Diseases to monitor in greenhouse tomatoes

Diseases to monitor in greenhouse tomatoes

Most foliar vegetable diseases require leaves to be wet in order for infection to occur; rain is often necessary for disease spread. Since greenhouse structures control rainfall on plants, these structures reduce leaf wetness. For this reason, diseases common in field tomatoes such as Septoria leaf blight and bacterial spot are less common in a greenhouse than they are in a field.

However, greenhouse tomatoes are not free from disease; they have their own set of problems. Greenhouse tomatoes often experience conditions of high relative humidity compared to field-grown tomatoes due to the enclosed nature of the structure. Under high relative humidity, the diseases discussed below are more likely to occur. In addition, greenhouse tomatoes grown in-ground to maturity are often not rotated with another crop, which increases disease pressure. Tomatoes grown in containers to maturity do not have the same crop rotation requirements. Tomatoes grown in a greenhouse for transplant production are exposed to greenhouse conditions for only a few weeks.

Four of the most common tomato diseases in the greenhouse are discussed below; however, this list is not exhaustive. Many more diseases may occur in tomato production in greenhouses.

Gray Mold (Botrytis cinerea)

Gray mold affects many vegetable and ornamental crops. Vegetable hosts include tomato, lettuce, pepper, and snap bean. If left uncontrolled, gray mold can cause severe symptoms on leaves, stems, and fruit. Gray mold lesions often start as small, water-soaked areas on leaves. Under dry conditions, the lesions turn a light brown. Lesions often are wedge-shaped with the wide edge on the leaf margin. One can easily observe the growth of the causal fungus with a 10x hand lens. Gray mold may also cause lesions on stems and fruit.

Since gray mold often infects injured tissue, growers should avoid practices that wound plants. Temperatures above 75şF decrease disease severity. Any practice that lowers relative humidity tends to lower the severity of gray mold and many other diseases. The addition of lime to soils to increase the calcium content of tomato plants may help to reduce the susceptibility to gray mold. The fungus that causes gray mold may survive season-to-season in crop residue.

Leaf Mold (Passalora fulva)

Although leaf mold produces quite noticeable leaf symptoms, the disease is usually not serious, depending on how far along into the season the disease occurs. Leaf mold causes bright yellow, blotchy lesions on the top of tomato leaves. On the undersides of leaves, the fungus that causes leaf mold can clearly be seen as an olive-green fuzz. Under severe conditions, the fungus can be seen growing on the top of the leaf as well. Lesions do not appear on stems or fruit. The spores are easily airborne, which spreads the disease throughout the greenhouse. Spore germination is favored by high humidity.

The optimum temperatures for leaf mold are between 72şF and 75şF. An excellent management strategy is to use tomato varieties that have resistance to leaf mold. However, some resistant varieties are not listed as such and variability in the fungus population may overcome the tomato plant's resistance. Other management options include taking measures to reduce humidity, increase airflow, and improve sanitation. The fungus that causes leaf mold may survive season-to-season in crop residue.

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Source: Greenhouse Grower


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