Ask Mr. Smarty Plants

The Pacific Northwest Fungi Database from Washington State University has a list of fungi that affect Sorbus species and Fraxinus species. Many of the fungi cause leaf spotting and cankers. The ones that produce fruiting bodies like those you describe generally indicate that the tree is extensively affected by heart rot. You can read about heart rot in Deciduous Tree Diseases from North Dakota State University (NDSU) Agricultural Extension site. See also the description of "heart rot" under NDSU Parasitic Diseases of Specific Trees for ash trees (Fraxinus spp.).

Unfortunately, there is no effective treatment to cure heart rot in trees. Prevention is the best defense. You can read discussions of treatment and prevention of heart rot and decay from the Florida Department of Agriculture Division of Forestry. The University of Arkansas Division of Agriculture Department of Plant Pathology says that by the time the fruiting bodies appear the damage to the tree is likely to be extensive. The tree may survive for many years after infection but its structural integrity could be questionable. Your best bet is to have a professional arborist check to see that is sound and, if not, have it removed. You can find a list of professional arborists near you by searching the Referral Directory of the American Society of Consulting Arborists. You might also get a recommendation for an arborist from your County Agricultural Agent. Visit the Montana State University Agricultural Extension web site to find the office closest to you.

Below are some of the fungi associated with Sorbus spp. and Fraxinus spp. that may be indicators of heart rot. Perhaps you will recognize the one that you have seen on your tree.

Fomes fraxinophila (synonym=Perennipiporia fraxinophila)

Fomes igniarius and other Fomes spp.

Polyporous sulphureus

Eutypella sorbi

Ganoderma lucidum, Ganoderma tsugae, Gandoderma applanatum and other Ganoderma species

Piptoporus betulinus

Fomitopsis pinicola

Sorbus scopulina

Fraxinus pennsylvanica