Hosts:  White fir, subalpine fir, ponderosa pine

Symptoms/signs:  Fruiting bodies of annosus are grey-brown on the upper surface with an undersurface that is chalky white with pores. They are found on the underside of decayed roots and slash, inside stumps (common in the Southwest), or under the duff at the base of infected stumps and trees (although rare in the Southwest). H. annosum leads to crown thinning and mortality or windthrow, the latter of which can occur before aboveground symptoms are evident.

Biology:  Spores of H. annosum are produced on conks in decayed stumps or on roots of windthrown trees. The most common means of initial entry of H. annosum into a site is via airborne spores that germinate on freshly cut stumps and basal wounds. Mycelium of H. annosum quickly colonizes the stump and grows into its roots. Transmission to adjacent trees occurs via root contacts. In live trees, the fungus decays woody root systems and then advances to the root collar where it may surface to the cambium and kill by girdling as in ponderosa pine, or progresses more slowly through roots to the stem and causes butt decay as in true fir.

Although most conifers are susceptible to infection, there appear to be two types of annosus in North America, each of which has specific host preferences. One type infects true firs, and sometimes spruce and Douglas-fir. The other infects ponderosa pine but may also infect juniper. Like Armillaria, H. annosum is a common decayer of dead woody material as well as a pathogen.

Effects:  This is one of the most important forest tree diseases in temperate zones worldwide. Damage to forests in North America has been most severe in the Southeast, where intensive forestry has the longest history. In the Southwest, annosus root disease is the second most common root disease of conifers. It is found most often on true firs; however, there are areas where it is common in ponderosa pine. Infected trees are predisposed to attack by bark beetles.

Similar Insects and Diseases:  The aboveground symptoms are similar to those caused by other root disease fungi.

References:  29, 53, 59, 73, 88, 92, 114