SCOUTING FOR PLANT DISEASES
Jean Williams-Woodward
University of Georgia
Scouting for Plant Disease
• Disease control relies on prevention• You cannot cure a plant of a plant disease• You cannot manage diseases by reacting to symptoms
• The time between infection and symptom development may be 21 days or more
• By the time you see symptoms, it is too late to manage the disease on that plant
Diseases come from…
• Infected plant liners, plugs, divisions• Off-shore production introducing exotic pathogens
into USA• Example: Daylily rust
• Residual plants from previous crop or previous season
• Plant and soil residues and debris
Pathogens causing disease:
• Fungi • Root rots, leaf spots, rusts, mildews
• Bacteria• Leaf spots, vascular wilts, soft rots
• Viruses• Nematodes
• mostly foliar nematode
• Phytoplasmas• Aster yellows
Disease Triangle
WATER! (wet foliage or soils, high humidity, poor air circulation)
Stressed or injured plant
Capable of causing disease (many are host specific)
Host Pathogen
Environment
Disease
Disease Management Principles
• Eliminate initial inoculum (pathogen survival)• Sanitation• Scouting for early detection
• Reduce pathogen spread• Water splash• Plant-to-plant contact• Wind dispersal• Vectors (insects, workers, tools)
Products available• Fungicides
• Primary disease management tool
• Bactericides• Coppers• Phosphonates (Xanthomonas sp.)
• Biological products• RootShield (Trichoderma harzianum)• Cease (Bacillus subtilis)
Use Fungicides to Increase Lag Phase of Epidemic
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20
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No fungicideSpray #1Spray #2Spray #3Spray #4
What to look for…• Concentrate on entryways into greenhouse (doors, open vents, walkways)
• Concentrate on incoming plants• Look for out-of-the-ordinary plants
• Stunted• Off-color• Yellowing• Wilting• Browning• Distorted• Leaf spotting
Root and Cutting Diseases• Common diseases are damping-off and root rot• Symptoms include wilting, leaf yellowing, plant stunting,
softening and discoloration of roots, plant death• Major pathogens: Pythium, Phytophthora, Rhizoctonia• Disease favored by wet rooting medium, high humidity,
poor ventilation, high soluble salts (EC)
Root Disease Symptoms
• Wilting, stunting, leaf yellowing and drop, softening and discoloration of roots and stems, branch dieback, plant deathPythiumPythium
RhizoctoniaRhizoctonia
PhytophthorPhytophthoraa
Black Root Rot (Thielaviopsis basicola)
• Disease favored by:• High potting medium
moisture, alkaline medium pH, excess fertilization
• Spread • Water movement and
contaminated pots, trays, rooting medium
• Survives as chlamydospores• Control
• Sanitation, fungicides
Botrytis Blight (Gray Mold)
• Spores dispersed by water impact and twisting from stalks upon drying
• Disease favored by • High humidity (>85%)• Moderate temperatures• Free moisture (4 hr at 79F)• Low light
• Control• Heating and venting,
sanitation, reduce plant wetness, fungicide sprays
Fungal leaf spot diseases • Wet leaves favor disease• Remove infected leaves, plants• Fungicides used preventively and to reduce spread
Leaf Rusts• Disease favored by:
• High humidity• Moderate to cool
temperatures• Spread by wind-blown
spores• Survives in living plants,
plant debris• Control
• Sanitation, discard plants, remove infected foliage
• Fungicides• Resistant cultivars
Daylily RustDaylily Rust
Snapgradon RustSnapgradon Rust Geranium RustGeranium Rust
Powdery mildew
Powdery Mildew Downy Mildew
• Cool, humid, wet• Sporulates on underside
• Local and systemic
• Warm, humid, dry• Sporulates anywhere
• Local infections
RoseRose ImpatiensImpatiens
LamiumLamium RudbeckiaRudbeckia
VeronicaVeronica
ArgyranthemumArgyranthemum
From U. Mass website
Bacterial Leaf Spots
• Disease favored by:• Free moisture on plant• Warmer temperatures• High humidity
• Spread• Water splashing• Plant-to-plant contact
• Survives in plant debris• Control
• Reduce water-splashing, sanitation!• Copper fungicides to reduce spread (limited)
Virus DiseasesTSWV, INSV, CMV, TMV, others
• Common symptoms• Tissue deformation• Stunting• Leaf necrosis• Mottling, Mosaic• Ring spots