The best approach for dealing with fungal problems is prevention: practicing good sanitation in and around the greenhouse. No weeds should ever be allowed to grow in or near the house, and the greenhouse floor should be kept clean with any plant debris removed promptly and composted far from the greenhouse air intakes. Any affected plants should be destroyed and not composted. If the greenhouse floor is dirt, sand, or gravel, consider covering the entire floor with a white plastic groundcover. This will accomplish several things: reduce pest and pathogens which reside in soil, reflect additional light to the plants and decrease humidity by preventing soil moisture from evaporating up from the warm greenhouse floor. Also, all growing media should be sterilized before use. Peat moss and peat products should not be assumed to be free from all disease organisms.
High humidity and poor air circulation can exacerbate fungal problems. The spread of a fungal problem can often be observed following the direction of air or nutrient solution movement within the greenhouse.
Below are some fugal diseases that can affect tomatoes in greenhouse environments:
Anthracnose (Colletotrichum sp.): manifests as small, sunken spot rot on ripe fruit. Also known as "Black dot root rot" due to spotted infections on white roots. Most prominent in high temperature and high humidity conditions. Control with fungicide spray.
Gray mold (Botrytis cinerea): appears as "ghost spots" on green fruit and moldy-looking stems, disease may girdle stems, killing the plants. Aggravated in cool, cloudy, humid conditions. Remove and destroy affected plants, raise temperatures and increase air circulation to reduce humidity. Fungicides are effective against this disease.
Buckeye rot (Phytophthora sp.): Large, dark brown soft spots on fruit. Also can cause damping off of seedlings, crown and root rot. Moderate moisture and temperatures favor this disease. Fungicides can help to control.
Damping off (Pythium or Phytophthora sp.): Blackening at the soil line of young seedlings or collapsed roots. Caused by soil-born fungi. Make sure seeding media is sterile and well drained.
Early blight (Alternaria sp.): Irregular brown spots on lower leaves or stems, enlarging and creating a "bulls-eye" pattern. Stem spotting and stem girdling may also occur. Symptoms may not appear immediately on green fruit, but become evident in transit. Increase air circulation, reduce humidity, and remove older leaves. Fungicides may not be very effective, choose resistant varieties.
Fusarium wilt (Fusarium sp.): Often called the "yellows" due to the overall yellow color of the plant, sometimes starting with older leaves. Browning of vascular tissue in stem indicates the fungus is clogging those tissues, causing wilting. Roots turn brown, and tap root may rot away. Choose Fusarium resistant cultivars and destroy all infected vines.
Leaf mold photo by M. Jensen
Leaf mold (Cladosporium fulvum or Fulvia fulva): Gray or brown spots on lower side of the leaf, leading to pale areas on upper surface. Can spread rapidly in greenhouses. Control by decreasing humidity and increasing the ventilation. Fungicides are also effective.
Late Blight (Phytophthora infestans): Irregular greenish-black watersoaked spots on older leaves, leading to leaf drop. Petioles bend downward. Under humid conditions, a white fuzzy growth may give plants a "frosted" look. Fruit lesions are brown, firm, and irregular. Most common when day temperatures are high and night temperatures are low. Control with fungicides.
Pythium rot (same organism as Damping off, but attacking at a later stage of the plant): Appears as a sunken brown spots on fruit, outlined with concentric rings. Recent research indicates that low concentrations of surfactants are effective in killing the mobile zoospores in recirculating (closed) hydroponic systems, preventing spread of the disease. Surfactants are not, however, effective in curing diseased plants.
Septoria leaf spot (Septoria lycopersici): Common in the Eastern US during late summer. Older leaves have many small, dark spots. Leaves may drop. Increase air circulation, and reduce humidity. Remove affected leaves. Treat with fungicide.
