From the February 1993 issue of "Forest Research West"

Root Disease the Unseen Killer

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.


The life cycle of the fungus begins with airborne basidiospores landing on freshly cut stump surfaces. It is suspected that some tree species such as true fir may become infected through basal wounds in addition to freshly cut stump surfaces. The spores than germinate, grow through the stump wood and enter the stump root system decaying the woody portions of the roots . Previously healthy trees become infected when their roots come into contact with stump roots containing the fungus. In this manner, the fungus spreads from tree to tree, often creating openings in the forest ranging in size from a single tree to openings of several acres. Survival of the fungus for up to 50 years in old, infected stumps under the climatic conditions found in the western United States is not uncommon. The fungus in these old stumps can then continue to kill regenerated trees once they grow large enough for their root systems to make contact with the fungus.

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.


When trying to detect the presence of the disease in the field, it is best to look at recently killed trees or trees with advanced symptoms. Some of the symptoms and signs associated with this disease are visible signs of crown decline, a gradual reduction in growth, a loss or shortening of foliage, yellow needles, dead branches, and trees that generally exhibit poor health and are not performing as well as other trees in the stand. At the other end of the tree, evidence of the disease can be found in the roots. Resin soaked roots and wood decay is often the signal of fungal infection. In pine stands, dead and dying trees in a group around old stumps are grounds for suspicion of annosum root disease. These signs are often not as evident in true fir stands, although experienced observers may detect reduced new branch growth in infected trees.


All things considered, tree mortality associated with annosum root disease in commercial forests usually means a loss in timber resource value. There are also intangible values at risk such as aesthetics and safety in recreational areas. As our population increases the demand for multiple uses of these forests will increase. Therefore, losses directly (tree mortality and growth loss) and indirectly (catastrophic insect outbreaks) linked to this disease will take on added importance as time moves forward.


For more than eight years, Dr. William J. Otrosina, a research plant pathologist (formerly with the USDA Forest Service's Pacific Southwest Research Station, headquartered in Albany, California) has focused his research on annosum root disease. He notes that annosum root disease is a constant factor in the forests, and a fundamental problem when it comes to forest health and productivity. "They (root diseases) are a highly underrated problem because we often don't see the dramatic effect on forests that we usually observe with insect attacks," says Otrosina. The goals of his research are to understand the basic biology, ecology, and genetics of the fungus as they relate to various forest tree species. Results of this research will enable the development of methodologies to detect, quantify, and predict losses to this root disease.

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.