Tri-County News

Tomato blight can be controlled


“Tomato blight” has attacked the tomato plants in many homeowners’ gardens again this year. The lower leaves of the plants first turn yellow, then brown and dry up before falling from the plant. This disease, is formally known as Septoria leaf spot, and is caused by a fungus. While it is really too late to do much about protecting this season’s crop, University of Minnesota (U of M) extension plant pathologist Janna Beckerman outlines management strategies that will protect the 2005 crop. Symptoms Septoria leaf spot primarily infects leaves, although stem, petiole and fruit lesions may occur. The disease can occur at any stage of plant development, but usually appears after fruit set. Initially, round, yellow spots develop on lower leaves. Later, these spots enlarge and turn brown or gray in color. Tiny black fruiting bodies eventually form in the center of the leaf spots. These fruiting bodies produce spores, which cause secondary infections throughout the plant, usually in an upward direction. Leaves that are heavily-infected will turn yellow, then brown and fall from the plant. Defoliation can cause sunscald damage to the fruit. How it spreads Under warm, wet conditions, splashing rain, insects and the hands and clothing of gardeners spread the spores. Septoria overwinters in the debris of diseased plants in the soil and on weeds of the nightshade family. Controlling tomato blight Beckerman points out that no single management strategy will cure the problem, but a combination of methods should be used to decrease its effect. Sanitation and crop rotation will reduce the number of spores that cause the primary infections. Sanitation involves the removal of all infected plant material and weeds throughout the season, as well as after harvest. To reduce secondary infections, water plants at the base early in the day and stake or cage plants to increase air circulation. Apply mulch around the base of the plants to minimize water splash, which will carry the spores upward onto the plant. Planting disease-free seeds and plants can help control Septoria, but currently there are no cultivars that are reliably resistant to the disease. Fungicide applications may be necessary in addition to these management strategies, particularly during rainy seasons. Beckerman cautions gardeners to read the label carefully and apply only as directed. Follow a regular fungicide application schedule, starting 2-3 weeks after transplanting. It may be unrealistic to expect good control if fungicide treatment begins after plants are badly infected. Removing the most severely infected foliage and applying fungicide to foliage that does not show symptoms will reduce the impact and spread of the disease. Applying fungicide Beckerman lists the following fungicides as labeled for control of Septoria leaf spot. Apply one of the following at seven to 10 day intervals starting at the first sign of the disease or immediately after bloom if an application program was not started earlier. o Copper containing products (Bordeaux 8-8-100, Kocide, TopCop). These products are considered acceptable for organic production. o Chlorothalonil containing products (Daconil 2787, Fungonil). o Maneb containing products (Bonide Manzate). Do not apply within five days of harvest. o Mancozeb containing products (Acme Maneb Tomato & Vegetable Fungicide, Greenlight Maneb Plus, Dragon Mancozeb Disease Control). There may be other products containing the fungicides listed. Be sure that any product you select is labeled for application to tomatoes and follow all directions and precautions. It is important to apply these products to the undersides of the leaves as well as to the surface.

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