how to prevent tomato blight
Apply a fungicide. Once infected, the application of sulfur dust may help prevent the spread of blight to other leaves or plants. Regular fungicide application 2 to 4 weeks after transplanting the tomatoes into the garden will help prevent blight development.
1 Water only at the base of the plant and early in the day. Long periods of moisture on foliage encourage blight.  Use a tomato staking system and remove suckers to increase air movement through the plant and to reduce moisture on the foliage. Staking also improves fruit quality and helps prevent root rot.