Answer: What you describe does sound like powdery mildew. Oaks are more susceptible to this disease while new shoots are developing and leaves expanding; mature leaves become more resistant. Unfortunately, cultural practices, such as trimming, and fertilizing that stimulate and prolong succulent plant growth encourage powdery mildew.
Powdery mildew may be treated with commercially produced fungicides. These products may include sulfur, fenarimol, dinocap, triadimifon or benomyl. Refer to the instructions on the package to determine method and timing of application. Weather conditions such as temperature and humidity are important factors when determining when to apply fungicides.
Hope your oaks regain their health!
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