Answer: It sounds like a fungal disease called powdery mildew. If the weather has been wet and humid, if your shrub is not getting good air circulation, if it is growing in a shady or partially shady area, and if it is otherwise under stress, it can develop powdery mildew. You can spray with a fungicide and/or you can prune away the infected plant parts. Pruning should open the plant up to better air circulation. Best wishes with your shrubs.
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