In order to answer your question, we need to know which diseases you're plants are likely to get in your area, also which ones you're trying to manage.

In general, preventive sprays aren't used.

Instead, action may be required at the very first hint of a problem. But, again, that problem must first be identified.

Spray leaves (underside also) once a week with fish/seaweed emulsion (from an organic gardening store). Put a tsp or so in a spray bottle with water.
Also get a good tomato food and sprinkle a capful around the base once every week or so, before watering. Or you can mix w/ water in watering can. Some people use organic tomato food, some like Miracle-Gro, Hasta-Gro, etc.
This year, I also started spraying a fungicide/insecticide/miticide on everything because in recent years the entire garden gets destroyed by stink/leaf-footed bugs, spider mites, and some kind of blight or wilt. Using organic AND non-organic to cover both bases. So we'll see if this helps in the long run.
But of course wait til you see signs before you start treating, and do some research to make sure of what you're dealing with. So it's not really a preventive thing, but you have to try to nip the issues in the bud before they get a lot worse.