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

(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)

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 Bactericides Biological products 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

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 death Phytophthora Pythium Rhizoctonia

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 Rust

Snapgradon Rust Geranium Rust

Powdery mildew

Powdery Mildew Downy Mildew Warm, humid, dry Sporulates anywhere Local infections Cool, humid, wet Sporulates on underside Local and systemic

Rose Impatiens Lamium Rudbeckia

Argyranthemum Veronica From U. Mass website

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)

Virus Diseases TSWV, INSV, CMV, TMV, others Common symptoms Tissue deformation Stunting Leaf necrosis Mottling, Mosaic Ring spots