You can't always prevent a disease from attacking a prized plant. The bacterial infection fireblight, for example, can enter blossoms readily if there is rain just at the time of bloom; you'd have to control the weather to stop it. A mosaic virus-infected bare-root rose won't exhibit symptoms until it leafs out.
Luckily, good gardening practices will fend off many diseases. To keep plant problems under control, take the following steps.
Keep plants healthy by giving them the water, light, and fertilizer they need to flourish.
Buy disease-resistant plants. You'll find tomatoes resistant to verticillium wilt and flowering pear trees less likely to succumb to fireblight, for example. Vegetable seed packets are labeled to indicate the particular plant's disease resistance; plant tags on fruit trees or ornamental trees and shrubs sometimes also include this information. Your Cooperative Extension Office can often provide information on plants resistant to diseases that may cause problems in your area.
Transplant carefully to minimize root damage. When broken, roots are susceptible to certain soilborne diseases.
Take care not to injure plants when you work in the garden. An open wound on a plant stem or tree trunk readily admits bacteria and fungi.
Avoid wet-weather garden work. You may unwittingly spread waterborne pathogens as you move about from one spot to the next.
Install a drip irrigation system or use soaker hoses to minimize the splashing water that can spread waterborne diseases.
Remove diseased plants. If certain plants are constantly afflicted by disease, eliminate them from the garden and replace them with less trouble-prone choices. This solution is simpler than attempting to control the disease, and it removes sources of further infection.
Dispose of infected plants and plant parts right away. Throw them out with the trash; don't compost them. Some pathogens may be killed by the heat generated during decomposition, but it's better not to take the chance.
Keep the garden clean. Do a thorough fall cleanup each year. Remove weeds, since pathogens may overwinter on them. In mild-winter areas, strip off any diseased leaves remaining on plants; rake up and discard all diseased leaves on the ground. You may also want to rake up other garden debris; though it can serve as a good mulch (if undiseased), it also shelters ground-dwelling pests.