
Downtoearthdigs
Answered on June 10, 2015
It sounds like powdery mildew.
Here is a link with information on how to control it.
https://www.gardeningknowhow.com/ornamental/shrubs/spirea/growing-spirea-shrubs.htm
https://www.gardeningknowhow.com/plant-problems/disease/powdery-mildew-homemade-and-organic-remedies.htm
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