Answer: If you can scrape the black off the leaves, or wash it off with water, it is most likely sooty mold. This is a by-product of feeding by insects such as aphids, scale insects and mealybugs. Typically, the pests will feed on the undersides of leaves and as they do, excrete a sugary substance called honeydew. This has a high enough sugar content that it favors fungal or bacterial growth. When that happens, it is called sooty mold. It doesn't hurt your plants but it does look unattractive. To get rid of it, you will need to get rid of the pests feeding on your plants. Inspect your plants for signs of insects and then control them with the appropriate sprays.
Best wishes with your garden.
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