Diseases of... Conifer Root

ANNOSUM

Pines Pinus spp.
Redcedars Juniperus spp.

Infected trees occur primarily in previously thinned pine plantations. They are characterized by a generally unhealthy appearance, thin, offcolor foliage, and mortality.
Infected trees are often leaning or completely uprooted while still alive due to loss of structural support roots, a result of the decay action of the pathogen. Infected trees tend to occur in groups or clusters referred to as infection centers which, without careful scrutiny, are sometimes mistakenly identified as bark beetle spot kills. Conks (sporophores) of the pathogen can often be detected on infected roots and stumps, or at the bases of infected trees in the duff or needle litter. In general, only a small percentage of infected trees actually support conks at any given time. In fact, conks are not always produced on infected trees. Therefore, the absences of conks does not necessarily mean the absence of infection. Sprophores are typically leathery in texture, with grayish brown to dark brown upper surfaces and, when actively growing, have creamy white, minutely porous undersurfaces.

Conks vary from smooth to deeply wrinkled and may occur as small pustules (buttons), or as brackets up to several centimeters across. Internally, infected roots display varying patterns of discoloration or staining, resin soaking, and decay, depending upon host and stage of disease development. Large clumps of soil often adhere to infected roots due to the frequent bleeding or resin exudation associated with infection. Advanced decay is characterized by small elongated white pockets, sometimes with attendant black spots or flecks, that eventually coalesce and reduce roots to a shredded or stringy condition. In the absence of conks or typically decayed roots, laboratory analysis is recommended for positive identification.

Annosum root rot is spread over long distances by way of airborne spores produced in the conks of the pathogen. Conks and spores are produced at various times during the year, but production is most abundant from fall to spring. Historical evidence suggests that spore production during the summer months is drastically reduced. Most infections are initiated as a result of spores being deposited on the surfaces of freshly cut stumps of susceptible host species, thus the intimate association between this disease and thinned plantations of pines.

Under suitable conditions of temperature and moisture newly deposited spores germinate and the fungus grows downward into the stumps and their attached roots. This infection process is severely limited by hot, dry weather which results in the rapid death of the very delicate spores. Once established in the stumps and roots the fungus continues its development and grows into the roots of adjacent standing trees which are near, in contact with, or grafted to the roots of the decaying stumps. The pathogen advances via this process at a rate of about 1-2 meters per year, often giving rise to pockets of infected trees which increase in diameter over time. A limited number of infections arise from direct root penetration by germinating spores which have washed into the soil through the action of rainfall.

Infections are usually present and active for 2-3 years prior to the onset of typical, aboveground symptoms. Sites characterized by deep, well drained sand or sandy loam soils with little clay and organic matter are considered high hazard for annosum root rot infections. Infections in stands on these types of sites are usually more frequent and more serious than on sites with shallow, poorly drained soils with high seasonal water tables and abundant clay and organic matter.

Annosum root rot infections can be prevented by simply applying granular borax to the surfaces of freshly cut stumps during thinning operations in commercial forest plantations or in tree removal operations in urban environments. Borax is toxic to the germinating spores of the pathogen and is very effective in preventing stump colonization. This practice is especially important when cuttings are performed during the fall, winter, and spring as these are the seasons when stump infections are most likely to occur.

Other control practices which are recommended for commercial forest stands on high hazards sites include: (a) thinning stands during low risk summer months (caution advised here due to potential hazards posed by bark beetles), (b) encouraging silvicultural prescribed burns to minimize the potential for buildup of fungus inoculum, (c) salvaging timber in severely diseased stands, (d) using less susceptible species (e.g., longleaf pine) when regenerating sites infested with the pathogen, or delaying regeneration of such sites for 1-2 years to reduce potential regeneration infections, (e) minimizing the number of thinnings during a stand rotation on high hazard sites, and (f) applying Phlebia gigantea (formerly, Peniophora gigantea), a commercially available, competitive fungus that reduces pathogen activity to stumps when thinning or harvesting stands already infected with Heterobasidion annosum.

Conk of Heterobasidion annosum at base of infected pine (needle litter and soil removed to expose conk). Note white undersurface of conk at tip of knife blade.