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Root-knot
Nematode Diseases
of Horticulture
Crops
Root-knot nematodes, Meloidogyne spp., are
the most important plant-parasitic
nematodes in our country because of their
wide host range and widespread distribution.
Root-knot nematodes cause large economic
damage. Yield losses is from 26% to 50%，
even no Yield.
 In addition, root-knot nematodes interact
with other plant pathogens, resulting in
increased damage caused by other diseases.

 Root-knot nematodes have an extremely
wide host range. All major field crops, most
vegetable crops, peaches, ornamentals, and
many weed or grass plants are susceptible to
one or more species of root-knot nematodes.
 Host vegetables include: carrot, tomato,
potato, eggplant, beans, peas, onion, shallot,
leeks, celery, cauliflower, cabbage, brussel
sprouts, turnip, capsicum, watermelon,
cucumber, gherkins, squash, pumpkin,
zucchini, lettuce, parsley, coriander, fennel,
radish, spinach, and over 2 000 other plant
species.
1. symptoms
Above-ground symptoms include wilting,
loss of vigor, yellowing, and other
symptoms similar to a lack of water or
nutrients. Infested plants grow more slowly
than neighboring healthy plants. Plants
often wilt during the hottest part of the day,
even with adequate soil moisture, and
leaves may turn yellow. Fewer and smaller
leaves and fruits are produced, and plants
heavily infested early in the season may
die. Damage is most serious in warm,
irrigated, sandy soils.
Below-ground symptoms are the swollen, knotted
roots. Root-knot galls can be single , or coalesce with
other root-knot galls and form a big one . It should not
be confused with nitrogen-fixing nodules that develop
on roots of legumes. Nitrogen-fixing nodules are
attached to the side of the roots, but root-knot
nematode galls are formed by enlarged root tissue .It
will not "flick off" as will nitrogen nodules. Nematode
galls on leeks, capsicum, shallot and members of the
grass family are usually small and even difficult to see.
We can see the pear-shaped mature female in the
Root-knot galls .
Eggplant
Cucumber
Carrot
2. Pathogen
The cause of disease is one or more
species of root-knot nematodes
(Meloidogyne). The main species are M.
incognita(南方根结线虫)， M.arenaria(花生根结线
虫)，M.hapla(北方根结线虫)and M.javanica(爪哇
根结线虫). M. incognita is the dominant type .
Meloidogyne(根结线虫）
Female nematode
Male nematode
3. Disease cycle
3.1 Dispersal
Nematodes are usually introduced into
new areas with infested soil or plants.
3.2 Survival
It is believed that root knot nematode
survives from season to season
primarily as an egg in the soil.
3.3 Infection and Life cycle
After the eggs hatch, the second stage
juveniles invade roots near root tips and
remain inside the root at one location
throughout their life. As larvae feed, the root
cells divide rapidly near the nematode's head.
This rapid cell division and enlargement
cause the swelling or knots on roots. The
female nematode undergoes three additional
molts and changes shape to become a pear
shaped adult.

The adult female may live two to three
months or longer and lay 500 to 1000 eggs
into a jellylike mass that extends out
through the root surface and into the soil.

The male nematodes eventually leave
the roots.

Eggs may hatch if temperature and
moisture are favorable.

In temperate climates, generation time is
four to six weeks and there are usually
three or four generations per year.
Life cycle
4. Management
 Management of nematodes is difficult.
The most reliable practices are preventive,
including sanitation and choice of plant
varieties.
 Existing infestations can be reduced
through fallowing（休耕）, crop rotation, or
soil solarization（曝晒）. Once an area or
crop is infested, try to minimize damage
by adjusting planting and harvesting
dates and irrigation or by the use of soil
amendments（改良剂）.
4.1. Sanitation


