Tree wounds and decay fungi

Karen Rane, Plant Disease Diagnostician, Botany & Plant Pathology Department, Purdue University

These ornamental cherry trees are showing symptoms of severe decline (Fig.1).  Several branches are dead, and leaves are falling prematurely.  On closer inspection, spore-bearing structures of the fungus Trametes versicolor (commonly called the turkey tail fungus) are visible protruding from the bark (Fig. 2).  This is one of a number of fungi that produce enzymes that break down lignin and cellulose in wood, resulting in a soft, stringy decay known as white rot. A second type of wood decay, called brown rot, is caused by fungi that degrade cellulose and hemicellulose, but not lignin.  Wood decay fungi are important forest recyclers, breaking down wood from fallen branches and dead trees and releasing nutrients and organic matter to the soil. Decay fungi can also invade living trees through wounds in roots, trunks or branches.  In this case, a pruning wound appears to be the entry point for this wood decay fungus (Fig. 3).

Wound response in trees is a complex process, and involves both physical and chemical changes that may continue for several years.  If the wound closure process is successful, the wounded area is eventually enclosed by layers of callus tissue.   While the wound remains open, microorganisms can colonize the exposed xylem tissue, and the wood can become decayed. Once the wound is completely closed, wood decay is halted.

Trees with wood decay can become structurally unsound, and may drop large limbs or blow over in storms.  Removal of the spore-bearing structures (called conks or brackets) from the bark surface does nothing to control wood decay – the threadlike hyphae of decay fungi are present in the wood beyond the immediate area of the conk. Wound treatment paints do not prevent invasion of the wound by wood decay fungi and therefore are of little benefit. Maintaining tree vigor through proper pruning, irrigation and fertilizing practices will help to reduce potential decay problems by promoting the tree’s natural wound response processes. The cherry tree in the photo has a restricted root zone and is not irrigated during dry summer weather – stress factors that are reducing tree vigor.  Landscape trees with fungal conks or large, open wounds on the trunk or large branches should be evaluated for structural soundness by a certified arborist.  Trees with extensive internal wood decay are hazardous and should be removed from areas where personal injury or damage to property could occur if the tree falls.

For more information on wood decay and the wound response process in trees, check out the following publications:

“Wood Decay in Trees” , Kansas State University

“Wood Decay Fungi in Landscape Trees” University of California

“Wood Rots and Decays”, University of Illinois (pdf file)

To find a certified arborist in your area, check out the website for the International Society of Arboriculture: http://www.isa-arbor.com/home.aspx