Early blight is a relatively common problem for the home gardener growing tomato plants. A fungal infection that can easily spread in wet weather, early blight can destroy the foliage of tomato plants, leaving exposed, undersized fruit that can be affected by sunscald. Since early blight infection can take place shortly after transplants are set, early prevention and treatment can help save your future tomato crop.
Early blight spores from the fungus Alternaria solani are commonly found in soil, especially if you've had an early blight problem in the past. Spores are transmitted to young plants in wet weather, when soil can splash onto plant leaves. Spore transfer can also occur when watering tomato plants from overhead instead of at the soil surface.
Early Blight Symptoms
Leaves will fall off of the tomato plant, starting from the bottom and working its way up the plant as the disease progresses. Diseased leaves will have brownish-black spots on them that measure 1/4 to 1/2 inch long. As the spots grow and merge, irregular blotches will appear on the leaves. Early blight can also affect growing tomato fruit. It will appear at the stem, leaving dark black concentric rings that almost look like velvet.
Foliage from infected tomato plants should be removed from the garden area and discarded. Because the spores can survive, don't use the foliage in compost. Spores can also survive winter temperatures. As a result, turning over infected plants will more than likely cause reinfection next year. Using a physical mulch barrier like black plastic can keep the soil from splashing onto leaves and transmitting spores. Straw and newspaper can also be effective barriers. You may also consider treating plants with a fungicide shortly after transplanting and repeating the treatment every seven to 10 days through the growing season. Additional applications of fungicide after wet weather are also beneficial.
The Mississippi State University Extension Service recommends rotating the location in the garden where you plant tomatoes annually to help prevent the spread of blight. Laying black plastic mulch or another physical barrier between the soil surface and the tomato plant can help prevent the spread of early blight. When watering tomato plants, water them at root level instead of from overhead to minimise soil splashing onto the plants.