The main symptom of early blight is round leaf spots with a characteristic target appearance due to the dark concentric rings that develop in most spots. They are about .5 inch in diameter. Spots first appear on older leaves near the base of the plant. The causal fungal pathogen also produces symptoms on stems and fruit. Young seedlings can be killed by stem lesions developing at their base. Sources of the pathogen are infested seed, debris from infected plants left in or on the soil (where it can survive at least 1 year), and spores from other affected plants dispersed typically short distances by wind, water, insects, or animals.
Above is quoted from http://www.longislandhort.cornell.edu/vegpath/photos/early_blight.htm where there are more photos and details.
