Blight can be difficult to spot at first, which in turn may allow it to spread and possibly cause significant damage before it is treated. In its early stages small black or brown circles begin to form on the lower leaves of the plant - a dark outer ring with a lighter center. Signs also include white, powdery spores; large olive green or brown spots on leaves; and brown or open lesions on the stems.
Unaffected plants in home gardens and commercial fields should be sprayed with fungicide to prevent the spread of the disease. As blight spreads, the spots multiply and the leaves begin to turn yellow. The fungus moves up the plant, damaging the leaves in its path and if left untreated, can destroy your entire tomato crop.
How can it be treated? First, gather all the diseased leaves. Don't let them rot on the ground - that will only continue to spread the disease to the unaffected plants. Dispose of the leaves away from the garden in a plastic bag. Wash your hands between plants in order to avoid further spreading of the disease. All affected plants should be pulled up by the roots, sealed in a plastic bag and thrown away. Do not compost it. If you have a large tomato garden or crop that is infected, pull up the plants and burn them, or turn the field of plants back into the soil. There is still enough daily sunshine in the season ahead to destroy the fungus.
Then go to your garden center and get an organic fungicide that destroys tomato blight and follow the directions with it carefully to treat the unaffected plants. Fungicides can protect unaffected plants, but there is no cure for late blight. You'll probably have to continue treatment weekly throughout the season. Watch for more diseased leaves and remove them as quickly as possible. You also may need to change your tomato growing spot for a few years to prevent the disease from coming back.