Common Name: Early blight of tomatoes
Scientific Name: Alternaria solani
One of the most severe tomato diseases affecting home gardeners, early blight can affect the stems, leaves, and fruit of tomato plants. It can also cause damping-off in seedlings. It doesn't occur in arid dry regions but is most active in warm, wet or rainy conditions.
Plant symptoms: Brown circular "bull's eye" rings appear as 1/2-inch diameter leaf spots. The spots appear first on lower, older leaves. As the disease progresses, stems and fruit may become infected. Leaves turn yellow and wither, starting at the base of the plant, until the whole plant is affected.
Fruit symptoms: Similar to the leaf spots, the spots on fruit appear as dark brown concentric rings with the presence of dark, dusty spores. As the disease progresses, the spots on more mature fruit may appear larger, dark, dry, and sunken.
The fungus survives in infested soil and plant residue. Therefore, planting successive years in the same soil can cause a recurrence of the disease. It is caused by different fungi than late blight. The disease can be carried by infected seed and can be spread by wind, water, insects, humans, and equipment. The spores that land on plants will affect the leaves when they are wet.
The Alternaria fungus is most severe on plants stressed by a heavy load of fruit or low nitrogen fertility and is worse during the rainy season.
A combination of practices can help keep the fungus under control. The best method is prevention. Once it's started the disease is very difficult to control. Fungicide may be your only option.
For more information on the Alternaria fungus and other tomato diseases, contact your county Extension office.
Technical content: Alfredo Rueda and Anthony M. Shelton, Cornell University