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. If your liliacs are thick and overgrown, you might want to consider some judicious pruning next spring. Powdery mildew spores do not overwinter in the soil. Good luck!
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