Download Introduction to Plant Diseases

Introduction to Plant Diseases
Damon L. Smith
Assistant Professor and State Extension Specialist
Department of Entomology and Plant Pathology,
Oklahoma State University
Plant Disease
Plant Disease: any malfunctioning of
the growth or reproduction of a plant
caused by continuous irritation by a
primary causal agent that results in the
development of symptoms
Plant Disease
Continuous irritation : Very important
point; this separates disease from injury
• Injury = acute
• Disease = chronic
Broad classification of Primary
Causal Agents
• Biotic (infectious): living organisms
(pathogens) that have the ability to infect
plants and cause disease
• Abiotic (non-infectious): disorders
induced by non-living causes such as
environmental conditions (85% of our lab
samples are identified as abiotic disorders)
Infectious Disease Agents
(Pathogens)
• FUNGI: cause the great majority of plant
diseases; thousands have been reported
• BACTERIA: six genera are known to cause
plant diseases
• VIRUSES: Submicroscopic obligate
parasites
• NEMATODES: small non-segmented
worms
Fungi
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Most common of plant pathogens
Attack all plant parts
About 16,000 fungal plant pathogens
Unit of infection = spore or mycelium
Multi-celled
Visible – microscopic
E.g., cedar-apple rust, black spot of rose
Spread by wind, insects, water, tools
Fungi
FUNGI grow apically by linear extension of their threadlike bodies from the tips. A single strand of this fungal
body is termed a hypha (plural hyphae), and the
hyphae taken all together are called a mycelium.
Nuclei are found in the hyphal cells.
Multinucleate
hypha
Most fungal hyphae have crosswalls;
some do not.
Mycelium on leaf surface
Examples of types of fungal
conidia (asexual spores).
These conidia are
microscopic
and easily carried by air
currents.
Botrytis
Verticillium
macroconidia
microconidia
chlamydospore
Aspergillus
Pencillium
Fusarium
Fusarium
Spore germination and infection
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Spores require high humidity for development (>80%)
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Spores require water film on the host tissue to
germinate
•
Many germinated fungal spores penetrate plant tissue
directly, or enter through the leaf pores (stomates) or
wounds.
•
Most conidia can survive from several hours to several
days if conditions necessary for germination are not
met.
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Resting spores with thick walls may survive for years.
Bacteria
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Quite small and single celled
Spread by water, tools and seed (sometimes)
Enter wounds or natural openings
E.g., soft rot of vegetables, fire blight, crown
gall
Bacteria
Crown gall of
grape
Bacterial scorch of
oak
Bacterial cells grow by dividing into two cells
from one (called binary fission)
Bacterial
cell
Dividing
cells
How Bacteria Spread
Plant pathogenic bacteria do not form spores
Bacteria are spread by:
• Water splash and runoff
• Insects (honey bees, insect larvae, chewing insects,
etc.)
• Tools (such as pruning tools, trowels)
• Contaminated flats, pots, or soils (in greenhouses
etc.)
• In contaminated or infected seed
• By cuttings taken from infected plants, or in infected
planting stock (tubers, bulbs, etc).
• Not spread by wind (exception of wind-driven rain
drops)
How bacteria gain entry into plants
Bacteria cannot penetrate plant tissue directly
Bacteria usually enter plant tissue by means of:
• Wounds
• Natural openings on plant leaves or stems: lenticels,
hydathodes, etc; Water saturation aids entry through
natural openings.
•
Insects or insect larvae are common vectors of bacterial
pathogens.
•
Bacteria may “piggy back” and gain entry through
cankers (open wounds) caused by fungi.
Viruses
• Very small, need electron microscope to see
• Spread by insect, seed, contact (handling,
propagation), nematodes, root grafts
Virus Diseases of Plants
Viruses = nucleic acid (RNA or DNA) surrounded by
a protein coat.
Viewed with electron microscope.
Shapes include: short rods, long flexuous rods, or
polyhedrons.
Exist and multiply only in living tissue.
