Best practices for avoiding tomato diseases in greenhouses

An increasing number of growers have built greenhouses to facilitate the early production of produce, with tomato being the most common and most profitable crop, as noted in the Greenhouse Management’s “Top 15” in the January issue. (By definition, a greenhouse is heated, whereas a high tunnel is not. This article will use the term greenhouse. The disease management issues are similar for both types of structures.)

The diseases common in field-produced tomatoes differ from diseases of greenhouse-grown tomatoes. Tomatoes grown in a greenhouse are often exposed to higher relative humidity than field grown tomatoes. High humidity in greenhouses is due to the covering over greenhouses that tend to keep moisture inside the greenhouse. Also, greenhouse tomatoes are often grown closer together than field grown tomatoes, decreasing airflow and increasing humidity.

Finally, tomatoes grown in greenhouses to maturity are not often rotated with another crop, increasing disease pressure. This article will discuss two common tomato diseases of the greenhouse, Botrytis gray mold, a foliar disease, and white mold, caused by a soil borne microbe. Finally, general disease management recommendations will be discussed.

Botrytis gray mold, caused by a fungus that survives in crop residue, is favored by cool conditions. Leaf lesions are often wedge shaped with the wide edge on the leaf margin. The growth of the causal fungus is easily visible with a 10X hand lens. Lesions may also be observed on stems and fruit. Gray mold affects many other crops such as flowers, lettuce, pepper, and snap beans. Since gray mold is often associated with injury to tissue, avoid practices that may cause wounding of plants. Any practice that lowers relative humidity tends to lower disease severity of gray mold and many other diseases (see suggestions at the end of this article.)

White mold (also known as timber rot) causes woody looking lesions on stems that can girdle and kill the tomato plant. The white growth of the causal fungus can often be found on the diseased tissue as well as dark, irregularly shaped fungal structures (sclerotia) (Figure 2). Sclerotia may survive for several years in the soil. Sclerotia germinate to form very small mushrooms. Spores from these mushrooms can infect a wide host range of plants. White mold is favored by cool temperatures (from 59°F to 70°F). High humidity and moist conditions contribute to the disease.
 

General management methods for greenhouse diseases

Crop rotation. A good crop rotation of 3 to 4 years away from tomatoes or other solanaceous crops will help to control most tomato diseases. However, many vegetable growers who operate greenhouses find that it is not practical to rotate away from tomatoes. In such cases, I recommend that the grower take as much of the tomato plant as possible out of the greenhouse and away from all potential production areas as soon as the last of the crop is harvested. I also recommend the use of landscape fabric between the plastic mulch on each row (Figure 3). The landscape fabric will help to reduce the amount of crop residue that enters the soil. Crop residue often harbors disease-causing microbes. The landscape cloth is changed or cleaned between seasons.
 

Ventilation. In general, reducing relative humidity and moisture in a greenhouse will reduce the severity of many tomato diseases. Here are some practices for doing so:

• Ventilate the greenhouse at dusk so that the drier air from outside replaces humid air inside the greenhouse.
• In high tunnels where heating is passive, it becomes important to close the vents before dusk to allow some heat to build up before temperatures start to drop. This allows warm air to be trapped overnight. Note, this is how to approach ventilation when cool temperature damage is a concern. Once cool temperatures are no longer a concern, one can manage the ventilation the same way you would for a greenhouse.
• Avoid the temptation to crowd as many tomato plants as possible into the greenhouse. Spacing of plants can vary depending on your situation, but a good starting point would be rows spaced 4-5 ft apart and 24 inches between plants within the row.
• Prune tomato plants to facilitate airflow. Lower leaves may be pruned periodically without reducing yields. This is especially true with indeterminate tomato varieties.
• Avoid pruning too much material at any one time. As a general rule indeterminate tomatoes should be left with at least twenty fully mature leaves after any pruning.
• To further facilitate reduction of humidity and leaf wetness, in greenhouses it is important to utilize appropriate air circulation fans. The placement and number of fans will be dependent on the size volume of air within the structure.

Fungicides. The use of fungicides can help to reduce the severity of tomato diseases in the greenhouse. However, fungicides will not substitute for good management practices. Read the fungicide label carefully and consult with your extension specialist with questions.

While greenhouse conditions may favor certain vegetable diseases, such as the ones discussed above, greenhouse conditions may also slow the spread of several diseases since there is no rain on greenhouse tomatoes to splash spores and bacteria from plant to plant. For example, early blight and bacterial spot diseases may occur under greenhouse conditions, but are likely to be limited due to the lack of rain that is necessary for disease spread.

The list of diseases given here is not exhaustive. Many more diseases may occur in greenhouses. However, the principles described here will help to lessen the impact of tomato diseases.

Dan Egel is an extension plant pathologist at Purdue University and Shubin K. Saha is an assistant extension professor, vegetable crops at the University of Kentucky.