Using Prescribed Fire to Control Annosus Root and Butt Rot

Annosus root rot is caused by the fungus Heterobasidion annosum (Fr.) Bref. (= Fomes annosus (Fr.) Karst) (Figure). It can directly kill infected pine trees, reduce tree growth, or increase the likelihood of damage from wind throw and bark beetles (Alexander 1981). Loblolly and slash are very susceptible. The disease is most common on sandy sites in closely spaced pine plantations where it spreads via root grafts. After an area has been thinned, basidiospores produced by conks can colonize stumps, and infection then spreads to adjacent living trees by root contact. Basidiospore production occurs year around and some infections may take place by spores leaching into the soil and contacting roots (Webb et al.1981).

Prescribed burning may reduce levels of this fungus due to the alteration of the microenvironment of the forest floor and removal of spore-bearing conks (Wade 1989, Froelich et al. 1978). Prescribed burning to reduce levels of H. annosum should be done in the winter with either a backfire or strip headfire (Crow and Shilling 1980). The burning should be conducted prior to thinning so that inoculum is reduced before fresh stumps are exposed.

The benefits of prescribed burning for disease control have not been consistently demonstrated. In the paper by Froelich et al (1978), the authors found greatest reductions in H. annosum in stands that were artificially inoculated and less effect in stands where H. annosum was already present. Other studies have indicated that prescribed fire may have no effect on H. annosum. Leptographium terebrantis, L. procerum,and H. annosum, all root pathogenic fungi, showed no reductions in areas where prescribed fire was applied during the winter (Otrosina et al. 2002). In a study of a 40-year old longleaf pine site that was thinned and burned, mortality actually increased from 16 trees/ha on unburned plots to 46 trees/ha on burned plots (Otrosina et al. 1995).

Prescribed burning may not be effective in controlling H. annosum for several reasons. If the disease is already present in the roots, burning would not be able to reduce inoculum. In addition, most of the fungus found in the soil by Otrosina et al. (2002) was present in the organic matter. A fire hot enough to affect this soil layer might also damage the fine roots, resulting in additional tree stress. Therefore, the disease is best controlled by increasing tree spacing, thinning stands in the summer when basidiospore production is low, and applying borax to thinned stumps (Stambaugh 1989).