University of Illinois Extension

Caring for Tomatoes

If you grow tomatoes and do not treat for diseases, then expect one of two foliar diseases to cause the loss of leaves, said James Schuster, a U of I Extension plant pathology specialist.

"The two diseases that cause tomato leaves to usually die from the bottom up are Septoria leaf spot (Septoria lycopersici) and Early Blight (Alternaria solani)," said Schuster. "Both are fungal diseases and both like free-standing water on the leaves. The water can be due to rains, dews or overhead watering. In addition, both diseases overwinter on the dead infected plant material on or just below the soil surface."

In the spring, splashing rains and blowing winds carry the pathogen onto the new plants. Fall sanitation can help delay serious infection the following year since the inoculum has been removed or reduced and the pathogen causing these leaf spots may have to blow in from another garden site.

"If you want to determine which of these diseases is on your plants, it will require close inspection of the disease symptoms," he said. "Septoria leaf spot first starts as tiny, water-soaked spots that soon enlarge to circular or angular lesions 1/8-1/4 inch across. As the spots enlarge, the lesions develop a dark margin with a grayish white center. The grayish white center usually contains tiny black spores. Leaves that are heavily infected turn yellow, wither, die and fall off in large numbers starting at the base of the plant. Septoria can infect other plant parts."

Early Blight spots are usually brown to black and develop target-like concentric rings as they enlarge to between 1/4 and 1/2 inches. Major leaf veins frequently limit the size of the lesions. The leaves yellow, wither, droop or fall off from the bottom up. This disease can and often does attack the stem, causing cankers that can kill the plant, and it can attack the tomato causing blackish, sunken and rotten spots on the fruit. Fruit lesions are often covered with a dark brown, velvety layer of spores. Schuster reports that Alternaria-caused diseases, such as early blight, can survive the winter not only on tomato debris but also on other plants.

Another disease, anthracnose, causes small, pale yellow or water-soaked lesions that quickly enlarge and become tan to dark brown or irregular and black. "Spots may merge and cause a blighted appearance or kill the leaf," he said. "This disease does best with frequent rains or heavy dews and temperatures between 68 and 85 degrees F. Crowding of plants and continuing warm, moist conditions favor the development and spread of anthracnose. In addition to tomatoes, other anthracnose fungi also infect and damage cucumbers, beans, peppers, and sweet corn."

Blossom-end rot is not an infectious disease caused by a pathogen. Instead it is a physiological breakdown of cell wall tissue due to insufficient calcium. "Wherever soils are very acidic, there may be insufficient calcium in the soil for the tomato to take up," Schuster explained. "However on alkaline soils, there are adequate amounts of calcium for the plants.

"Blossom end rot occurs when tomatoes are watered shallowly and often. The rapid wetting and drying of the soil inhibits the uptake of calcium - leading to blossom-end rot. Mulch the soil area where the tomato roots are growing and water deep and infrequently to avoid blossom-end rot."