Pensacola News Journal

For: Saturday, August 3, 2002

By: Daniel E. Mullins

Extension Horticulture Agent

Santa Rosa County

Black Stuff on Crape Myrtle Leaves Revisited

“The leaves on my crape myrtles are turning black. What can I do?” This is a common question at this time of year. Though the problem is not as serious as it looks there is a need to understand what is happening, and provide control when it is required.

The primary cause of sooty mold on crape myrtles is the crape myrtle aphid. This is a very specialized aphid. It was apparently brought into the United States at the same time that its host was introduced. No other aphid species infests crape myrtle and it is not known to occur on other plant species.

In north Florida, crape myrtle aphid populations generally peak between late June and early August. Feeding upon buds and on the undersides of leaves, these aphids secrete a sugary solution known as “honeydew.” Drops of honeydew fall from the aphids onto leaves and stems below. This sweet solution promotes the growth of sooty mold fungi, or Capnodium species.

Sooty mold appears as a black film or powdery coating on leaves and stems. The blackened plant parts are often the most obvious sign of aphid infestation. When this is seen, check the undersides of leaves and if aphids are present, you will see many of these small yellow or greenish insects.

Although it is unsightly, sooty mold itself does not directly harm crape myrtle. This layer of black fungus can however shade the leaves and interfere with photosynthesis. When plants are under heavy aphid infestation there is a potential for reducing long term vigor.

Control of the crape myrtle aphid will stop further development of sooty mold. Once there are no more aphids present to provide honeydew, this black layer will break down through the actions of sun, rain and wind. Sprays containing insecticidal soap or ultra-fine spray oil for aphid control also help to loosen and remove sooty mold. Other aphid control products for use in the landscape include these active ingredients: acephate, bifenthrin, abamectin, cyfluthrin, azadirachtin, malathion, permethrin or pymetrozine.

Entomologists have learned some interesting things about this aphid and what is known as the “sooty mold complex” that results from its feeding. The presence of the honeydew and mold attracts over 30 species of beneficial insects to the landscape. Once attracted to the infested crapemyrtles, these beneficials move to other plant species and help to control damaging pests.

Based upon this information, the gardener is faced with a decision. If the sooty mold can be tolerated there are some benefits to letting nature take its course. If on the other hand, it is extremely objectionable the aphids and resulting sooty mold can be controlled with one of the recommended insecticides.

Question of the Week: I plan to cut back the tops of my Amaryllis plants, divide and transplant them this month. Is this the right time?

Answer: First, make sure that your Amaryllis plants need dividing. If they have developed many new bulbs and have become crowded, plan to do this in September. Allow the tops to die back naturally before digging them. When transplanting, place the tip of each bulb even with the top of the ground.