Tomato plant blight is caused by fungus spores that live in infested soil and plant residue. It can also be caused by a nitrogen deficiency in the plant. The best remedy for blight is prevention, because once started, the blight can be hard to control.
One of the first preventive measures one can take against early blight is to make sure that seeds are not planted in the same soil that peppers, potatoes, eggplant and tomatoes have been planted in within the last two years. Since blight fungus lives in soil, mulching tomato plants also helps keep foliage from contact with the soil. Grass does not host blight, so planting in an area with a grassy perimeter gives plants a measure of protection.
Other preventive measures consist of using certified disease-free seeds, using fresh soil with good drainage, and sterilizing the soil with hot water before planting seeds. Increasing the amount of organic material in the soil and using natural, nitrogen-rich fertilizers also reduce the potential for blight.
Chemical control is possible as well, however, one should check with the local county agricultural extension office to see what chemicals are legal for use. When early signs of blight are detected, protective fungicides should be applied to every part of the plant. Rain or overhead watering washes away chemical treatments, so they require reapplication within seven days or 10 days in dry weather.