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Scouting for Plant Diseases
Jean Williams-Woodward University of Georgia
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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
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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
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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
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(wet foliage or soils, high humidity, poor air circulation)
Disease Triangle Host Pathogen Stressed or injured plant Capable of causing disease (many are host specific) Disease Environment WATER! (wet foliage or soils, high humidity, poor air circulation)
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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)
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Products available Fungicides Bactericides Biological products
Primary disease management tool Bactericides Coppers Phosphonates (Xanthomonas sp.) Biological products RootShield (Trichoderma harzianum) Cease (Bacillus subtilis)
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Use Fungicides to Increase Lag Phase of Epidemic
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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
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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)
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Root Disease Symptoms Wilting, stunting, leaf yellowing and drop, softening and discoloration of roots and stems, branch dieback, plant death Phytophthora Pythium Rhizoctonia
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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
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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
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Fungal leaf spot diseases
Wet leaves favor disease Remove infected leaves, plants Fungicides used preventively and to reduce spread
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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 Rust
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Snapgradon Rust Geranium Rust
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Powdery mildew
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Powdery Mildew Downy Mildew
Warm, humid, dry Sporulates anywhere Local infections Cool, humid, wet Sporulates on underside Local and systemic
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Rose Impatiens Lamium Rudbeckia
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Argyranthemum Veronica From U. Mass website
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Bacterial Leaf Spots Disease favored by: Spread
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)
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Virus Diseases TSWV, INSV, CMV, TMV, others
Common symptoms Tissue deformation Stunting Leaf necrosis Mottling, Mosaic Ring spots
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