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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60

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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