Depends on the disease. Some are fungus cause and require regular fungicide use. Others are bacterial and fungicides won't help. And some are environmental caused and require changes in the growing conditions.

So what disease are we talking about? Early Blight, Septoria Leaf Spot, Alternaria, Bacterial Spec/Spot, one of the wilts, mildew, etc. etc. See link below or you can search each of the most common ones here and get recommendations for prevention on each.

Dave

Here is a link that might be useful: TAMU Tomato Problem Solver

Thanks for the link , Dave . I was thinking about prevention versus cure . Is that possible ?

All tomato diseases are primarily prevention oriented. There are no real "cures" once the disease is established. But there is no blanket prevention for all diseases either and if we sprayed on all the various mixes that would be needed to try to prevent everything we'd likely kill the plants. :)

Daconil fungicide sprayed weekly from the first day of plant out has a pretty good record at preventing fungus diseases or at least stalling them. But it does little for bacterial diseases and nothing for viral diseases. Copper sprays can 'help' prevent bacterial problems but does little for fungus and nothing for viral diseases. See the problem?

And there are many preventative environmental controls that folks would argue are as effective as using fungicides. Things like planting the more resistant varieties, increased spacing between plants, off-season soil treatment, positioning to improve air circulation in among the plants, heavy mulching, drip irrigation, increasing sun exposure, etc.

So again that all depends on what disease we are talking about? What diseases are prevalent in your garden, your area, your region? What have you had problems with specifically in the past?

The link I gave you gives some recommendations for each of the many diseases but sadly there is no "one for all". If there was we'd all be using it.

Dave