Root Disease
Annosus Root Rot
Heterobasidion irregulare Garbelotto & Otrosina

Host: Ponderosa Pine

Heterobasidion occidentale Otrosina & Garbelotto

Hosts: White fir, subalpine fir, Engelmann spruce

Figure 270. Figure 270. Fruiting bodies of Annosus root diesease are often flat with the pores facing down.

Symptoms/signs:  Fruiting bodies of Annosus are grey-brown on the upper surface with an undersurface that is chalky white with pores. They are found on the underside of decayed roots and slash, inside stumps (common in the Southwest), or under the duff at the base of infected stumps and trees (rare in the Southwest). Annosus root disease leads to crown thinning and mortality or windthrow, the latter of which can occur before aboveground symptoms are evident.

Figure 271. Figure 271. Annosus fruiting bodies are commonly found on underside of roots and inside stumps.

Biology: Spores are produced on conks in decayed stumps or on roots of windthrown trees. The most common means of initial entry of Annosus root into a site is via airborne spores that germinate on freshly cut stumps and basal wounds. Mycelium of Annosus root quickly colonizes the stump and grows into its roots. Transmission to adjacent trees occurs via root contacts. In live trees, the fungus decays woody root systems and then advances to the root collar where it may surface to the cambium and kill by girdling, as in ponderosa pine, or progress more slowly through roots to the stem and cause butt decay, as in true fir.

There are two species of Heterobasion that cause root disease in North America, each with specific host preferences. In Arizona and New Mexico, H. occidentale infects white fir and sometimes spruce and subalpine fir, and H. irregulare infects ponderosa pine. Like Armillaria, the Heterobasidion fungi are common decays of dead woody material as well as pathogens.

Figure 272. Figure 272. Characteristic white pocket rot caused by Annosus root disease.

Effects: This is one of the most importatnt forest tree diseases in temperate zones worldwide. Damage to forests in North America has been most severe in the Southeast, where intensive forestry has exacerbated the disease due to providing stumps for colonization. In the Southwest, H. occidentale is a common root disease of white fir, but H. irregulare is fairly rare in ponderosa pine by comparison. For both species, infected trees are predisposed to attack by bark beetles.

Figure 273. Figure 273. Wood discoloration is an early sign of Annosus decay.

Similar Insects and Diseases:  The aboveground symptoms are similar to those caused by other root disease fungi.

References:  30, 54, 60, 74, 89, 93, 115