1. Select healthy plant material. Annual flowers and vegetable transplants should be vigorous, show good color, and have no dead or yellowed areas. Avoid cell packs with stunted, sickly or missing plants. Remember to check the roots: they should be white and vigorous looking. If using seeds, take care that they are fresh or have been stored carefully in a cool place. Trees and shrubs should have a pleasing form so that heavy pruning is unnecessary. Avoid specimens that have mechanical damage to the trunk or larger limbs. Bulbs, tubers, roots and corms should be firm and have no obvious mechanical damage or mold.
    

2. Look for plant varieties with built-in disease resistance. Often, the plant label will indicate what resistance the plant possesses. If not, check with a knowledgeable source such as a county extension office or a recent garden reference book for the names of disease-resistant plant varieties. This is especially important for plants such as crabapples and roses, where careful plant selection can makes a big difference in later maintenance cost.
    

3. Put plants where they will thrive. Know your soil type, pH, soil drainage, and the garden's exposure to sun and wind. Then choose plants that will thrive in these conditions.
    

4. Water plants early in the day. Water infrequently but deeply, and, if possible, only at the base of the plant. An extended period of wet foliage encourages plant diseases. Stay alert to weather conditions during the growing season so as not to over water or underwater your plants -- it will increase their susceptibility to disease. The appropriate interval between watering will vary with the soil and climate characteristics where you live.
    

5. Provide for good air circulation in the garden and landscape. Allow adequate space when planting, remove spent flowers promptly, stake plants, and prune as necessary.
    

6. Avoid excessive fertilization. Too much fertilizer can reduce plant health by producing lavish, succulent growth that is more susceptible to disease.
    

7. Mulch your soil (let it warm up first in northern gardens). Mulching will conserve moisture and reduce the time needed for weeding and watering. Be careful to keep the mulch away from contact with the stem, in order to avoid encouraging fungal infections.
    

8. Keep the garden clean. Scout your plantings every few days to look for the first signs of disease or insect infestations that may require treatment or removal. Remove dead or diseased plant parts or entire plants to minimize disease spread. It's best to prune during dry weather.
    

9. Clean tools after use. Soil on garden tools can harbor plant pathogens.
    

10. Apply fungicides, if necessary, but do it correctly and at the right time. Most fungicides are effective at protecting plants against disease, but are not instant "cures." They should be applied in advance of an anticipated disease problem, or as soon as possible after symptoms are noticed. They provide temporary protection and may need to be reapplied. Always read and follow label directions. Remember that many cultural problems can cause symptoms on plants, so analyze the situation thoughtfully before reaching for the sprayer. Fungicides will not be of use on plants that are declining due to poor site conditions, improper care, or damage caused by insects, spider mites, bacteria, or viruses.
     

11. Rotate plants. Annual flowers and vegetables species should not be planted in the same location each year, in order to prevent the build-up of diseases over time.

The American Phytopathological Society (APS) is a non-profit, professional scientific organization dedicated to the study and management of plant disease with 5,000 members worldwide.

Source: American Phytopathological Society

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