Answer: Early blight can cause the symptoms you describe. Early blight, caused by the fungus Alternaria solani, is also known as Alternaria leaf spot or target spot. Like Septoria leaf spot, early blight is common in tomato plantings, and the two diseases may attack the same plants. Premature loss of lower leaves is the most obvious symptom of the disease. Brown to black spots, 1⁄4to 1⁄2inch in diameter with dark edges, appear on lower leaves. Spots frequently merge, forming irregular blotches. Dark, concentric rings often appear in leaf spots, resulting in the ?target? appearance suggested by the common name. Leaves turn yellow and dry up when only a few spots are present. The fungus occasionally attacks fruit at the stem end, causing large, sunken areas with concentric rings and a black, velvety appearance. Warm, wet weather favors rapid spread of early blight. Early blight can infect plants at anystage during the growing season but usually progressesmost rapidly after plants have set fruit.
You can avoid these diseases by rotating crops. Plant tomatoes in the same place only once in three or four years. Remove and destroy tomato vines in the fall. Plow or rototill to bury the remaining crop refuse. Use healthy transplants. Remove badly diseased lower leaves, as these are a source of leaf spot fungus spores that help spread the disease.
Water at the base of the plants to avoid splashing water, which spreads the spores. Avoid watering with overhead sprinklers in late afternoon or evening. If the plants stay wet all night, leaf spot infections are likely to occur.
Use fungicides when needed. These diseases spread rapidly and are difficult to control once established. Fungicides must be applied before the disease first appears and reapplied throughout the growing season. Chlorothalonil fungicide, sold as Ortho Multi-Purpose Fungicide, can be applied up to the day of harvest.
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