Early blight is a fungus that affects leaves throughout the summer. As plants begin to grow and leaf material fills in and touches other plants, air circulation is reduced. The humid microclimate that develops as a result of reduced air circulation is ideal for the early blight fungus. When the fungus is present, lower leaves begin to turn yellow and contain dark brown circular spots. To avoid the problem, be sure to allow adequate space between plants. Dusting with sulfur may help control severe disease problems.
Virus disease symptoms also may appear late in the season. Small white or yellow rings on ripening fruit indicates the presence of tomato spotted wilt virus (TSWV) which is spread by western flower thrips (insects). Although the fruit is edible, the disease may affect flavor and fruit production. Seed and plant varieties can be purchased that are TSW resistant. (Soil-borne fungi can cause vascular wilt disease.)
There is no control for viral diseases, other than plant removal. Whether or not disease is present, it's wise to rotate tomatoes into another part of the garden.
For more information, see the following Colorado State University Extension fact sheet(s).
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Updated Tuesday, July 22, 2014