I am getting calls from people having problems with their tomato plants showing some blight and other fungus, specially for those who grow them in the same garden year after year (same with potatoes). The best way to fight this is to rotate tomato plants to a new garden area each year but this is not a possibility for most growers. Probably the best way to fight this is pick off effected leaves and dispose of them and then mulch below the plants (with anything really...straw, grass clippings, etc.). Fungus usually effect the lower leaves first as the spores are splashed from the ground onto these lower leaves during rain. Mulch helps put a barrier between the plants and the infected soil (and conserves moisture and adds organic material to the garden). Now... to stop the spread of the fungus, you need to coat the unaffected leaves with something to prevent the fungus spores from infecting leaves that are not yet diseased. Copper based fungicide sprays work pretty good for this, is all natural and safe. Another way to fight this is with Serenade, an all natural microbial bio-fungicide, Bacillus subtilis. All natural, safe and can be sprayed up to day of harvest. Both copper fungicides and Serenade need to be sprayed on a regular basis and can be alternated for best results. Make sure new growth is coated to protect it. Even fairly damaged plants will "grow out" of fungal attacks if taken care of.