Disease Control in Greenhouse Tomatoes

Greenhouse grown tomatoes are subject diseases that occur only rarely in field grown crops including: Corky Root Rot, Fusarium Crown and Root Rot, Ghost Spot (Gray Mold), Leaf Mold, and Powdery Mildew. These diseases tend to predominate due to the unique environment in greenhouses and the time of year when tomatoes are grown in greenhouses.

Corky root rot caused by Pyrenochaeta lycopersici is a common and serious problem in spring grown crops, where the media may be too cold. It occurs in soil-grown, rockwool, and nutrient film crops. Symptoms first appear on fine roots as light brown lesions. Plants show a loss of vigor, yellowing, and stunting. Larger roots develop brown, corky lesions and the root cortex can be easily separated. Lesions can also develop at stem bases and massive root failure can occur. The pathogen is soilborne and while it is not known how it infests artificial substrates, it can survive for up to two years as microsclerotia. The fungus grows slowly and disease increases slowly as well. Management consists of transplanting into warm media, avoiding mulches until the soil warms, good ventilation of stem bases, resistant rootstocks, and soil pasteurization.

Fusarium crown and root rot caused by Fusarium oxysporum f.sp.  radicis-lycopersici also occurs in field grown tomatoes. The disease mainly infects fruit bearing plants and appears as thinning of plant tops, wilting, and a brown cortical rot at the soil line. A red-brown vascular discoloration occurs, leaves yellow, and roots may have dark lesions. Fruit loses color and firmness. The pathogen survives as chlamydospores in the soil and is seedborne. Chlamydospores or microconidia can be spread by workers, machinery, crates, crop residue, and soil amendments. The pathogen increases rapidly after introduction into fumigated or steam-sterilized soil. Microconidia may also be spread in irrigation systems and by fungus gnats. Management of Fusarium crown and root rot consists of resistant varieties, planting into warm soils, soil pasteurization, and mounding of soil around the bases of infected plants which stimulates the formation of adventitious roots.

Gray mold or Ghost Spot caused by Botrytis cinera is a common disease of greenhouse tomato that is managed by manipulation of the greenhouse environment. The pathogen has a wide host range, attacks senescing tissue, and survives as mycelium in infected crop debris or sclerotia in the soil. The fungus attacks leaves, stems, and fruit and causes light brown to gray circular lesions. Affected leaves collapse and dry up; infected stems can be girdled. Ghost spot occurs on fruit as a tiny, brown, raised necrotic spot surrounded by a white halo and results in a decrease in quality. Fruit may also become infected through flower parts and become s water-soaked with a brown, gray decay. Control is only achieved by providing adequate heat and ventilation in the greenhouse, especially at night. Relative humidity should be kept below 80% and air circulation improved by removing lower leaves and drip irrigation. Crop debris should be removed promptly. Fungicide (copper, mancozeb, PCNB, iprodione, thiophanate-methyl) sprays can be used, but resistance to many common materials has been reported in Botrytis populations. Fungicide rotations and combinations (Banrot, Spectro 90) should be practiced.

Leaf mold caused by Fulvia (Cladosporium) fulva is most important in poorly ventilated plastic greenhouses. The disease is equally severe in hydroponic production. Symptoms occur on the foliage as yellow-green areas on the upper surface and brown to purple, velvety fungal growth on the lower surface. Symptoms occur on lower leaves first then progress to younger leaves. Affected leaves yellow, curl, day and drop prematurely. Infected blossoms die before fruit set and fruit can develop a black, leathery stem-end rot. Disease is favored by high relative humidity and free water on the leaves. The pathogen produces abundant conidia and the disease spreads rapidly. F. fulva survives as conidia, sclerotia, or mycelium in the soil and in crop debris and may be seedborne. Management of this disease consists of improving air circulation by row and plant spacing, removal of lower leaves, and surface watering. Relative humidity should be kept below 80% by heating and ventilation. Diseased leaves should be removed and destroyed. If Leaf mold has been a problem, all crop residue should be removed after harvest and the entire greenhouse disinfested. Resistant cultivars are available.

One or more species of Oidium cause Powdery mildew on tomato. Severe disease can result in early senescence and reduced yields. Symptoms consist of white powdery growth on both leaf surfaces. Spots may yellow, become necrotic, and the leaves twisted and deformed. The pathogen is an obligate parasite and survives on overwintering tomato, alternate hosts, or fruiting bodies (cleistothecia) in infected crop debris. Conidia are spread by wind and do not require free water to germinate and infect plants. Control consists mainly of fungicide (sulfur, copper, mancozeb, potassium bicarbonate) sprays.