Disease prevention and management is an inescapable part of growing crops in a greenhouse. Growers who scout thoroughly and know their crops well enough to spot the first signs of a plant disease developing will have the greatest likelihood of curtailing its impact on crop quality and yield.

Three components determine whether a plant disease develops: 1) a susceptible host, 2) an environment conducive to the pathogen, and 3) an appropriate pathogen. The goal of disease management is to keep the level of disease underneath the economic damage threshold or level of disease needed to cause a resulting yield/quality loss. There are many steps you can take to prevent and manage disease in the greenhouse. A good start is to understand what environmental conditions the plant disease needs to develop and recognize the first signs of disease development. This handy guide is a good refresher for those who are more experienced with plant diseases and a nice place to begin for those who are less familiar with diseases they might encounter in the greenhouse.

ROOT, STEM, CROWN, AND COLLAR DISEASES

Pythium

Crops affected: Ornamentals, vegetables, cannabis

Seasonal Timing: Year-round

Conditions: Wet soils, poor drainage, cool temperatures, excess soluble salts

Symptoms:

• Damping-off
• Wilting, browning, or yellowing of lower leaves
• Leaf drop
• Stunting
• Mushy, brown roots (outer tissues slough off)
• Death

 

Phytophthora

Crops affected: Ornamentals, vegetables

Seasonal Timing: Summer

Conditions: Wet soils, high temperatures, high humidity, overgrown plants, propagation under mist, water splashing

Symptoms (vary with host plant):

• Attacks at the root crown or stem
• Large, wet-looking leaf spots
• Blackened cankers at the stem base
• More likely to kill plants than stunt them
• Root rots
• Brown lesions at the base of leaves attached to cankered stems

 

Rhizoctonia

Crops affected: Ornamentals, vegetables

Seasonal Timing: Summer

Conditions: Warm, wet weather; poor drainage, wet leaves, high temperatures

Symptoms:

• Damping-off (affects seeds and new seedlings)
• Attacks plant stem at the soil line
• Brown to black cankers at stem base girdle or kill plants
• Infected stems have a dry, shriveled, or wiry appearance
• Sunken cankers near the soil line

 

Thielaviopsis (Black Root Rot)

Crops affected: Ornamentals

Seasonal Timing: Year-round

Conditions: Temperature stress, high pH

Symptoms:

• Chlorotic lower foliage
• Poor rooting
• Stunted growth
• Blackened roots
• Rotted root hairs and secondary roots

 

FOLIAR DISEASES

Powdery Mildew

Crops affected: Ornamentals, vegetables, cannabis

Seasonal Timing: Spring and Fall

Conditions: High humidity, cool nights, and warm days

Symptoms:

• Starts with pale-yellow leaf spots
• White, powdery spots on upper and lower leaf surfaces
• Irregular, chlorotic, purple areas, or necrotic lesions, followed by a white, powdery appearance on leaves
• Fruit ripens prematurely
• Pre-mature leaf drop, shriveling
• Distorted new shoots
• Stunted growth
• Leaf rolling

 

Downy Mildew

Crops affected: Ornamentals, vegetables

Seasonal Timing: Year-round, depending on the plant

Conditions: High humidity, cloudy, poor air circulation

Symptoms (vary depending on host plant):

• Small, yellow spots develop on the upper sides of leaves while white to bluish and grayish, fuzzy-looking spores and mycelium develop on the lower leaf surfaces
• Infected leaves and branches may distort and die
• Spore colonies can grow systemically throughout a plant

 

Fungal Leaf Spots

Crops affected: Ornamentals, vegetables

Seasonal Timing: Spring to Fall

Conditions: Wet leaves, moderate temperatures

Symptoms:

• Tan to reddish-brown to black circular lesions with distinct borders
• Lesions may run together to form irregular shapes or blighting of the entire leaf
• Spots can enlarge and grow onto branches and stems

 

Bacterial Blight

Crops affected: Ornamentals, vegetables

Seasonal Timing: Summer

Conditions: 72⁰F to 95⁰F, high humidity, over-irrigation

Symptoms:

• Dark-brown necrotic (dead) leaf spots with yellow halos
• Leaf curling and twisting
• Necrotic blotches start at the leaf margins and advance inward
• Death of branch tips, leaves, and blossoms in severe cases

 

FUNGAL DISEASES

Botrytis (Gray Mold)

Crops affected: Ornamentals, vegetables, berries, cannabis

Seasonal Timing: Year-round

Conditions: 38⁰F to 77⁰F; high humidity, cloudy or rainy, poor air circulation

Symptoms:

• Gray, fuzzy coating on aging flower blossoms and soft, ripe fruits
• Brown to gray circular spots that later become fuzzy
• Spotting and decay of flowers, leaves, fruits, and berries
• Damping-off
• Leaf spots and blights
• Bud rot
• Can cause cankers on roses and other ornamental plants

 

Fusarium Wilt

Crops affected: Ornamentals, vegetables, cannabis

Conditions: High temperatures, low pH, high levels of ammonium nitrate

Symptoms:

• Wilting
• Stunting
• Lower leaves turn yellow and then brown and die
• Browning of vascular tissue
• Stem canker
• Root rot diseases