Answer: If the black substance is shiny and can be scraped off with a fingernail, it is sooty mold. This substance is the result of insect feeding higher up in the plant. As insects such as aphids feed, they exude a sticky substance called honeydew. There is just enough sugar content in the excretion to grow mold and the substance is then called sooty mold. Insecticidal soap is effective against aphids so once you get the aphid population under control you shouldn't have any more problems with sooty mold. If the black substance cannot be scraped off but in fact the leaves are turning black and falling off, it indicates over watering or overly soggy soils. Correct the water stress problems and your plants should rebound. Best wishes with your garden!
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