Early blight, caused by the fungus Alternaria solani, is one of the most common diseases of tomatoes in Virginia. It occurs to some extent every year wherever tomatoes are grown. In spite of its name, the disease may occur at any time during the growing season. The fungus attacks leaves, stems and fruit. It can also cause disease on potato, pepper, and eggplant.
Alternaria solani may cause damping-off in the seedbed and a stem canker or collar rot that is destructive to transplants in the field (Fig. 1). However, the first and most noticeable symptoms are usually the small, irregular, brown spots that form on older, lower leaves. These spots may enlarge until they are one-half inch in diameter. Spots have concentric rings or ridges that form a target-like pattern and are often surrounded by a yellow halo (Fig. 2). Some spotting of the leaves may appear early in the season, but the greatest damage usually appears after fruit set. Severe defoliation may occur during periods of high temperature and high humidity and expose fruit to sunscald.
On stems, lesions are initially small, dark, and slightly sunken. Small spots enlarge to form elongated lesions with concentric markings similar to those on the leaves (Fig. 3). Large spots that occur on the stem near the ground line can cause partial girdling or collar rot. Plants that survive the early stem cankers usually remain small and produce few fruits.
Early blight also affects the fruit. Dark, sunken, leathery lesions appear on the stem-end of the fruit. On older fruit these lesions reach considerable size and the rot extends deep into the flesh of the fruit. Heavily infected fruit usually drops. Infected tomatoes that reach maturity are not marketable. Under environmental conditions favorable for disease development, the fungus may also cause spotting of the fruit stems and blossoms and drop of young fruit.
|Refer to the current Virginia Pest Management Guide for Home Grounds and Animals (VCE Publication 456-018), http://pubs.ext.vt.edu/456/456-018/456-018.html, for details on the proper use of pesticides.|
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Issued in furtherance of Cooperative Extension work, Virginia Polytechnic Institute and State University, Virginia State University, and the U.S. Department of Agriculture cooperating. Alan L. Grant, Dean, College of Agriculture and Life Sciences; Edwin J. Jones, Director, Virginia Cooperative Extension, Virginia Tech, Blacksburg; Jewel E. Hairston, Administrator, 1890 Extension Program, Virginia State, Petersburg.
May 1, 2009