Answer: White powdery stuff sounds like the fungal disease called powdery mildew. Powdery mildew covers the upper surfaces of leaves with a white powdery growth. Later, the white mildew disappears leaving dark, reticulate markings, which appear from the upper side as yellowish areas. It is usually not serious enough to require control measures. This disease is most prevalent during the dry season. Malathion is an insecticide and won't control the fungal disease so you probably won't need to use it again on your tree. Be sure to water deeply once or twice a week. Hope your avocado tree perks up soon!
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