Answer: The first action you can take to keep diseases from spreading to new, healthy plants, is to remove and destroy all affected plant parts at the end of the season. Powdery mildew is a fungal disease that attacks plants when environmental conditions are right (warm days, cool nights, some humidity) and air circulation is poor. You can help avoid powdery mildew by giving plants the required exposure to sunshine, and plenty of elbow room so there's good air circulation all around them. Some plants are more resistant to powdery mildew, so choose carefully and avoid those that are mildew-prone. Powdery mildew spores do not overwinter in the soil. Verticillium wilt is also a fungal disease, but it attacks both the roots and stems of plants and can remain in the soil for years. If your soil is harboring verticillium, you'll need to grow resistant plants in those plots, as well. Crops to avoid include tomatoes, strawberries, potatoes and melons, unless you can find some resistant varieties. (Look on the seed packet and plant only those that are noted as being resistant to verticillium.) If you continue have problems, you may have to garden in containers for a few years.
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