My rhododendron has a black, sooty substance on its leaves and stems. What is it and how can I correct this problem?

That black substance is probably sooty mold growing on the deposits of honeydew (excrement) left by an insect. It is often a homeowners’ first sign of trouble in the landscape. Aphids, scales, whiteflies and mealybugs produce this sugary, sticky substance. They all feed by piercing and sucking a plant’s juices. The sooty mold itself does not injure the plant. But if left untreated, it will shade the leaves, reduce growth, and invite other unwanted guests to dinner.

You can simply wipe off the sooty mold. More importantly, you need to find the source of the honeydew. Be sure to consider where your rhododendron is planted. If it is positioned as an understory feature with taller trees, your plant may not be infested itself, but merely be the honeydew recipient of an “abovestory” tree problem.

Examine your plant carefully, using a magnifying glass if necessary to identify the culprit. Aphids are a maximum of 1/8-inch long, pear-shaped, and green, pink, black, dusty gray or white fluffy-coated insects that cluster on leaves, buds, and young stems. To see if you have whiteflies, just shake a branch. The winged adults are visible and will temporarily fly away. Scales species appear as white, gray or brown bumps on the undersides of the leaves. Mealybugs are soft, oval, 1/10-inch long insects with distinctly segmented pinkish bodies.

Once you’ve identified the insect, you can select a control. Follow the control product’s directions for repeat applications. The watchwords for you are vigilance and diligence to keep the insects from making your plant a five-star restaurant!