The black film you are seeing may be sooty mold, a fungus that grows on the honeydew secreted by plant-sucking insect pests such as aphids, mealybugs, or scales that feed on the sap inside leaves. The water and sugars ingested by the insects are excreted as honeydew, which adheres to leaves and also drops to objects below. Sooty molds are not plant parasitic,
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