INTRODUCTION

Armillaria root disease is attributed to a group of fungi that occurs worldwide in boreal, temperate and tropical forests. The fungi affect a broad variety of tree species (4). These fungi were previously thought to be one species, known as Armillaria mellea, however, research has shown that there are actually a number of species within the Armillaria group. This helps to explain the variation in disease characteristics found worldwide. Each species is now known by its specific species name (e.g. A. ostoyae, A. sinapina). There is one Armillaria species that retained the name A. mellea. This species is not found in B.C.

GLOBAL DISTRIBUTION OF ARMILLARIA SPECIES

Globally, there are approximately 36 known species of Armillaria (4). These species vary widely in characteristics. In Europe, species occurrence and geographic distribution has been well documented (4). Five of the European species are identical to, or closely related to, certain North American species (1). Armillaria species have been recognized as a component of the mycoflora of indigenous forests in New Zealand since 1879 (3). Seven species of Armillaria occur in Australia, mostly in the temperate areas. In one Australian study, Armillaria was found to occur on nearly all stumps after clearcutting of a first rotation radiata pine plantation (2). Some studies have been conducted on Armillaria root diseases in Japan, Korea, Africa and Siberia. There is still little information about the occurrence and effects of the fungi in South America and China.

DISTRIBUTION OF ARMILLARIA SPECIES IN NORTH AMERICA

Nine species of Armillaria have been identified and described in North America. The species and their effects on trees differ between deciduous and coniferous forests (8). The various Armillaria species found in deciduous forests act primarily as secondary pathogens of stressed trees. Armillaria ostoyae, the predominant Armillaria species in coniferous forests, is pathogenic with the degree of pathogenicity being determined by the inoculum potential of the fungus. Inoculum potential is the energy available to the fungus for infecting new hosts. When inoculum potential is low, the amount of damage, and the rate of spread, is also low. Conversely, when inoculum potential is high, the fungus acts as an aggressive primary pathogen capable of colonizing and killing living trees.

DISTRIBUTION OF ARMILLARIA SPECIES IN BRITISH COLUMBIA

There are six Armillaria species found in B.C., but most of these have only a limited occurrence and are not a concern to forest managers.

Armillaria ostoyae is the predominant root pathogen of conifers in the southern half of the province. Susceptible host species also include deciduous trees, certain shrubs, and possibly some herbaceous plant species.

The behaviour of A. ostoyae varies considerably between coastal and interior areas of B.C. The effects of A. ostoyae are much more severe in the southern interior of B.C. than in coastal areas. On the coast, mortality due to A. ostoyae occurs primarily in young forests (both plantations and natural regeneration). In the southern interior, the pathogen can kill trees throughout the rotation (6). Reasons for this variation may be related to the greater ability of coastal trees to callus root lesions and therefore limit the spread of the fungus (Morrison, pers. comm.). Site productivity and tree growth may also play a role.

Another commonly occurring species, Armillaria sinapina, is found throughout most of B.C., primarily on aspen and other hardwoods. A. sinapina is only weakly pathogenic and is believed to exist mainly as a saprophyte (8). There is some question as to whether or not the fungus is pathogenic in younger trees. Studies in Alberta have shown that A. sinapina may be pathogenic in younger trees (5), however, there is no evidence to suggest that this is the case in B.C. (D. Morrison, pers. comm.).

Armillaria gallica (previously named A. bulbosa) and Armillaria nabsnona are found on hardwoods on the lower mainland and southern Vancouver Island (6). Neither species is of significant concern to forest managers. Occurrences of Armillaria cepistipes have also been recorded in B.C.. While the range of this species appears to be very broad (i.e. found in Stewart and Hope B.C.), its occurrence is rare and it is currently not a concern for forest managers. One other member of the Armillaria group has been found in B.C. but the species has not yet been identified. It is currently referred to only as NABS X. NABS X has been found on both coniferous and deciduous trees. Its extent appears to be limited to a very small area near Nelson, B.C. and a site near Fernie, B.C., hence it is not of significant concern to forest managers.

To date, only the two species of Armillaria: A. ostoyae and A. sinapina have been found to occur on a widespread basis in B.C., and are a concern to forest managers. A. ostoyae, which attacks all coniferous species, is the most pathogenic, and therefore the most damaging.

IMPLICATIONS OF ARMILLARIA OSTOYAE

Armillaria ostoyae may lower timber productivity by reducing tree growth, causing direct mortality of host trees, or predisposing trees to attack by insects or pathogens (9). The extent to which forests are vulnerable to the fungus varies considerably from ecosystem to ecosystem (4), site to site, and tree to tree. The Interior Cedar Hemlock zone (ICH) is one of the most productive and diverse biogeoclimatic zones in the province yet it is heavily impacted by Armillaria infection (9). The role of Armillaria root disease in shaping and/or maintaining the characteristics of these forests is poorly understood. Much more research is necessary before we fully understand Armillaria root disease, and the effect of forest management activities on its dynamics.

REFERENCES

1. Anderson, J.B., K. Korhonen and R.C. Ullrich. 1980. Relationships between European and North American biological species of Armillaria mellea. Experimental Mycology 4: 87-95.

2. Anon. 1982. Forest pathology - conifer plantations. In: Report on Research Activities for 1980-1981. Australia: Queensland Department of Forestry, Division Technical Services: 52-54.

3. Hood, I.A. and C.J. Sandberg. 1987. Occurrence of Armillaria rhizomorph population in the soil beneath indigenous forests in the Bay of Plenty, New Zealand. New Zealand Journal of Forestry Science 17(1): 83-99.

4. Kile, G.A., G.I. McDonald and J.W. Byler. 1991. Ecology and disease in natural forests. In : Shaw, C.G., III; Kile, G.A. 1991. Armillaria Root Disease. Agricultural Handbook No. 691. U.S. Department of Agriculture, Forest Service. 233 pp.

5. Mallett, K.I. 1990. Host range and geographic distribution of Armillaria root rot pathogens in the Canadian prairie provinces. Can. J. For. Res. 20: 1859-1863.

6. Morrison, D.J. 1981. Armillaria root disease. A guide to disease diagnosis, development and management in British Columbia. Information Report. BC-X-203. Environment Canada, Canadian Forest Service: 16 pp.

7. Morrison, D.J., D. Chu and A.L.S. Johnson. 1985. Species of Armillaria in British Columbia. Can. J. Plant Pathology. 7: 242-246.

8. Wargo, P.M. and C.G. Shaw III. 1985. Armillaria root rot: the puzzle is being solved. Plant Disease 69: 826-832.

9. Woods, A.J. 1994. The behavior and impacts of Armillaria ostoyae in mature stands and plantations in the Shuswap region of British Columbia. Master of Science thesis. University of British Columbia. 119 pp.

February 1995