One of the keys to growing tomatoes is eliminating or reducing stress. Stressful growing conditions weaken tomato plants and make them targets for disease. To help cut down on competition from weeds and to help the soil hold moisture better, spread a couple of inches of mulch over the soil surface. Mulch can also create a protective barrier that helps stop soil-borne diseases from splashing up onto and infecting plant leaves.
Wet foliage can encourage fungal diseases, so it's especially helpful to water your tomatoes with a soaker hose or drip irrigation instead of with a sprinkler or spray nozzle on your garden hose. If you have to water from the top of the plant, do so before noon so the leaves have plenty of time to dry before temperatures cool down at night.
Black rot and blossom rot are two of the most common tomato issues gardeners face. They are caused by a lack of calcium in the soil. Rotating crops will help this problem some, because it gives the plants new soil new fertile soil each year. However, you can go one step more by adding in a few crushed egg shells at planting time. The eggs shells provide much-needed calcium to the tomatoes as they break down. This is a huge help in eliminating black rot and blossom rot. Just crush two or three egg shells up in your hands, and mix into your planting hole around the roots.
As your plants grow, keep the bottom of each plant trimmed up to at least 6″ above the soil line. This allows light and circulation into the plants – a key in keeping mold and fungal disease at bay. It also makes it harder for many insects to find a ride up onto the plants. Be sure as well to prune off any foliage that shows signs of blight or disease. By walking through the garden each day and trimming back, you can keep disease from spreading quickly to other parts of the plant or adjacent plants.