Answer: Powdery mildew develops under certain weather conditions and when other conditions are right. The best defense is to grow varieties with built in disease resistance to it, such as "David". Next comes siting the plant in a location with good air circulation and spacing them out to allow good air flow between the plants. In the spring, thin out some of the new growth to allow air to move inside the plant. When you first see powdery mildew begin, remove the worst affected leaves and dispose of them in the trash and rinse the plant gently with water. Do this in the day so it has time to dry before the cooler night temperatures occur. Also, in the fall, clean out all of the plant debris after frost. Remove all of the frost killed stems and foliage from the garden to try to limit reinfection. Sometimes despite all these efforts, it still happens. That is why it is better to grow the resistant varieties, especially in your climate where mildew is so likely to occur. I hope this helps.
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