The good news about Sooty Mold, a fungus, is that it usually doesn't harm the plants, but it does indicate a serious infestation of piercing-sucking insects like aphid, scale, or mealybug.
Leaves and other plant parts, even tree trunks, show a dark gray or black sooty layer over their surfaces. Sooty mold can be easily wiped off to show healthy tissue below. Sometimes the fungus forms a papery black layer that can be peeled away from the leaf surface. The mold grows on the sugars in the sticky honeydew that is excreted by piercing-sucking insects like aphid, mealybug, whitefly and scale. Fortunately, the mold does not get into the plant's tissues, but it can however, if severe enough to the point where the entire leaf surface is covered, block light to the leaf which can produce poor, stunted growth, or even collapse of the plant. Sooty mold spreads very quickly in warm, moist conditions.