Older leaves on your tomatoes turning yellow? It's probably early blight.
Posted: June 30, 2011
Early blight is an extremely common disease on tomatoes. By late July, I can usually find it getting started in just about any planting of tomatoes that hasn’t been routinely sprayed by a fungicide. But in years like this one, when we’ve had plenty of rainfall throughout the spring and early summer, early blight starts early and spreads quickly.
Caused by a fungal pathogen, Alternaria solani, early blight gets started on the lower leaves of tomato and potato plants. Besides leaf yellowing, symptoms include spots or lesions that start out small but increase in size. Larger lesions often have a target-spot or bull’s eye appearance, with concentric rings visible within the lesion. Usually, fruit are unaffected, but when the disease starts early in the season and conditions are favorable for continued infection, fruit can show lesions near the stem end.
Don’t confuse early blight with late blight, a far more devastating disease that usually gets started in the upper portions of the plant, and is characterized by large, dark, greasy-looking lesions on leaves and stems. As of late June, we have not had any confirmed cases of late blight in our area, but
What can be done about early blight? A number of cultural practices can help keep this disease at bay. These include rotating away from tomato or potato for a minimum of three years, tilling under any plant residue from the previous season, spacing and pruning plants to allow good air drainage through the plant canopy, and avoiding getting leaves wet when watering. For the most effective control, especially during rainy periods, apply a fungicide, taking care to get good coverage on the upper and lower leaf surfaces, especially on older leaves. Most fungicides that are labeled for homeowner use on vegetable gardens will be effective on early blight, particularly those with chlorothalonil as the active ingredient. Chlorothalonil will also protect the plants from late blight infection.
For more information, see the Early Blight of Potato and Tomato factsheet from Ohio State University.