Question
Leaves on Gardenia Bushes have a dry, black powdery coating on them.  It will come off by rubbing.  Is there something I can spray on them and wash it off ... or should I cut them back?  

Answer
The black is sooty mold that is growing on the excrement of one of the sucking insects more than likely white flies. These insects suck the juice from the plant and secrete a substance called honeydew which is high is sugar content--the sooty mold grows on the honeydew. Control the white flies and the mold will go away after a couple of rains. Or you can wash the leaves after the insect is controlled.

As the name implies, whiteflies are small, fly-like insects with white-colored wings. They hide on the underside of leaves, where they multiply rapidly. Whiteflies suck on plant juices and, in large numbers, can consume a considerable amount of nutrients. Like aphids, they excrete lots of honeydew, on which black sooty mold fungus grows.

Whitefly eggs are spindle-shaped and stand vertically on the underside of leaves. After hatching, the "crawler" stage of the insect moves about on the leaf looking for a feeding site. At temperatures between 65 and 75 degrees F., it takes less than 40 days to develop from egg to adult. Mating is not necessary for egg production and each female may lay up to two hundred eggs.

With that reproductive rate, it抯 no surprise how many are present this spring.

WHITEFLY CONTROL The key to whitefly control is not a specific insecticide: the key is persistence. Almost any insecticide will kill a whitefly. Insecticidal soap, neem oil, horticultural oil, permethrin, bifenthrin and lambda-cyhalothrin are deadly to the bug. Getting the insecticide in contact with the insect involves some agility and perseverance though.

Since whiteflies live under gardenia leaves it is critical to direct your spray upwards under the leaves. Plan to spend several minutes with each gardenia, working to achieve complete coverage. One spray won抰 kill all of them. Nearby plants harbor the creatures and they will move back in within a few days. If you have citrus whiteflies on your gardenia, mark your calendar to apply an insecticide to the plant every seven days for a month. If the bugs reappear during the summer, repeat the four sprays.