Use good sanitation. Diseased plants and plant parts can spread disease to healthy plants. Removing diseased plants or dead foliage from the garden helps limit the spread of the disease. Control weeds and insect pests. Weeds crowd plants and compete with them for nutrients, moisture, and light.
Some insects are responsible for spreading plant diseases as they move from plant to plant.
Many viral diseases are spread by sucking insects such as aphids. Recognize that not every disease needs to be controlled. Some diseases, while unattractive, are not particularly damaging to the plant. Some plant disorders are caused by lack of plant nutrients. Before using any chemical controls, be sure you have correctly identified the problem.