A potentially devastating disease in tomatoes, blight is the same disease that caused the epic Irish potato famine in the 1840s. Three main types of blight occur in tomatoes, namely Septoria leaf spot, early blight and late blight. As the saying goes, prevention is the best treatment, and this strategy particularly applies to blight in tomato plants. In many instances, you may recognize blight symptoms too late to do anything to save your plants. Checking your tomatoes regularly for disease will help you catch signs of blight as soon as possible so you can treat the disease promptly.
Apply fungicide to treat Septoria leaf spot in your tomato plants. Look for signs of Septoria like small, grayish leaf spots with light-colored centers and distinctive dark edges; tiny, black spots may also appear in the middle of the light-colored areas. Apply fungicide made with chlorothalonil or mancozeb. Follow application directions exactly to ensure that you treat the disease correctly.
Spray fungicide on tomato plants affected by early blight. Early blight usually occurs toward the end of the tomato growing season. Look for small, brown lesions on lower leaves and foliage that gradually enlarge and spread to include leaves higher in the tomato vine; these spots often develop dark, circular rings, a distinctive feature of this type of blight. Spray your tomato plants as soon as you identify signs of the disease. Apply fungicide with maneb, mancozeb, triphenyltin hydroxide or chlorothalonil every seven to ten days.
Spray tomato plants affected by late blight with chlorothalonil or mancozeb. Look for irregularly-shaped, dark green lesions on both the foliage and the tomato fruit. Remove diseased parts of your tomato plant as soon as you see signs of infection and destroy them promptly to reduce disease transfer. Follow the directions on the fungicide and provide treatment as soon as you see signs of infection.
Apply a fungicide approved for organic use to treat blight. According to the National Sustainable Agriculture Information Service, copper-based fungicides are a natural treatment option approved for organic use on tomato plants infected with blight, including formulations such as copper hydroxides, copper sulfates and copper oxides.