Nematodes are usually introduced into new
areas with infested soil or plants. Prevent
nematodes from entering pathogen-free garden
by using only nematode-free plants purchased
from reliable nurseries. To prevent the spread of
nematodes, avoid moving plants and soil from
infested parts of the garden. Do not allow
irrigation water from around infested plants to
run off, as this spreads nematodes. Nematodes
may be present in soil attached to tools and
equipment used elsewhere, so clean tools
thoroughly before using them in your garden.
Remove the roots of infested plants as through
as possible .
4.2 Resistant or Tolerant Varieties and
Rootstocks
 One of the best ways to manage
nematodes is to use vegetable varieties and
fruit tree rootstocks that are resistant to
nematode injury.
 Consider replacing severely infested
plants with plant species and varieties that
are more tolerant of the nematodes present.
Unfortunately, resistant varieties are not
available for many crops and ornamentals.
4.3 Rotation / Fallow
 Growing a crop on which the nematode pest
cannot reproduce is a good way to control
some nematodes. Unfortunately, rotation is
not as easy for controlling root knot
nematodes because so many vegetable crops
and weeds are hosts of the pest. Annual crops
that are useful in a rotation plan for reducing
root knot nematode populations include small
grains such as wheat and barley, sudangrass,
and resistant tomato and bean varieties.
 With a nonhost crop such as sweet
corn and other grain crops will be
effective in controlling the northern rootknot nematode.
 Fallowing is the practice of leaving the
soil bare for a period of time. Fallowing
is hardly used in our country.
4.4 Planting and Harvesting Dates
Most nematode species are active during the warm
summer months and cannot penetrate roots at soil
temperatures below 64°F. Therefore, we can reduce
nematode injury to fall-planted crops such as
carrots, lettuce, spinach, and peas by waiting until
soil temperatures have dropped below 64°F. Plant
summer vegetables as early as possible in spring
before nematodes become active. It is also helpful to
remove annual vegetables (including their roots) as
soon as harvest is over, to prevent nematodes from
feeding and breeding on root systems.
Nematode-suppressive Plants
Certain marigolds (Tagetes)(万寿菊） suppress root
knot and lesion nematodes. French marigolds are
most effective. Marigolds do not work well against
the northern root knot nematode (Meloidogyne
hapla). The effect of marigolds is greatest when they
are grown as a solid planting for an entire season.
 When grown along with annual vegetables or
under trees or vines (intercropping), nematode
control is usually not very good.
Soil amendments and Irrigation
Various organic amendments can be added to the
soil to reduce the impact of nematodes on crop plants.
The amendments, which include peat（泥煤）,
manure, and composts（混合肥料）, are useful for
increasing the water- and nutrient-holding capacity of
the soil, especially sandy soils. Increasing the soil's
capacity to hold water can lessen the effects of
nematode injury. Likewise, more frequent irrigation
can help reduce the damage caused by nematodes. In
either case, there will be just as many nematodes in
the soil, but they will cause less damage.
4.5 Pesticides
Chemical control involves the application of certain
volatile and nonvolatile nematicides to the soil to
reduce nematode populations.
 methyl bromide (溴甲烷）
 Dozomet（棉隆、必速灭） ai 6kg/ 667m2
 Ethoprophos （ethoprop，灭克磷)20% ai
3kg/667m2
 1.8%Avermectin（阿维菌素）1ml/㎡treat soil，or
3%Miral granules（米乐尔/氯唑磷）60kg/ hm² row
replacement （沟施)or hole replacement(穴施）
4.6 Biological control
Paecilomyse lilacinus（淡紫拟青霉），
Cylindrocarpon(柱孢属）, heteronema （异丝藻属），
Verticillium chamydosporium（厚垣轮枝菌）
preparations have been found to be highly effective
against the nematode.
Bulb and Stem Nematode in
Onions and Carrots
 The bulb and stem nematode (Ditylenchus
dipsaci) occurs in many regions around the
world and is a particularly destructive pest on
vegetables grown in temperate climates.
 Although many different crops are
susceptible to invasion by this nematode, more
than 20 biological races have been identified.
Only a few of these races are know to occur in
