December 17, 2011

1 - Development of black coloration on tree bark may indicate insect problems that should be managed.

Yard and Garden
December 17, 2011

Q. I have a Washington hawthorn tree, seventeen years old, that I presume to be diseased. The bark of the limbs and of the trunk have turned blackish. Otherwise it seems okay and turned a beautiful color this fall. I live in northeast Albuquerque. Is there anything I could, or should, do to help the situation?

Name Withheld

A. There are several possible causes for the blackening of the bark on your tree. The most likely is the development of sooty mold. Sooty mold is a rather benign fungus that grows on bark and leaves as a result of a substance (honey dew) excreted by some insects. A heavy coating of sooty mold may reduce photosynthesis. The insects are a more significant problem if they are the cause of the sooty mold and should be treated. As the insects feed on the sap from the tree, they excrete a syrup-like substance. This is what feeds the growth of the sooty mold. The insects responsible are most likely aphids or scale insects. Aphids are fairly easy to manage by washing them from the leaves and twigs in the summer with strong streams of water. They will return, but by frequently washing them away without use of pesticides you reduce the damage they do and encourage the development of beneficial insects that feed on the aphids. Scale insects are more difficult to manage. Insecticides or dormant/horticultural oil sprays may help manage scale insects. The scale insects will probably be found on the twigs in the tree. Aphids are probably not evident at this time of year. A sample of the twigs taken to your local NMSU Cooperative Extension Service office may allow for a positive identification of any insect problems. The NMSU Extension Service agent can recommend appropriate treatments once they identify the problem. Another possible problem could be development of southwest injury. This is damage to the bark that occurs most commonly on the southwest side of the trunk and branches directly exposed to direct winter sunlight. Thin-barked trees, such as your Washington hawthorn, apple trees, other trees and young saplings are subject to this problem. However, your description of the problem does not clearly indicate this problem, but it is worth considering when talking to your NMSU Extension agent.

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.

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