On Crops: Tomatoes
Worldwide, in temperate climates
The most common cause of dark spots on tomato leaves, early blight begins on the oldest leaves closest to the ground. The irregularly-shaped spots have concentric rings around their edges, sometimes with a dark dot in the centre. Affected leaves gradually wither to brown and hang on the stems, while leaves higher up on the plant remain green and healthy. Early blight spreads fastest in humid weather when temperatures are between 27-29C (80-85F).
Tomatoes can tolerate losing some of their low leaves to early blight, but if persistent rain causes the disease to move more than halfway up the plants, they may be doomed. Most of the time early blight weakens plants but does not kill them.
Try resistant varieties, which are new but very worthwhile. Grow tomatoes at wide spacing in full sun, so their leaves dry promptly each morning and after rains. In early summer, clip off the first spotted leaves when you see them, and compost them in an active compost pile. Use mulches to keep soil from splashing onto tomato leaves. When watering tomatoes, avoid wetting the foliage.
Invisibly small spores are spread to new leaves by wind, rain and insects, where warm, damp conditions encourage them to germinate and grow. Pruning off low leaves can slow the spread of the disease by reducing the number of spores present.