Pathology > Forest Pathology > Root Disease > Armillaria Root Disease

Armillaria Root Disease

The causal fungi, their distributions and hosts

Six species of Armillaria cause root disease in British Columbia and the northwestern United States. Armillaria sinapina is the most widespread and common species in British Columbia, occurring from 49° N to about 57° N. The hosts of this species are primarily broadleaved trees and shrubs, and plants that are very susceptible or stressed may be killed. Armillaria sinapina often colonizes conifer stumps in managed coastal forests. The host range of A. nabsnona is identical to that of A. sinapina, but it has been collected only in the southern part of the coastal region. Armillaria cepistipes appears to be similar to A. sinapina and A. nabsnona in behaviour and host range; however, it is rare, having been collected only twice in British Columbia, at Hope and Stewart. Armillaria gallica occurs within the range of Garry oak, its principal host, in southwestern British Columbia. The four species of Armillaria listed above are weakly pathogenic, and they spread slowly on their host's root system; usually, only stressed hosts are killed.

A fifth species, Armillaria ostoyae, occurs in British Columbia from 49° N to about 53° N, where its principal hosts are conifers. This fungus also attack broadleaved trees, shrubs and some herbs. Ponderosa pine (Pinus ponderosa), western larch (Larix occidentalis) aged 15 years and older, and birch (Betulapapyrifera), are species with the greatest tolerance to A. ostoyae infection. The sixth species, an undescribed one termed North American Biological Species X (NABS X), occurs in southeastern British Columbia, primarily on conifers; it is uncommon.

Mushroom fruiting bodies of Armillaria ostoyae.

The problem

In British Columbia, A. ostoyae is the only species causing significant damage, including growth repression and mortality. Throughout its range in the Province, A. ostoyae kills coniferous trees in plantations and natural stands beginning 5-7 years after their establishment. Incidence of mortality is usually 1-2% per annum. In the coastal region, mortality usually stops by stand age 15 years. However, in the southern interior mortality does not stop, although its incidence may decline. In juvenile, immature, and mature stands, mortality caused by A. ostoyae creates unstocked or understocked openings that may occupy as much as 30% of the stand area.

Current state of knowledge of Armillaria ostoyae

In British Columbia, most of the research on A. ostoyae has been done in the southern interior because most of the damage occurs there. Armillaria ostoyae is a normal component of many ecosystems across its range in the southern interior. It occurs in the Interior Douglas-fir (IDF), Montane Spruce (MS), Interior Cedar-Hemlock (ICH), Engelmann Spruce-Subalpine Fir (ESSF), and Sub-Boreal Spruce (SBS) biogeoclimatic zones but is often absent from very dry sites in the IDF zone and from high elevations in the ESSF zone.

Transfer of Armillaria ostoyae occurs at root contacts between infected stumps and healthy trees.

Armillaria ostoyae is a facultative parasite, killing hosts during the parasitic part of its life cycle and utilizing the host as a food source during the saprophytic phase. Following a stand replacement event, such as a wildfire or harvesting, the fungus is carried over to the next rotation in colonized root systems, which are called inoculum. Spread from carry-over inoculum to regeneration is primarily by rhizomorphs. Rhizomorphs are root-like structures produced by the fungus in colonized root systems that grow through soil and may contact and infect young trees. In juvenile, immature, and mature stands, where root systems of adjacent trees overlap, spread from diseased to healthy trees occurs by mycelial transfer at root contacts. At a point of infection on a root initiated by a rhizomorph or root contact, the fungus initially spreads in the outer bark and then penetrates to the cambial zone. The host responds to infection by producing resin and new tissue (callus). The outcome of the host-fungus interaction is determined by the resources available to each. A vigorous host will overcome the pathogen with low inoculum potential, and vice versa. On a susceptible host, the fungus spreads proximally on the root to the root collar and kills the tree by girdling the root collar. Vigorous hosts or species more tolerant of the fungus may live for many years with dormant root infections, but their growth may be reduced, the amount of reduction depending on the proportion of roots infected.

Usually, only coniferous trees with more that two-thirds of their roots affected by A. ostoyae show above-ground signs and symptoms. Consequently, the actual (below-ground) incidence of disease in a stand is difficult to determine. The proportion of diseased trees that show above-ground symptoms depends upon the climatic region (dry, moist or wet) in which the stand is growing. In juvenile stands in the dry region of British Columbia, one-half of trees with diseased roots can be detected whereas only one-quarter of root-diseased trees show symptoms in the moist and wet regions. In most undisturbed mature stands 15-25% of diseased trees are detectable, regardless of climate region.

