Plum trees are susceptible to a black fungus or mold called sooty mold. Aphid infestations and their secreted honeydew is a host for the fungus to live on. Spray the aphids with an insecticide suitable for plum trees, to stop the sooty mold from occurring. Wash the moldy areas of the foliage with a mild solution of dish soap and water.
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Joyce Starr is a freelance writer from Florida and owns a landscaping company and garden center. She has published articles about camping in Florida, lawncare, gardening and writes for a local gardening newsletter. She shares her love and knowledge of the outdoors and nature through her writing.