I mentioned tomato foliar diseases in the last post. I mentioned them in a generic fashion, because there are so many, and because, franky, it's often hard to tell them apart. Here's a link to tomato disorders; click on "leaf" and you'll see what I mean. http://aggie-horticulture.tamu.edu/tomatoproblemsolver/index.html
They are often either fungal or bacterial. There are man-made sprays such as Daconil that many tomato growers use to prevent fungal infections. The older standard sprays were often copper based, and somewhat less effective. I don't use synthetic garden products anymore if I can help it, definitely not Daconil (it works great, I just don't like eating poison - that's why I grow my own food). Last year I tried preventative neem oil sprays, and it certainly seemed to keep foliar diseases to a minimum until the routine was interrupted. Perhaps this year I'll stick to the spray schedule...
So there is hope for preventing fungal infections, but what about bacterial? They recommend crop rotation and general cleanliness in the garden, but it's pretty hard to keep a sterile environment outdoors. I suspect there is a way to encourage soil microorganisms that keep the problem bacteria populations lower, which would have at least some impact. I don't know the answer yet.
I do know that tomato plants have more problems when they are stressed. To avoid stressing them, you want to provide them steady moisture and proper nutrients. The easiest way to ensure this is to mix plenty of compost in the soil, feed with a low nitrogen fertilizer, and mulch. Compost contains many micronutrients, helping to prevent deficiencies, and dramatically improves soil structure. Too much nitrogen causes lush green growth, which attracts disease-carrying insects and can slow flower production. Mulching tomato plants helps keep the soil from drying out and prevents infected soil particles from splashing onto the foliage during rain.