Annosus root and butt rot (Heterobasidion occidentale)

Micro-habitat(s)

Base of tree

Distribution

British Columbia

Damage, symptoms and biology

The infection begins in autumn when the spores germinate on the surface of fresh stumps. At first, the disease causes incipient decay (e.g., yellow-brown to red-brown stain in wood). Later, as the disease evolves,  moderate to advanced decay can be seen. Then, wood may separate along rings with many small black flecks evident in pits or pockets; eventually wood becomes a spongy or white stringy mass. Finally, it causes root rot, which can kill young trees quickly and very large trees after a few years.

Once the stump has become infected, the fungal hyphae seek to colonize the stump roots. The disease spreads to other trees through the contact between the roots of the infected stump and the healthy roots of nearby trees. After a site has been infected for a few years, this method of transmission gives rise to a more or less circular mosaic of dead trees. The French common name of the disease, "maladie du rond", comes from this circular pattern of areas of mortality.

Trees younger than 15 years that have a major portion of their root system killed by Heterobasidion occidentale exhibit crown symptoms typical of other root diseases (i.e., reduction in leader and branch growth, chlorotic foliage, and a distress cone crop). In more mature trees, however, the fungus causes a butt rot and external symptoms are not readily discernible. Trees with extensive decay in the structural roots are subject to windthrow, and groups of windthrown trees may indicate the presence of pockets of annosus root rot.

The fungal fruiting bodies are present at the base of the stumps and at the base of dead or dying trees. They are white on the underside and contain many pores, and the top is brown. The spores produced by these fruiting bodies become dispersed and induce new infections. Immature fruiting bodies ( 'popcorn conks') may appear on the surface of recently upturned roots.

Other information

This disease probably migrated from Mexico to the northern regions of North America.

Spores are present in the air throughout the year and can be carried by air currents for many kilometres. Infection by spores occurs through wounds on stems or roots, or through cut surfaces of fresh stumps. The fungus moves to surrounding trees through root grafts or root contact. Stand thinning treatments or damage to trees from logging operations can therefore exacerbate annosus root rot problems. H. occidentale is able to survive in stumps for several decades.

There are few registered products for treating stumps following cutting. For information on the products registered for controlling specific insects or diseases, please contact the Pest Management Information Service of the Pest Management Regulatory Agency (PMRA), at the following toll-free number: 1-800-267-6315.

Other control recommendations include shortened rotations and avoidance of wounding during logging. Immature fruiting bodies of this fungus could be confused with Fomitopsis pinicola but can generally be distinguished by the location on the tree. In addition, F. pinicola has very regular pores (pin-hole like) whereas the pores of H. occidentale are irregular in shape.

Reference

Linzer, R. E.; Otrosina, W. J.; Gonthier, P.; Bruhn, J.; Laflamme, G.; Bussières, G.; Garbelotto, M. 2008.  Molecular Phylogenetics and Evolution 46:844-862.

Myren, D.T.; Laflamme, G.; Singh, P.; Magasi, L.P.; Lachance, D. 1994. Maladies des arbres de l’est du Canada. Ressour. nat. Can., Serv. can. for., Administration centrale, Dir. générale des sciences et du développement durable, Ottawa (Ontario). 159 pp.

Morrison, D.J.; Johnson, A. L. S. 1978. Stump colonization and spread of Fomes annosus 5 years after thinning. Can. J. For. Res. 8:177-180.

Otrosina, W. J.; Chase, T. E.; Cobb Jr, F. W.; Korhonen, K. 1993. Population structure of Heterobasidion annosum from North America and Europe. Can. J. Bot. 71:1064–1071.

Otrosina, W. J.; Garbelotto, M. 2010. Heterobasidion occidentale sp. nov. and Heterobasidion irregulare nom. nov.: a disposition of North American Heterobasidion biological species. Fungal Biology 114:16-25.

Otrosina, W.J.; Scharpf, R.F. (tech. coord.), 1989. Proc. Symp. on research and management of annosus root disease (Heterobasidion annosum) in western North America. USDA For. Serv., Gen. Tech. Rep. PSW 116.

Main host(s)

Douglas-fir, Sitka spruce, western hemlock

Secondary host(s)

Alders, amabilis fir, apples, bigleaf maple, grand fir, lodgepole pine, ponderosa pine, western redcedar, white spruce