Canada and the federal government regulates
the importation of potential hosts of exotic
races to prevent their entry.
 Some biological races are host or crop
specific while others are capable of invading
more than one host. Crops infected by bulb
and stem nematode include onions, garlic,
leeks, chives, shallots, peas, beets, carrots,
celery, tomato, cucumber, narcissus, hyacinth,
tulip, oats, rye, alfalfa, red clover, phlox and
strawberry. Onions are susceptible to several
of these races of bulb and stem nematode.
1. Symptoms
Onions
Symptoms of bulb and stem nematode
infestations on different crops vary depending on
crop and population of nematodes.
In onions, the base of infected seedlings appears
swollen and leaves appear twisted, malformed and
may bear slightly raised pimple like spots (Figure 1).
Figure 1. Young onion plants
infected with bulb and stem
nematode. Initial symptoms
include irregular swelling and
twisted leaves (A).
As the disease progresses, the outer
leaves begin to die back from the tips,
the outer bulb scales split and the
neck may become spongy in texture
(B).
Severely infected plants eventually turn
yellow and die. Scales on older infected
bulbs often split and appear swollen or
bloated (Figure 2).
Symptoms frequently look similar to
damage caused by onion maggot. Infected
bulbs are also very susceptible to
secondary infections by bacteria and fungi.
Figure 2. Symptoms of severely infected onions with bulb
and stem nematodes (left) and uninfected healthy onions
(right). The outer layers of scales of the infected bulbs are
split allowing the inner layers to bulge through. When the
splits extend into the root region, leaf-like shoots often grow
out where roots normally grow.
Carrots
Although strains of the bulb and stem
nematode that infect carrots have not been
confirmed in Ontario, growers should become
familiar with the damage symptoms.
Young carrot seedlings develop distorted
leaves and by harvest time, infected crowns
become spongy and crumbly making
mechanical harvesting difficult. The symptoms
often resemble those caused by Aster Yellows.
Infected carrots are also more prone to infection
from crown rot fungi and soft rot bacteria.
2. Biology
Bulb and stem nematode activity begins early in
the spring when they leave the soil under
conditions of high humidity caused by a recent
rain or heavy dew. The nematodes travel in the
film of water covering the surface of stem and
leaves. In onions the nematodes will move down
inside the leaf sheath until they reach the bulb and
feed between onion scales. During feeding, the
nematode injects saliva-containing a toxin
(enzyme) into the cells, which creates the lesions
and stimulates distorted growth.
They prefer moist muck soil（腐殖质土） and
are less likely to cause damage during hot, dry
seasons. However, irrigation can encourage the
invasion of susceptible crops during drought
periods. The nematodes can spread through
irrigation water, infected bulbs or seed, and on
contaminated equipment. They can survive
freezing or extreme dry conditions in a dormant
state in dried infested plant residues.
3. Recommendations for Control
Vegetable growers should consider having a
nematode test done on fields in the fall after harvest
or in the spring before cultivation to determine if the
bulb and stem nematode is present.
 Clean Seed, Sets, Bulb
Make every effort to insure that seed, sets from
onions, garlic, shallots and flower bulbs are clean to
prevent contaminating your farm. Sets and bulbs
should be examined carefully and only plant if they
appear perfectly healthy. Infected sets tend to be soft,
shrunken and discolored (dark brown) near the neck
of the plant and are lighter in weight.
Sanitation
Dispose of cull onions and carrots away
from the field, where they will not
contaminate irrigation canals. Clean off soil
adhering to equipment before moving
between fields. Remove infested plants and
debris from both field and storage, then
destroy them.
Rotation
Rotate all fields, including those with low
populations, with a non-host crop for 3 years.
Fumigation
Field fumigation with a registered fumigant
in the fall prior to planting a susceptible crop
the following spring has also been shown to
effectively control these nematodes.