Only a few diseases really do much damage to container-grown plants, and most of those can be prevented or at least reduced in severity with good cultural practices or by planting resistant varieties. If you know a certain disease is a problem on a particular plant in your area, simply growing something else is the easiest solution.
Some cultural practices can help avoid plant diseases:
Choose resistant plants: If you know a certain disease is common in your area, choose plants that resist it. Plant labels often will tell you if a variety is resistant to a common disease. For example, some varieties of roses are very susceptible to black spot, a disease that causes, oddly enough, black spots on the foliage. Other varieties are very resistant to infection.
Remove infected plants: As soon as you notice a plant with a problem, give it the yank. Even picking off infected leaves helps prevent a disease from spreading.
Avoid wetting foliage: Most plant diseases require moisture to spread. Avoid overhead watering, and apply water to soil, not foliage. Don’t handle plants when the leaves are wet. If you have to use overhead watering, do so in the morning so the leaves dry quickly in the sun, rather than sitting moist all night.
Space plants properly: Planting too close reduces air circulation between plants — a condition that invites disease. Set containers so the air can circulate between them. Unfortunately, planting close together is often the norm with flower-filled planters, so just be aware that diseases are more common under these conditions.
Keep your garden clean and tidy: Many diseases spread on plant debris, so pick up fallen leaves and remove dead plants. Keep the spaces under containers clean.
Provide drainage: Make sure your pots can drain properly. You know that your pots must have holes in the bottom so that water can drain out. But frequent checks also allow you to be sure that the openings aren’t clogged with roots. And if you have a saucer under your pots, make sure you drain it after watering so plants don’t sit in soggy soil — overly wet soil leads to root-rot diseases.
Keep tools clean: Disinfect pruning tools with a 10-percent-bleach solution (1 part bleach to 9 parts water) between cuts to prevent spreading diseases.
Control insects: Many diseases are spread by insects, including the infamous Dutch elm disease, which is spread by elm bark beetles.
Grow healthy plants: Give your plants the right amount of light and water by putting them in a location they’re well suited to. For example, when sun-loving plants are grown in shade, mildew and other diseases are more likely to take hold. When plants get exactly what they need, they stand up to disease much better.
Avoid excess nitrogen fertilizer: Yes, you want to keep plants growing vigorously, but don’t supercharge them with nitrogen. This nutrient makes plants grow quickly, but the rapid growth is weaker and susceptible to disease. Consider using slow-release or organic fertilizers to maintain healthy growth.
Use fresh soil mix: Don’t replant in used soil mix, especially if you’re growing plants that may be susceptible to diseases. Compost the old, and refill with the new.