Issue: December 6, 2003

Black sooty mold fungus on tree branches

I have two 80-foot trees that I think are a species of ash. There is a black fungus-like powder on the limbs from the bottom to the top of the tree. This condition has been present for at least 2 years. Nothing I have done has made a difference. Please advise. G.F. Luna County

You are probably describing sooty mold fungus. This is a minor fungus that grows on the syrupy excretions of some insects that feed on trees. The fungus does little physical damage to the tree unless it is so severe that leaves coated with sooty mold cannot photosynthesize and create food for the tree. The insects may be a greater problem, but even then they may not be a major problem. The greatest effect is the aesthetic damage. Some people do not want to have dark bark on a tree that should have a lighter colored bark. Others are just worried that this is a problem that could permanently damage or kill their tree.

While permanent damage is possible, it will take years, so you have time to deal with the problem. The method of managing this problem is to manage the insects. The problem is probably due to aphids that feed by sucking juices from the leaves and twigs without chewing on them. They may be treated with several insecticides. However, if the damage to the tree is not too severe and the need for removal of the insects is not too urgent, try alternative methods to allow natural predators to assist. In the spring before the leaves appear, spray the branches and trunk with a horticultural oil spray (according to label direction). This will remove some overwintering insects with minimal damage to the beneficial insect population. As the tree begins growing and the insects begin excreting honeydew, it is time to spray. Use a strong jet of water to wash aphids from the leaves. Aphids will drop easily from the leaves when disturbed. Many will land on lower leaves, but washing will remove them to the ground. You may not be able to reach the top of an 80-foot-tall tree without a power washer or commercial spray device, but even if you donât reach the top of the tree, you can reduce the severity of the problem. This will also reduce the damage being done to the tree. Beneficial insects will help manage the problem in the top of the tree since you are not using insecticides that will kill them. The next line of defense will be to employ insecticidal soap (don't use in the heat of the summer) or other least-toxic insecticides. The purpose is to affect the aphid population with minimum impact on the population of beneficial insects.

Any time you use insecticides, organic or synthetic, it is important that you read, understand, and follow the label directions. If you need help reading and understanding the label, contact your local Cooperative Extension Service office, which provides such information to all residents.

Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: desertblooms@nmsu.edu, office: 505-865-7340, ext. 113.

For more gardening information, visit the NMSU Extension Horticulture page at Desert Blooms and the NMSU Horticulture Publications page.

Send gardening questions to Southwest Yard and Garden - Attn: Dr. Marisa Thompson at desertblooms@nmsu.edu, or at the Desert Blooms Facebook.

Please copy your County Extension Agent and indicate your county of residence when you submit your question!