Minimize Irrigation. Tomato plants have surprisingly low water needsand overwatering can promote disease. Once the fruit has started to form, water only when the top three inches of soil becomes dry and the leaves look limp in the heat of the day.
Water in the Morning. This way, the moisture will evaporate quickly from the surface of soil, giving the roots the water they need, but keeping the humidity down around the plants.
Remove Infected Leaves Immediately. Don’t hesitate to clip off leaves as soon as any spots or deformation is apparent – it may save the rest of the plant from succumbing to the disease. Dispose of these clippings far away from your tomato plants.
Prune Out Dense Foliage. Tomatoes tend to grow more thickly than is necessary, reducing air flow and producing more foliage than their immune systems can support. Prune out new sprouts that emerge from the main stems once fruit has begun to develop and train the plants to an open, spreading form.
Disinfect Tomato Tools. Anything that you use to prune diseased tomato plants or to work the soil around them should be disinfected before using on or around healthy tomato plants.
Control Insect Pests. Tomatoes are rarely destroyed by insects, but they are frequently attacked on a small scale, which weakens the plants and makes them more susceptible to disease. Some insects are also responsible for spreading diseases.
Fertilize. Tomatoes are heavy feeders and will have greater disease resistance with a few boosts of fertilizer during the growing season. Once the fruit has set, apply a high phosphorus fertilizer every three weeks.