The Q&A Archives: Sooty Canker Infestation

Question: I have a fairly large cottonwood tree in my yard that has become infected with sooty canker. I need to know how to eradicate this disease, or at least halt its contamination to my other trees.

Answer: The following information is taken from a publication written by Mary W. Olson and Deborah J. Young, both Extension Plant Pathologists from the University of Arizona. The complete publication is available online at

Sooty canker causes cankers, wilting and dieback in tree branches. Leaves on affected branches are often small and wilt and die during the summer. Brownish, moist areas appear on limbs during the first stages of disease, then the bark in these areas cracks or peels away revealing black masses of fungal spores.

At lower desert areas in Arizona, infections may take place at any time when there is sufficient humidity for the fungal spores to germinate and penetrate wound sites.

The pathogen infects the host only through cracks in the bark or wound sites such as those created by heat stress, sunburn, pruning or freezing. Trees with smooth bark that is easily wounded are particularly susceptible. The fungus grows into the bark where it becomes established and produces masses of spores for which the disease is named. It is well adapted to hot, dry weather, and once established, grows best at 91 o F to 97 o F.

Disease can be prevented by maintaining tree vigor and avoiding unnecessary pruning or wounding. Since sun-burned bark is the most common place that the fungus enters, careful pruning techniques should be used so that limbs that shade the trunk and scaffold branches are not removed.
When infections are found in upper branches, remove infected limbs by cutting at least six inches below infection sites. If severe pruning is needed, be sure to remove limbs in the fall or early spring when the remaining tree will not be sunburned as a result of pruning. Treatment of wounds with pruning paint or compound is no longer recommended, and the best assurance of preventing re-entry of the fungus is to prune during cool, dry periods when the wound will seal quickly. Rinsing pruning tools in a 20% bleach solution between cuts will prevent spreading the fungus with tools.

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