Hosts:  Douglas-fir, ponderosa pine, southwestern white pine, spruce

Symptoms/signs:  Trees infected with P. schweinitzii do not typically show outward symptoms, so the presence of disease is usually not realized until a tree breaks or is windthrown due to loss in structural support. The fruiting bodies of P. schweinitzii often emerge through soil from decayed roots. They have a velvety surface which is yellow to orange at first, then turning brown with a yellow margin. It turns a dark red brown upon aging, is crumbly and can be mistaken for a cow pie at this point, hence the nickname “the cow patty fungus.” Incipient decay appears as a light yellow to pale reddish-brown color. As the decay advances, the color intensifies and the wood rapidly loses strength. In advanced stages of decay, the wood breaks up into large cubes that are crushed easily, and the butt portions of infected trees can be swollen.

Biology:  Spores likely initiate most P. schweinitzii infections. P. schweinitzii is unusual in that the fungus establishes itself in forest soil and enters trees through root tips. There is no root-to-root spread as used by other root disease causing fungi. Decay is confined to the roots and lower 1-2 m of the butt log, rarely extending much higher.

Effects:  Schweinitzii root and butt rot is a major disease of older trees. It causes decay of the roots and lower stem and the resulting strength loss predisposes trees to windthrow and breakage.

Similar Insects and Diseases:  The fruiting bodies of I. tomentosus and I. circinatus are similar to Phaeolus schweinitzii since they are all velvety on top when fresh and have a pore surface beneath. However, P. schweinitzii fruiting bodies are larger, greater than 15 cm in circumference, while the other two are about 5 cm maximum.

References:  30, 92