFICATION AND CONTROL WATERMELON DISEASES
References:
Page
38
1. Egel, D. 2001. Bacterial fruit blotch. In: Vegetable Crops Hotline. Purdue University Cooperative Extension Service
2. Hopkins, D. 1996. Blocking fruit blotch. The Grower 29(8): 9-10
3. Hopkins, D., and R. X.Latin. 2001. Diseases. In: Watermelons, Characteristics, Production, and Marketing (D. Maynard, editor). Alexandria, Va.: American
Society for Horticultural Science Press
4. Isakeit, T. 1999. Bacterial fruit blotch of watermelon. Texas Agricultural Extension Service Publication L-5222
5. Latin, R. X. 1993. Diseases and pests of muskmelons and watermelons. Purdue University Cooperative Extension Service Publication BP-44
6. Champaco, E. R., Martyn, R. D., and Miller, M. E. 1993. Comparison of Fusarium solani and F. oxysporum as causal agents of fruit rot and root rot of
muskmelon. HortScience 28:1174-1177.
7. Nash, S. M., and Alexander, J. V. 1965. Comparative survival of Fusarium solani f. cucurbitaeand F. solani f. phaseoli in soil. Phytopathology 55:963-966.
8. Compendium of Cucurbit Diseases. T. A Zitter, D. L. Hopkins, and C. E. Thomas, eds. American Phytopathological Society, St. Paul, MN.
A PRACTICAL GUIDE TO IDENTIFICATION AND CONTROL WATERMELON DISEASES
"For better or for worse, Plant Pathology
had its genesis in fields and granaries
more than in halls of ivy. Society needed
agriculture and agriculture need plant
pathology."
"Plant pathology has helped reveal
profound and useful truths. It was among
the pioneers in revealing the vast and
variable world of microorganisms and in
identifying man’s friends and foes
amongst them. It has shown how to
combat many of the bad ones and how to
utilize some of the good ones."
Page
39
E. C. Stakman. 1959