Early Blight
Early blight begins on the lower branches of a tomato plant. Foliage will exhibit one or two spots per leaf, approximately 1/4 to 1/2 inch in diameter. Eventually, the spots grow larger and run together. Spots have tan centers with concentric rings in them and yellow halos around the edges. The stems will develop dark, sunken cankers at or above the soil line, and some fruit will start to show dark, sunken spots on the stem end of fruits. There is no treatment for early blight. Once you discover it on a plant, the best course of action is to remove infected branches immediately. You can still harvest unaffected fruit and the plant should continue to grow and blossom. Remember to sanitize your pruners when you have finished removing infected branches. If the disease overruns the plant, pull up and dispose of the plants in the trash—do not compost or burn blighted plants. Next year, plant your tomatoes in a different area of the garden.
Late Blight
Late blight was responsible for the Irish potato famine in the 1840s. If your tomato plants are infected, the symptoms are leaves with large, dark brown blotches with a green gray edge. Stem infections are firm and dark brown with a rounded edge. Spots can become mushy as additional bacteria invade. In periods of high humidity, thin powdery white fungal growth appears on infected leaves. Fruit damage in the form of dark, leathery patches that appear on the top and sides of unripe fruit is yet another sign of late blight. Late blight is a terminal disease, and the method for dealing with it is to pull out and destroy the plants. Do not place the plants in your compost pile. For the next growing season, purchase late blight-resistant varieties, and grow your them in a different part of the garden.
Septoria Leaf Spot
Septoria leaf spot is aptly named because the primary symptom of the disease are the numerous brown spots that appear on the leaves, approximately 1/16 to 1/8 inch in diameter. The spots lack a yellow halo, and, upon close inspection, have black specks in the center. Unlike early and late blights, there is no stem or fruit damage. If your tomatoes are infected, they will continue to produce fruit. Remove infected foliage immediately and disinfect your pruning tools when working from plant to plant. To prevent the disease next year, plant your tomatoes in a different part of the garden because the fungus overwinters on infected tomato debris and in the soil, Purchase disease-resistant varieties, do not water overhead, use mulch to prevent soil from splashing up on foliage, and increase air circulation between plants.
