Brown spots on tomato leaves, called early blight, are caused by the fungus Alternaria alternata or solarno. The fungus may also attack stems and fruit. Brown spots on the leaf are circular and 1/2 inch in diameter with a pattern of concentric rings. When the fungus is present, the leaf may turn yellow and drop off. According to Colorado State University, the best way to treat this condition is to make sure it does not occur from the beginning.
Plant certified disease free seeds or transplants. Practice plant rotation. Don't plant tomatoes in the same spot that you did last year. If you had any kind of fungus it can carry over to the new crop.
Water the tomato plants early in the day and do not wet the leaves. Water the ground around the plants or use drip irrigation. Drip irrigation applies the water directly to the root area. It is supposed to provide your plants with the appropriate amount of moisture to promote healthy growth.
Remove all weeds around the tomato plants. Look out for horse nettle and black nightshade. Keep your plants free from these weeds. Cleaning and maintaining the area within and around the tomato bed helps prevent infection. Fertilising the soil also contributes to healthy plants.
Treat your plants with fungicides containing the active ingredient chlorothalonil. The University of New Hampshire suggests these fungicides are effective in preventing infection when the plants show no sign of infection. However, if the plant is already infected, treating it with a fungicide will not help. According to Ohio State University, fungicide should be applied two to three weeks after the tomatoes begin to grow.
Check your plants regularly for spots on leaves, stems and fruit, once they start to grow. If you notice spots on any part of the plant, immediately remove the entire plant including the roots. Put them in a plastic bag and throw them away.
Remove or destroy debris after harvest or plough under deeply if you have a big tomato bed. Don't compost spotted plants.
Always clean your hands after handling infected plants. Handling infected plants then touching healthy plants may spread the infection.
There is also what is known as late blight (Phytophthora infestans). Rainy, overcast weather contributes to the development and spreading of the spores from this fungus. Signs are dark green to brown spots and white fungal growth on the underside of the leaf. This fungus does not have the yellow outline around the edge.