TOMATOES

There are a number of diseases that effect tomato plants, some common ones are: 

Early Blight which causes concentric spots on both the leaves and the fruit. Usually the lower leaves and stems are effected first, and then it spreads. Spray the infected plants with sulfur and/or copper to spot the further development of the disease. Sometimes spraying the plants with a mixture of 1 tablespoon of baking soda to a quart of warm water will prevent the disease. 

Blossom End Rot which causes the fruit to rot on the bottom end. This is caused by a calcium deficiency and can be aggravated by drought, uneven soil moisture, high salt levels in the soil and excessive nitrogen. 

Late Blight, treat same as Early Blight.

Anthracnose which causes the fruit to rot. Spray with copper as soon as the fruit appears, if you have had this problem before. 

Probably the best advise to prevent any diseases, is to take good care of your garden soil, by fertilizing with a good balanced natural/organic fertilizer. Rotate your garden plants every year. Plant a cover crop of rye grass after harvest and plow it under in the spring as a green fertilizer.