Prevention is key to tomato blight control

Published on Wednesday, 30 November -0001 00:00
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Many home gardeners have seen stress and slow growth in their tomato plants this spring because of the abnormally cool air and soil temperatures. Within the past 10 days, however, the temperatures have warmed considerably providing favorable growing conditions for tomatoes and other warm-season vegetables. The succulent growth in combination with warm, humid weather makes tomato plants susceptible to "tomato blight," the common name for Septoria leaf spot and Early Blight which are two of the most destructive fungus diseases of tomatoes. Like all fungus diseases, these two leaf-spot diseases cannot be cured, only prevented.

Septoria leaf spot is the most prevalent of these diseases and normally infects the leaves and stems, but not the fruits. The disease is characterized by small spots with a dark outer ring and a lighter center. Tiny black fruiting bodies eventually form in the center of the leaf spots and produce spores which continue to spread the infection, climbing the plant like a ladder. Heavily infected leaves turn yellow, then brown and fall from the plant. The resulting defoliation can expose the fruit to the sun, which results in blistering and sunscald damage.

Early blight is identified by yellowing of leaves with gray-colored spots consisting of concentric rings. This disease infects not only leaves, but also stems and fruit, causing defoliation of the plant and fruit rot near the petiole.

These diseases survive in the soil on plant debris and on perennial weeds, particularly members of the nightshade family. The spores are spread by splashing water, insects, soil particles in the wind, and even on the hands and clothing of gardeners. The disease starts on the lower leaves of the plant and moves upward. Moisture on the leaves encourages the germination of the spores and development of the diseases.

As with many diseases, no single management strategy will cure the problem. Efforts should be concentrated on preventing the disease from infecting plants. Sanitation and crop rotation will reduce the number of spores that cause primary infections. Sanitation involves the removal of all infected plant material and weeds throughout the season as well as a complete cleanup of all plant material in the fall.

The diseases can be carried on seeds so purchase disease-free seeds or save seed only from healthy plants. There currently are no varieties of tomatoes that are reliably resistant to the disease.

To prevent secondary infections, concentrate on reducing moisture on the leaves. Plants should be spaced far enough apart and staked or caged to encourage free air movement and more rapid drying of the leaves. Use drip irrigation or a soaker hose so that the water is not applied to the leaves; also mulch around the base of the plants to minimize water splash which carries the spores upward onto the plant.

In addition to these management strategies, fungicide application may still be necessary, particularly if the diseases have reduced the tomato harvest in previous years. Again, prevention is the key and it is unrealistic to expect good control if fungicide treatment begins after the plants are badly infected.

However, the impact of the diseases infected plants can be reduced by removing the most severely infected foliage and applying a fungicide to the foliage that does not show the symptoms. Follow a regular fungicide application beginning 3-4 weeks after planting or, at the very latest, immediately after bloom begins and when the temperatures are 60-65 degrees F. Apply a fungicide labeled for tomatoes at 7-10 day intervals. It is important to apply the product on the undersides of the leaves as well as the surface. Products containing the following fungicides are labeled for tomatoes; copper (acceptable for organic production), chlorothalonil (Daconil), maneb and mancozeb. Read and carefully follow all directions and precautions when using these products and be sure to check the interval before harvest.