Alternaria solanii, Alternaria tomatophila
Early Blight is the most common tomato disease in more humid areas. It is a form of Alternaria Blight that first manifests itself as irregular shaped dark brown concentric spots on the shaded lower leaves. These slowly enlarge and merge and badly infected leaves eventually die (those at the bottom of the plant first and then progressing upward). There may also be dark sunken lesions on the stem and the fruit may rot from the stem end and drop. It likes warm (85 degrees F) wet weather.
Early Blight most commonly attacks tomato and potato, but occasionally pepper and eggplant are affected too (though not usually as badly). Potatoes usually show signs of infection as they enter the flowering stage.
This fungus overwinters on weeds and decaying crop debris and spores may be transferred by rain, wind, tools, flea beetles and other insects. Minimize its effects by keeping your plants well fed and watered, as stressed plants are more susceptible. Mulch and stake tomato plants to keep soil from splashing onto the leaves. Staking also helps by improving sunlight penetration and air circulation (some gardeners deliberately prune off the lower leaves too). Avoid wetting plant leaves when irrigating (or water in the morning or early evening so they can dry out quickly). Rotate crops for 3 years. Remove infected plants.
This disease can also be transmitted on infected seed, so if you suspect a problem give them a hot water seed treatment at 127 degrees F for 25 minutes (or 1 part bleach to 10 parts water for 30 minutes).
Image: Howard F. Schwartz, Colorado State University, Bugwood.org