Answer: Leaf drop in August accompanied by reddening and browning of the foliage would typically indicate overly dry soil causing the plant to defoliate as a protective measure. This plant is tolerant of dry soils, but it would prefer average soil moisture for the best performance, especially in an area with high summer temperatures. Using several inches of mulch over the root zone and perhaps an occasional slow deep soaking during summer dry spells should solve the problem. With regard to the white material, it is possible it is powdery mildew which can develop in certain weather conditions and does not actually require moisture. In the early stages sometimes it can be washed off with a stream of water from the hose; some gardeners also report success using a spray made of milk mixed with water at a rate of roughly one part milk to three parts water. Usually mildew of this sort is unsightly but not dangerous to the plant. If it is truly overwhelming, you might wish to consult with your county extension for a more specific diagnosis of the problem and the most up to date recommendations for your state on how to handle it. In the meantime, also clean up and destroy the worst affected and any fallen leaves to limit reinfection. Good luck with your burning bush next summer.
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