My experience is that shade in the morning tends to result in more powdery mildew problems. Also, peony stems seem to be more susceptible to it when they start to yellow and brown later in the season.
I also have more problems in areas where I plant too tightly and air flow is too poor to facilitate proper drying. Another way to get it going is for a lawn mower to blow wet grass into the peony. The grass carries it with it AND reduces air flow - sort of a one-two combo.
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