Answer: Both of these plants are potentially subject to infection by powdery mildew which will cause wilting and defoliation. Powdery mildew occurs under certain weather conditions, but you can try to avoid it by thinning the plants and spacing them to allow for good air flow and by watering as needed to keep the soil evenly moist (damp like a wrung out sponge, not sopping wet and not dried out). Garden phlox in particular is frequently subject to disfiguiring infections of mildew and in the northeast region -- with its humid summers -- it is important to plant varieties that are specially noted for their resistance to it. Good sanitation is also important, clean up and remove prunings and do a thorough clean up of these plants in the fall. In severe cases, fungicide treatments such as sulfur may be needed.
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