Pathology > Forest Pathology > Root Disease > Annosus Root Disease

Annosus Root Disease

The causal organism, its distribution and hosts

Heterobasidion annosum (formerly called Fomes annosus) is endemic in many temperate zone forests, where it causes mortality and butt rot of conifers. Two varieties of the fungus occur in western North America, the spruce and pine types. Only the spruce type has been detected in British Columbia. In British Columbia, studies have shown that the range of H. annosum is approximately coincident with that of western hemlock. Within its range, the fungus has been found on all commercially important conifer species and on alder and broadleaf maple.

The problem

In old growth forests, H. annosum causes butt rot. In second growth, trees whose roots contact H. annosum inoculum in old-growth stumps may be killed when less than 15 years old, windthrown when roots are decayed, or butt-rotted. The incidence of Annosus root disease and the damage it causes in managed forests are related to the frequency and intensity of thinnings. The fungus infects freshly cut stumps by means of airborne spores, and may colonize the stump, grow into its roots, and spread to adjacent healthy trees at root contacts. Airborne spores also infect stem wounds on thin-barked species such as hemlock and fir (Abies spp.); extensive decay can develop above and below the wound.

Butt rot caused by Annosus Root Disease can weaken tree stems and predispose them to wind breakage.

Current state of knowledge

Root and butt rot and decay associated with stem wounds are commonly seen throughout the range of Heterobasidion annosum. In old growth forests, the incidence of H. annosum can be as high as 5% (mean 1.5%) in western hemlock, amabilis fir, western redcedar, Sitka spruce, Douglas-fir, and subalpine fir. However, the volume of decay attributed to H. annosum in these stands may be small, especially compared to volumes of decay caused by true heartrot fungi such as Phellinus pini and Echinodontium tinctorium. In unthinned second-growth stands, up to 5% of trees may have butt rot caused by H. annosum, with disease centers containing up to five trees in Douglas-fir plantations, and up to 20 trees in hemlock stands.

Studies of tree wounds indicate that about one-half of wounds larger than 1000 square centimetres on the lower bole of second-growth hemlock can be colonized by H. annosum and the fungus can decay 0.5-0.7% of the tree's volume annually. Results of thinning studies show that up to 20% of Sitka spruce and amabilis fir stumps in precommercial thinnings may be colonized by H. annosum, compared to 6% and 10% of Douglas-fir and hemlock stumps, respectively. The incidence of stumps colonized by H annosum varies with stump diameter, season of thinning, stand age and biogeoclimatic subzone.

Young trees that have most of their roots killed by the fungus usually exhibit typical symptoms of root disease, including reduced leader growth, faded foliage, and distress cones. In older trees the fungus causes butt rot, and external symptoms are not evident. Trees with extensive decay in their roots are susceptible to windthrow and one or more windthrown trees may indicate the presence of an Annosus root disease center, especially in hemlock stands. Fruiting bodies (conks) of the fungus are produced on colonized stumps and on the decayed exposed roots and the underside of the stem of windthrown trees. The early stage of H. annosum decay is a yellow-brown to red-brown stain, while in advanced stages the wood is reduced to a white stringy mass containing black flecks running parallel to the grain. A useful test to confirm the fungus in a wood sample is to wrap it in moist paper, enclose it in a plastic bag and store it at room temperature. After 7 days, the asexual fruiting structures of the fungus will form on the wood surface.

Decay of Heterobasidion annosum in a western hemlock stump. Like other forest root diseases, Annosus Root Disease slows tree growth and weakens root and stems making infected trees a safety concern.

Management tactics for H. annosum focus on cut stumps and tree wounds. Any increase in inoculum resulting from spore infection of thinning stumps can be minimized by thinning before age 15, by cutting only trees less than 10 cm in diameter, and by thinning during wet seasons. Also, the incidence of wounding during selective harvesting can be reduced by favoring thick barked species as crop trees, suspending operations in spring during sap flow, using buffer trees or pads at turning points on skid trails, harvesting during winter on snow, and thinning stands as young as practicable.

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

Pertinent publications by CFS research staff

Filip, G.M. and Morrison, D.J. 1998. Impact, control and management of Heterobasidionannosum root and butt rot in Europe and North America: North America. pp. 405-427 In Heterobasidion annosum: Biology, Ecology, Impact and Control. Eds. S. Woodward, J. Stenlid, R. Karjalainen and A. Hüttermann. CAB International, Oxon, UK.

Morrison, D.J. and Johnson, A.L.S. 1970. Seasonal variation of stump infection by Fomesannosus in coastal British Columbia. For. Chron. 46: 200-202.

Morrison, D.J. and Johnson, A.L.S. 1978. Stump colonization and spread of Fomesannosus five years after thinning. Can. J. For. Res. 8: 177-180.

Morrison, D.J. and Johnson, A.L.S. 1999. Incidence of Heterobasidion annosum in precommercial thinning stumps in coastal British Columbia. Eur. J. For. Path. 29:1-16.

Wallis, G.W. and Reynolds, G. 1970. Fomes annosus root and butt rot: a threat in managed stands in coastal British Columbia. For Chron. 46: 221-224.

Wallis, G.W. and Morrison, D.J. 1975. Root rot and stem decay following commercial thinning in western hemlock and guidelines for reducing losses. For. Chron. 51: 203-207.