Common Diseases of Greenhouse Plants

Powdery Mildew

Foliage Diseases

Powdery Mildew: whitish, powdery growth on upper leaf surface. More prevalent on newer growth. May cause leaf distortion.

Downy Mildew: whitish growth on undersides of leaves; pale green or yellow patches may appear on upper leaf surface. Symptoms usually appear first on older leaves. Leaves often turn brown and die.

Botrytis (Gray Mold): round or triangular fuzzy grayish brown spots that eventually engulf the entire leaf. Petiole and stem may turn brown, eventually girdling the stem. Plant wilts and dies above stem lesions. Small whitish or tan spots may appear on flowers and buds, spreading to include the entire flower. Botrytis can cause damping off of seedlings.

Rust: underside of leaf dotted with white, orange or brown spores. Yellow or necrotic spots may appear on upper surface.

Impatiens Necrotic Spot Virus (INSV) and Tomato Spotted Wilt Virus (TSWV): varied symptoms include spots, zonate spots, ring, mosaic and line patterns, necrosis at petiole end of leaf, stunting, tip dieback, stem cankers, wilt. Patterns may be brown, black, yellow or white depending on the host plant. Spread by thrips.

Leaf Spots: Pseudomonas, Alternaria, Xanthomonas, Septoria: Symptoms vary with host plant and disease organism, but usually begin as brown or black spots that may spread and include the entire leaf, causing leaf drop/death. Flowers may also be affected.

Stem Diseases

Fusarium Stem and Root Rot: foliage turns off-color and begins to wilt, usually starting on one side of the plant. Eventually entire plant wilts and dies. Dark brown, rotted lesions on the crown and stem. Often a whitish fungal growth is seen with the lesions.

Root and Crown Diseases

Pythium Root Rot: roots dark brown, cortex sloughs off when pulled, leaving white inner root. Stem may turn black, lower leaves turn yellow and brown, eventually plant wilts and dies. Can cause damping off of seedlings.

Phytophthora Root Rot: stunting of plant, brown roots, stem cankers, leaves turn brown where attached to stem, wilting rapidly and eventually dying. Can cause damping off.

Rhizoctonia Stem and Root Rot: dark brown or black lesions at base of stem, eventually girdling the plant causing wilting and death. Most common cause of damping off.

Thielaviopsis (Black Root Rot): yellowing of younger growth, often looks like nutrient deficiency, distinguished from other root rots by blackened root tips.

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