Michelle Grabowski, UMN Extension Educator
When shade trees are stressed by environmental conditions or wounded by storms, ice, insects, sunscald or mechanical damage, sapwood rotting fungi often show up to take advantage of the trees weakened state. There two sapwood rotting fungi commonly found in Minnesota, Schizophyllum commune or Cerrena unicolor. These fungi infect the tree through wounds. They then rot the sapwood and kill the bark. When severe, the infection can grow to completely encircle a branch or the main trunk, killing all leaves and branches above the infected area.
Sapwood fungi infect a wide variety of common shade trees. Maple (Acer), linden (Tilia), willow (Salix), elm (Ulmus), oak (Quercus), ash (Fraxinus), poplar (populus), and many more may all suffer from this disease.
How to Recognize Sapwood Rotting Fungi
Gardeners often first notice dead branches throughout the canopy of infected trees. Cracked and peeling bark may be seen on the main trunk or branches. Upon close examination, clusters of small white shelf fungi can be found growing on the infected wood. These are reproductive structures of the sapwood rotting fungi and can be easily seen this time of year in Minnesota. A gardener can determine which of the two fungi is causing the problem by closely examining these fungal spore producing structures.
The spore producing structures of Cerrena unicolor are ½ to 3 inch semi circular shelf fungi. They are white to green gray in color and often rings of growth are apparent on the top surface of the fungi.
Teeth like ridges can be seen on the lower surface of the fungi.
Schizophyllum commune also produces small shelf fungi on infected wood. These shelf fungi are smaller (1/4th to 2 inches across), cupped downward, and pure white. When examined up close, the upper surface appears fuzzy. Gills can be seen on the lower surface of these shelf fungi.
Keep Trees Healthy and Strong to Avoid Sapwood Rotting Fungi
Prevention is the best management strategy to control sapwood rotting fungi. Avoid wounding trees with weed whips, lawn mowers or other lawn equipment. If trees are damaged in a storm or by ice, remove broken and cracked branches with a clean pruning cut. This cut will be easier for the tree to heal than a long jagged rip caused by a storm. Water trees during times of drought. Mulching the base of the tree out to the canopy drip line with an organic mulch like wood chips can help maintain soil moisture and reduce competition with turf grass.
If infection appears on a branch, prune the infected branch on a cool dry day and remove it from the area. If the infection appears in the main trunk, the only thing that can be done is to reduce stress on the tree and hope for the best.