Heterobasidion annosum

Heterobasidion annosumis a species complex of perennial root rot and heart rot pathogenic fungus found throughout North America and Europe. In the Pacific Northwest it primarily infects forest and commercially-planted conifer trees. Within Europe and North America there are three distinct strains of host-specific H. annosum that do not interbreed. The three resulting intersterility groups (ISGs) are the S, P, and F strains. In the Pacific coastal forests of North America the P strain and S/F cross strain have been identified(Asiegbu et al, 2005), with the P strain attacking Pinus (Korhonen et al., 1998a), and the S/F cross strain attacking Abies, Tsuga, Picea, Pseudotsuga, Sequoiadendron (Harrington et al. , 1989; Korhonen and Stenlid, 1998; Korhonen et al., 1998a; Otrosina et al., 1993).

Infection and Hosts

Primary infection occurs when monokaryotic basidiospores, or often times conidiospores,  land on freshly cut stumps or open stem wounds of the host, where the hyphae colonize the heartwood and sapwood. This mode of infection will initiate butt rot and heart-wood staining in the host, decreasing tree health and productivity in forests and causing some commercially planted trees to become unmarketable. Once a compatible monokaryotic hyphae is contacted a dikaryotic mycelium forms that can spread from the stem to the roots. The resulting annosus root rot may spread vegetatively between host trees through root to root contact underground (citation).  Annosus root rot will decrease productivity of and prematurely kill a host tree (Hadfield et al, 1986).

In the Pacific Northwest H. annosum infects many species of native conifers such as grand fir (Abies grandis), noble fir, white fir (A. concolor), Douglas-fir (P. menziesii), and western hemlock (Tsuga heterophylla). The susceptibility of these species to annosus root rot varies widely. It is known to cause aggressive damage in young conifer stands, stands weakened by other pathogens, or stands that have been partially cut and have fresh stumps next to living trees. It is directly linked to managed conifer stands (Smith, 1984) of commercial timber or christmas trees, however it can also be found in old growth forest near the west coast. Douglas fir is resistant, whereas grand fir, noble fir, white fir, and western hemlock are susceptible (Hadfield et al, 1986).  Although vegetative spread between hosts through root to root contact is commonly reported in mature, less intensively managed forest settings, specific genets are confined to single trees in more intensively managed, first rotation commercial plantings (Swedjemark and Stenlid, 1993). However, Spores commonly infect stumps after a thinning. In this way it can get into a completely new stand that doesn’t yet have it. It grows down the roots of the stump, crosses over to contacting roots of living trees, and attacks. Stumps are susceptible for several weeks after cutting (http://www.forestpathology.org/root.html#annosum)

Ecological Role

Heterobasidion annosum, like all wood decayers, influences species composition, ecosystem diversity, stand structure, stand density, and direction and rate of forest succession (Goheen and Otrosina,1998). In addition, as a saprotroph, the fungus is thought to contribute substantially to nutrient recycling by returning vital nutrients locked up within wood tissues back to the soil and providing habitat for a wide variety of animals (Filip and Morrison, 1998). Mortality caused by the root rot is important in creating gaps in the forest canopy (Filip and Morrison, 1998). Forest gaps change the light, moisture and temperature in the forest and thus change the habitat for a diverse range of plants and animals.

Identification

Presence of the fungal fruit bodies. Basidiocarps have whitish pallid margins, upper surface is tan to dark brown and sometimes reddish, usually irregular shaped with furrows, grooves, knobs, and warts, 3.5 (−7) cm thick and up to 40 cm in diameter; pores 5–19, 7–22 and 13–26 mm2 for the P, F and S (ISG) groups, respectively. Small brownish non-sporulating postules develop on the outside of infected roots. Asexual spores (conidiospores) are 3.8–6.6 × 2.8–5.0 µm in size. Spore print is white. Mating tests are necessary for identification of intersterility groups (Asiegbu et al, 2005).

Incipient Decay

At first yellowish to reddish brown stain, later becoming white pithy rot. Yellow-cream mycelial pustules develop on roots. Irregular white pockets of mycelium with black flecks present on wood and under bark. Fully degraded wood will be riddled with small holes parallel to the grain.