PH, I share your distaste for spraying, it is the only part of growing dahlias I dislike. Wearing a respirator and protective clothing is miserable but I don’t know of any other way to raise dahlias commercially. Hopefully, the disease/pest pressure will not be too great in your area and you will not have to spray often. Here, I spray every 7-10 days.
I spray preemptively for powdery mildew. PM does best under humid conditions but it does almost as well in dry, hot weather. It can spread fast and cover an entire field within a week. As you noted, some varieties are more susceptible than others. PM is not easy to control once it is established so the best bet is to spray fungicides regularly as a preventative.
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