Caused by a number of different fungi, this disease can afflict any plant. It shows up as a powdery, dark brown or black coating on leaves--hence the name "sooty mold." The responsible fungi live on a plant's natural secretions and on the honeydew excreted by aphids, mealybugs, and scale. While fairly harmless on its own, sooty mold may weaken a plant if combined with extensive insect damage.

To control the problem, reduce the population of honeydew-excreting insects. Rinse small, ornamental plants by hand; hose down larger infected areas (such as trees or expanses of ground covers).

There is no chemical control.