Splashing water is one of the most common ways disease organisms are spread. Many fungi and bacteria require a certain period of wetness in order to infect leaf tissue. Watering early in the day allows time for the leaves to dry prior to the evening. Adequate spacing between plants will help provide good air circulation and rapid drying of leaves. Avoid too much water.
Wet soils provide ideal conditions for many of the root rot fungi.
If soil drainage is poor, you may want to consider raised beds to avoid excess soil moisture. Finally, avoid fluctuations of too little water and then too much. Blossom end rot of tomato is an example of a problem that may develop when such moisture fluctuations occur. Control insects and weeds. Certain insects, especially aphids and leafhoppers, may transmit virus and bacterial diseases to healthy plants.