On Older Plants: Dark spots with concentric rings develop on older leaves first. The surrounding leaf area may turn yellow. Affected leaves may die prematurely, exposing the fruits to sun scald.
Dark lesions on the stems start off small and slightly sunken. As they get larger, they elongate and you will start to see concentric markings like the spots on the leaves. Spots that form near ground level can cause some girdling of the stem or collar rot. Plants may survive, but they will not thrive or produce many tomatoes.
On Tomato Fruits: If early blight gets on the fruits, spots will begin at the stem end, forming a dark, leathery, sunken area with concentric rings. Both green and ripe tomatoes can be affected.
On Seedlings: Affected seedlings will have dark spots on their leaves and stems. They may even develop the disease on their cotyledon leaves. Stems often wind up girdled.
Early blight is hard to battle, but there are some methods to control it and even some resistant varieties of tomatoes, as explained on the next page.