The goal should be to quickly grow a relatively large, healthy plant and then back off on the fertilizer as it reaches the reproductive, or fruiting stage. Much tomato production is missed because some gardeners continue to provide excessive fertilizer too late in the season. This practice often leads to a big bush or vine and low yields. Most major tomato diseases are caused by fungi, while a couple are caused by bacteria. Because these organisms are microscopic and early infection is not seen with the naked eye, control measures should begin before visual symptoms are present.
Chemical control includes regular applications of tomato fungicides.
Recommended products for the most common diseases include maneb, mancozeb and chlorathalonil. Control bacterial spot with basic copper sulfate plus maneb or mancozeb. Thorough spray coverage of both leaf surfaces, stems and fruit is required. Follow label directions carefully. There are several other cultural practices that help to control diseases whether a spray program is followed or not.