Transmitted from plant to plant
1. insects (mostly aphids and leafhoppers)
2. mechanically through small wounds
3. parasitic plants like dodder
SYMPTOMS CAUSED BY VIRUSES
1. Rarely kill plants
2. Some degree of stunting of entire plant
3. Symptoms usually systemic (throughout the plant) rather
than localized
4. Symptoms usually appear on leaves as
a. mosaic (yellow areas intermingled with normal
green areas)
b. ring spots (chlorotic or necrotic rings)
c. line patterns (chlorotic or necrotic lines)
d. lighter colored areas mingled with normal color
on flowers or fruit.
e. yellows (overall chlorosis)
f. tumors
g. pitting or flattening of stems or fruits
Viruses
http://www.oznet.ksu.edu/dp_hfrr/extensn/pr
oblems/tomswv.htm
TSWV on Tomato
Leaf roll virus of
grape
TSWV on Peanut
Nematodes
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Non-segmented, round worms
Most feed on roots; some on foliage & stems
Usually microscopic
E.g., root-knot nematode
Nematodes
• 15,000 described species. Size ranges from 3/1000th of
an inch to 27 feet in length.
-Most are nonpathogenic, but several hundred species (10% of
described species) are parasitic on living plants.
-Plant parasitic nematodes typically measure from 1/25th to 1/50th
of inch in length
• Females lay eggs that hatch into small nematodes.
• Characteristically cause stunting of plants, may also
cause cysts, galls, or lesions on roots.
• Spread by anything that moves soil. i.e. wind, water,
tools, animals, man
Plant Parasitic Nematodes
Root Lesion Nematodes
Endo-migratory
Cyst Nematodes
Ecto-sedentary
Root Knot Nematodes
Endo-sedentary
Ring
nematode
Sting Nematode
Ecto-migratory
Spiral Nematode
Plant pathogenic nematodes have hollow stylets or
spears as mouthparts with which they withdraw
their nutrients from plant cells.
Root Knot Nematode:
Meloidegyne incognita
Stylet (spear)
Symptoms Resulting from Nematode
Feeding
Root knot nematode
G
Root knot nematode;
two small, individual galls
(G)
G
Watermelon: advanced stage,
heavy infection
Plant Disease Management
Principles and
General Concepts
Plant Disease Triangle
Host
Pine Trees (many
species)
Diplodia Tip Blight
Pathogen
Fungus: Diplodia pinea
Environment
Wet Conditions; cool to
moderate temperatures
Manipulate the Host Component
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Genetic resistance most powerful disease
management tool
Use native plants (fewer disease problems)
Select ornamentals and trees adapted to
Oklahoma conditions (Oklahoma Proven
Varieties)
Plant stress management – Stress
predisposes plants to disease
Host
Pathogen
Environment
Manipulate the Environment
• Do not over fertilize – many diseases are more
severe when there is an excess of nitrogen
available
• Reduce competition for water, light, nutrients
• Provide adequate air movement and light
penetration
• Irrigate in morning
• Apply irrigation water to the soil
(e.g. drip irrigation)
Host
Pathogen
Environment
Reduce Amount of Pathogen
• Exclusion – Preventing the
introduction of a pathogen (beware
of exotics!)
• Eradication – Constant observation
and early diagnosis is critical
• Sanitation – Removing pathogen
infested plant material from the
area
• Pesticide Use
Host
Pathogen
Environment
Fungicides
• Protectant/contact
– remain outside the plant and only protect the tissues to which they are
applied
– chemical does not move after it dries
– removed by pruning, often degraded by the environment
– Mancozeb (Fore, Formec)
• Systemic
– chemical moves through the plant
– slightly misleading – fungicides move upward
– only true systemic is fosetyl-aluminum (Aliette, Flanker, etc.)
• Local systemic (penetrant)/mesotemic
– Triazole and strobilurin chemistries
– absorbed into tissues but have limited movement
– Propiconazole (Propiconazole, Fertilome Systemic Fungicide, Banner),
trifloxystrobin (Compass)
Fungicide Resistance Management
•Fungicide resistance can be a problem if MOA
rotation is not used, proper spray volume is not
used, off-label rates are used, etc.
Recommended Fungicide Use
Patterns
= Sensitive Individuals
Non-recommended Fungicide
Use Patterns
= Resistant Individuals
Accurate Diagnosis
• Submit sample to OSU Plant
Disease Diagnostic Lab
• Proper identification will lead to the
right management methods
• Send a good sample!
Jennifer Dominiak-Olson, Plant Pathology Diagnostician
Contact:
Room: 119 NRC Phone: (405) 744-9961
FAX: (405) 744-6039
http://www.ento.okstate.edu/pddl/