Early Blight on Tomatoes
Early blight is a common tomato disease that infects the leaves, stems and fruit. This fungus is most prevalent during warm wet weather.
Early blight first appears as brown spots with dark concentric circles on lower leaves, then spreads up the plant. Badly infected leaves eventually turn yellow and drop.
Minimize the risk by growing blight resistant or tolerant varieties. Ask your local extension service for a list of disease resistant tomatoes suited to your climate. Further reduce the risk by keeping plants healthy and properly fertilized.
Rotate plantings if possible.
Properly space and stake or tower the plants to increase airflow to help reduce the risk of disease. Mulch the soil, water early in the day and if possible apply the water directly to the soil with a soaker hose or watering wand.
A bit more information: Early blight is spread by wind, air and water. The fungus overwinters in debris in soil for several years. Remove susceptible weeds and volunteer tomato plants to further reduce the risk of this and other diseases. Review the symptoms and treatment for other leaf spot diseases when diagnosing tomatoes problems.