yellow leaves on tomato plants

Asked July 9, 2015, 10:07 AM EDT

Last year and this year the leaves on my tomato plants keep getting yellow and there are black spots. They are all small tomatoes - red, yellow, orange cherry-type tomatoes. I have a sprinkler system but turn it off when there is rain. They are growing in a raised bed on my front lawn.What to do? Is it the soils?

Allegheny County Pennsylvania

1 Response

Early blight is an extremely common disease on tomatoes. By late July, it is often found on many tomatoes that haven’t been routinely sprayed by a fungicide.

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.

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.