The Q&A Archives: black soot

Question: I live in region #10. I have Crotons and other leafy shrubs planted in the ground around my pool patio. T Last year I started to notice the crotons were loosing leaves due a black soot almost like mildew. This now is spreading to other newer shrubs. Do you know what this is and how can I stop it?

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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