There is an unseen killer lurking in the forest - and it's called Annosum Root Disease. This root disease is caused by the fungus Heterobasidion annosum , a fungus belonging to the group that forms brackets or conks. These conks are the reproductive structures of the fungus, producing spores that are wind disseminated. H. annosum has existed in conifer forests long before man started to manage them; however, as man increased utilization of the forest the fungus has become more widespread and damaging. This fungus causes serious problems in coniferous forests throughout North America, Europe and Scandinavia. One of the first finds of this disease in the West was recorded in the early 1900's in Monterey, California, on Monterey Pines. The fungus is widespread in California affecting pine, true firs, and other conifer species in most national forests in the State. Because of the varied and often use of California forests for timber production and recreational activities; and the diversity in climatic conditions and tree species associated in the State, the biology of this fungus and the disease it causes is quite complex.
In addition to mortality, this disease has other effects as well. In some tree species, such as true fir, infection by the fungus may result in slow decay of root systems lasting decades before the tree succumbs. These trees are often wind thrown because of the decay of their anchoring roots. Also, this type of chronic infection may result in considerable growth loss long before symptoms appear.
Another important consequence of infection by H. annosum is that it renders infected trees susceptible to bark beetle attack. During periods of normal precipitation, trees infected with H. annosum, because they are under stress maintain endemic levels of certain tree killing bark beetles. When protracted drought occurs, as we are currently experiencing, these endemic beetle populations build up rapidly and cause extensive, catastrophic mortality in affected stands. Thus, if we can control the extent of root disease in forest stands, we may be able to lessen the effects of drought induced insect outbreaks.
Recent contributions to our knowledge of this disease by Otrosina's research unit centers around the existence of two biological species of H. annosum in the western United States. These biological species were first discovered in Finland in 1978. Our research, in cooperation with the University of California, Berkeley, has shown that these two biological species of H. annosum are pathologically different fungi, each infecting specific tree species. These findings were determined through a coordinated study of genetic characteristics of the fungus, field sampling, and greenhouse inoculation experiments. One biological species, known as the P group, was shown to attack primarily pines, junipers, and incense cedar while the other biological species, known as S group, infected true fir and sequoia. Methodology was also developed to rapidly distinguish the two biological species. This represents a significant breakthrough in that the land manager, by knowing which biological species of the fungus predominates on an affected forest site, has the option of reducing losses to this disease by silviculturally encouraging species that are not susceptible to the biological species of H. annosum present.
Otrosina believes that if we can get a handle on the annosum root disease problem, we may be able to lower endemic insect populations at least to the point where they may not increase as rapidly during periods of drought and other agents of stress, such as air pollution. Otrosina also feels that continued research and technology transfer will enable land managers to make more informed decisions when it comes to dealing with annosum root disease.