Answer: It's difficult to tell from your description whether your Jade plant has been attacked by mealy bugs, or has developed a fungus. Mealy bugs are small insects which are covered with a white, cottony fluff. They feed on the leaves and stems of plants, sucking out fluids and causing a blackish area of dead tissue. Try to inspect the affected parts of your plant with a magnifying glass to see if you can identify the white fuzz as mealy bugs. If so, remove them by rubbing them off with a cotton swab dipped in plain water. The symptoms you describe could also indicate a fungus, called Botrytis, or gray mold, a disease indicating poor air circulation and too much humidity around your plant. If your Jade plant has developed this disease, the best course of action would be to cut away the infected parts and then improve the growing conditions to include better air circulation and less humidity.
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