Answer: Sounds like early blight to me. This disease is caused by the fungus Alternaria solani and is first observed on the plants as small, black lesions mostly on the older foliage. Spots enlarge and concentric rings in a bull?s eye pattern can be seen in the center of the diseased area. Tissue surrounding the spots may turn yellow. If high temperature and humidity occur at this time, much of the foliage is killed. Lesions on the stems are similar to those on leaves, sometimes girdling the plant if they occur near the soil line (collar rot). On the fruits, lesions attain considerable size, usually involving nearly the entire fruit. Concentric rings are also present on the fruit. Infected fruit frequently drops.
The fungus survives on infected debris in the soil, on seed, on volunteer tomato plants and other solanaceous hosts, such as Irish potato, eggplant, and black nightshade.
To prevent, use resistant or tolerant cultivars. Use pathogen-free seed and do not set diseased plants in the field. Use crop rotations, eradicate weeds and volunteer tomato plants, fertilize properly, and keep the plants growing vigorously.
To treat, select one of the following fungicides: maneb, mancozeb, chlorothalonil, or fixed copper. Follow the directions on the label.
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