The white coating on the crapemyrtle could be a couple of things. Sometimes, high alkaline water will leave a white residue on leaves. Have you noticed it with other plants in your garden? It could also be powdery mildew. To fight powdery mildew, there are products available at the garden center or you can use baking soda combined with dish soap and spray on the leaves. To avoid it in the future, water the ground around the plant rather than on top of the leaves and give the plant good air circulation.
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