In British Columbia, the incidence of trees affected by A. ostoyae has been determined in studies where all trees in plots were removed from the soil and their root systems examined for disease. Incidence of A. ostoyae in the Province varies with climatic region and stand age: in juvenile stands incidence is about 10% in the dry region and 30-35% in the moist and wet regions; in mature stands, incidence of the fungus is about 10% in the dry region, 80% in the moist region and 35% in the wet region. The probability of a tree being affected by A. ostoyae increases with increasing tree diameter.

Dead and dying trees due to Armillaria Root Disease. In uneven aged stands, leave trees generally die before regenerating trees because their roots are the first to contact infected stumps.

Forest management practices that create stumps on root-diseased sites may increase the incidence of trees with A. ostoyae, the amount of disease on a root system, and subsequently, the amount of damage. When a tree with one or more active or dormant A. ostoyae root lesions is cut, the stump and root system are rapidly colonized by the fungus. In stands that have been spaced, commercially thinned or selectively cut, these new sources of inoculum can upset the balance that may have existed between host and fungus such that the increase in the inoculum potential of the fungus is sufficient to overcome host resistance. This effect has been observed in spaced and selectively cut stands in the dry and moist climatic regions of British Columbia. The increases in disease incidence and damage are largest at sites in the dry climatic region.

Measures to reduce damage by A. ostoyae include: stump removal after clearcutting, pop-up spacing and commercial thinning (i.e. whole tree removal), use of mixtures of species when regenerating sites and selection of the most resistant species as crop trees.

Current research includes monitoring long term disease development and control installations and determining growth repression on diseased trees in juvenile and mid-rotation stands.

More information on this particular root disease is available on the Common Tree Diseases of British Columbia web site.

Posters

Detection of a Chitinase-like protein in the roots of infected Douglas-Fir trees

Growth reduction of Douglas-fir infected by Armillaria ostoyae

Root and Butt Rot : A Sustainable Forestry Issue

Pertinent publications by CFS research staff

Bloomberg, W.J. and Morrison, D.J. 1989. Relationship of growth reduction in Douglas-fir to infection by Armillaria root disease in southeastern British Columbia. Phytopathology 79:482-487.

Cruickshank, M.G., Morrison, D.J. and Punja, Z.K. 1997. Incidence of Armillaria species in precommercial thinning stumps and spread of A. ostoyae to adjacent Douglas-fir trees. Can. J. For. Res. 27:481-490.

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

Morrison, D.J., Wallis, G.W. and Weir, L.C. 1988. Control of Armillaria and Phellinus root diseases: 20-year results from the Skimikin stump removal experiment. Canadian Forestry Service, Pacific Forestry Centre, Information Report BC-X-302. 16pp.

Morrison, Duncan, Merler, Hadrian and Norris, Don. 1992. Detection, recognition and management of Armillaria and Phellinus root diseases in the southern interior of British Columbia. FRDA Report 179. 25 pp.

Morrison, Duncan and Mallett, Ken. 1996. Silvicultural management of Armillaria root disease in western Canadian forests. Can. J. Plant Pathol. 18:194-199.

Morrison, D.J., Pellow, K.W., Norris, D.J. and Nemec, A.F.L. 2000. Visible versus actual incidence of Armillaria root disease in juvenile coniferous stands in the southern interior of British Columbia. Can. J. For. Res. 30: 405-414.

Morrison, D.J., Pellow, K.W., Nemec, A.F.L., Norris, D.J. and Semenoff, P. 2001. Effects of selective cutting on the epidemiology of armillaria root disease in the southern interior of British Columbia. Can. J. For. Res. 2001:59-70.

Peet, F.G., Morrison, D.J. and Pellow, K.W. 1996. Rate of spread of Armillaria ostoyae in two Douglas-fir plantations in the southern interior of British Columbia. Can. J. For. Res. 26:148-151.

Robinson, R.M., Sturrock, R.N., Davidson, J.J., Erkamoddoullah, A.K.M. and Morrison, D.J. 2000. Detection of a chitinase-like protein in the roots of Douglas-fir trees infected with Armillaria ostoyae and Phellinus weirii. Tree Physiology 20: 493-502.

White, E.E., Dubetz, C.P., Cruickshank, M.G. and Morrison, D.J. 1998. DNA diagnostic for Armillaria species: within and between species variation in the IGS 1 and IGS 2 of British Columbia isolates. Mycologia 90:125-131.

Frontline Express Bulletin #6 : Epidemiology of Armillaria Root Disease in Plantations. Great Lakes Forestry Centre, Canadian Forest Service.

Frontline Express Bulletin #7 : Growth Loss Resulting from Infection by Armillaria Root Disease. Great Lakes Forestry Centre, Canadian Forest Service.