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Anthony (Tony) Keinath,
Professor of Plant Pathology
Clemson Coastal REC, Charleston
Sustainable Disease Management for Vegetable Growers
Organic Richland IPM Workshop, Sep. 13, 2012
Today’s Outline What causes diseases on plants?
The disease triangle shows that disease is an interaction between ___, ___, and ___.
5 groups of pathogens
How is disease managed?
5 control strategies
Management of specific vegetable diseases
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The Disease Triangle: Disease is an Interaction
Host Plant
Environment
Disease
Pathogen
fungi,
water molds,
bacteria,
viruses,
nematodes
Rain, dew, fog;
Temperature
Sunlight
Wind
Disease Triangle for Phytophthora Blight
Host Plant Phytophthora
Environment
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Damping-Off, Root and Crown Rots Damping-off
Pre-emergence
Post-emergence
Seed rot (extreme case)
Root rot, crown rot on older plants
Soil fungi Pythium (most common)
Rhizoctonia
Use transplants Older plants have stronger cell
walls = resistant
3 Bacterial Diseases
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Cercospora leaf spot (fungal) (left) and bacterial blight (right) on turnip greens
Which leaf is infected by a fungus and which by a bacterium?
Bacterial Diseases “The Essentials”
Need specialized tests to ID bacteria
Worse in wet weather
Generally difficult to control
Copper is the only effective material available to organic growers.
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Virus Diseases
Virus Diseases General symptoms are distorted growth or irregular
yellowing
Can be confused with herbicide injury
Many viruses spread by insects
Aphids, whiteflies, thrips
Need specialized tests to ID viruses
Difficult to control
Tomatoes with resistance to ToMV, TSWV See Reimer Seeds (reimerseeds.com)
Summer squash with resistance to mosaic viruses See Harris Seeds (harrisseeds.com)
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Root-Knot Nematode Primary symptoms—galls
Secondary symptoms— Yellowing Stunting
MANY hosts Mainly dicots
Parsley family
Okra
Use marigolds to control by starving young ‘todes
Use resistant tomato and So. pea cultivars
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The Importance of Plant Disease Prevention
Post-harvest
Crop management
At-planting decisions
Pre-plant soil preparation
5 General Control Strategies
Avoidance
Protection
Resistance
Eradication
Exclusion
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What is EXCLUSION?
Keeping a pathogen out of a disease-free area, e.g., a garden, a county, a country.
Quarantine of affected areas
What to do: Use disease-free
transplants Be careful when saving
seed Don’t move or spread
infested soil Clean equipment purchased
from other states
How Pathogens Are Spread
Wind
Splashing rain
Moving water/soil
Insect vectors
Infected plants/parts
Transplants
Seeds
Tubers
Contaminated tools, hands, shoes
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What is ERADICATION?
Eliminating or destroying a pathogen after it is established
Remove plants that carry a pathogen
Sanitation is a VERY Important Eradication Method
Difficult to achieve 100%
eradication
Eradication may have negative
side effects, e.g. fumigation
Examples of Eradication
Remove diseased plants or parts --including roots.
Clean contaminated tools.
Grow marigolds to eradicate root-knot nematodes.
Eliminate wild radish (host for cole crop pathogens)
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What is the Difference Between…
Resistance
Genetic ability of a plant to defend itself from pathogens
Opposite is susceptible
Resistance is (usually) the most effective strategy.
Tolerance
“Ability of a plant to endure disease without serious damage or yield loss”
Sometimes used by seed companies to mean “a low level of resistance”
Heirloom varieties may have tolerance
What is PROTECTION?
Placing a barrier between the plant and the pathogen to prevent infection.
Spray fungicides
Identify diseases to select the best fungicide.
Most effective when applied before plant is infected.
Especially organic-approved fungicides
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Common Vegetable Fungicides
Conventional Organic and Biofungicides
Protectants Chlorothalonil
Mancozeb
Copper
Sulfur
Systemics Strobilurins (e.g. Quadris,
Cabrio)
Tebuconazole and related
Group 3 fungicides (e.g.
Monsoon)
Potassium phosphites
Copper
Sulfur
Potassium bicarbonate
Serenade
Root-Shield
Organocide (5% sesame oil + 92% fish oil)
Neem oil
Who Decides What Fungicides Are “Organic”?
Organic Materials Review Institute (OMRI)
Reviews products against the National Organic Standards
National Organic Program (NOP)
Part of USDA, Agricultural Marketing Service
National Organic Standards Board National List of Allowed
and Prohibited Substances
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NOP The National List of Allowed Synthetic Substances
As plant disease control:
1) Coppers, Fixed
2) Copper Sulfate
3) Hydrated Lime
4) Hydrogen Peroxide
5) Lime Sulfur
6) Oils, Horticultural
7) (fruit only)
8) Potassium Bicarbonate
9) Elemental Sulfur
10) (fruit only)
11) (fruit only)
What is AVOIDANCE? Preventing contact between
pathogen and host to prevent disease.
Includes modifying the environment to make it less favorable for disease.
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Avoidance Techniques
Don’t grow RKN-susceptible plants in summer.
Stake tomatoes to prevent fruit rots.
Raise beds to prevent damping-off.
Don’t wet leaves when watering.
The Disease Triangle: An Aid to Management
Host Plant
Environment
Disease
Pathogen
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General Cultural Practices
Reduce leaf wetness
•Drip irrigation •Increase row/plant spacing •Avoid edges of woods •Row orientation
Improve soil drainage
• Raised beds • Subsoil • Add organic matter
• Compost • Cover crops
Controlling Downy Mildews Reduce leaf wetness
• Increase row/plant spacing • Avoid edges of woods
Cole crops • Late planting date • Resistant broccoli, cabbage • Potassium phosphite
Cucurbits • Early planting date • Tolerant: ‘Aladdin’ pumpkin, C. maxima, C.
moschata • Fungicides
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Controlling Powdery Mildews Biopesticides
• Organocide • Potassium bicarbonate
Sulfur
• Do not apply at 90ºF.
Resistant summer and winter squash and pumpkin cultivars from Harris Seeds
English pea—early planting date
Powdery and downy mildew on the underside of a pumpkin leaf.
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Controlling Tomato Diseases Early blight
• Resistance • Increase row/plant spacing • Fungicides • Serenade for organic production
Southern blight • Deep plowing • Grafting
• ‘Big Power’
Bacterial spot
• Avoid fall planting • Increase row/plant spacing • Copper sprays
Bacterial spot on tomato fruit and foliage. Note copper residue on fruit.
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Sustainable Disease Management
Use organic soil management
• Cover crops • Crop rotation • Compost
Use as many cultural practices as possible to
• Exclude, • Eliminate and • Avoid disease.
. Check plants weekly for problems
Identify diseases to select the best control method.
• Bacterium or fungus? • Water mold or true fungus?
Apply least toxic fungicide when needed