Black stain root disease creates ponderosa pine snags of all sizes by causing tree mortality. It also commonly creates dead patches of small Douglas-fir trees in young forests west of the Cascade Mountains. Trees killed by G. wageneri eventually contribute to levels of down wood when they break or fall over. Black stain root disease creates canopy gaps, facilitating a more diverse stand structure and at times a more diverse plant species composition, as less-susceptible or non-host trees, shrubs, and forbs are released or become established in the openings. Bark beetles frequently are attracted to trees infected with G. wageneri, providing good foraging habitat for woodpeckers.
Distribution in Oregon and Washington: Found throughout Washington and Oregon; most prevalent in southwestern Oregon and along the southern edge of the Blue Mountains in northeastern Oregon. (See also Important Habitats and Spread Dynamics).
Douglas-fir is the most common host species west of the Cascades crest. East of the Cascades crest, ponderosa pine is most commonly affected. Occasionally found on lodgepole pine, western white pine, sugar pine, knobcone pine, western hemlock, and mountain hemlock.
Trees with black stain root disease usually have sparse, chlorotic crowns and reduced terminal growth. Some may also have distress cone crops and basal resinosis. The best diagnostic feature is found by chopping into the sapwood of lower stems and roots near the soil line. Infected trees will exhibit a dark-brown to purple-black stain extending down into the roots that often is not visible in the outermost ring of the sapwood but will be apparent in the older sapwood. When viewed in cross-section such as on a stump surface, the stain usually remains within the confines of one or two growth rings, following their curvature. Other stains that may be present and potentially confused with black stain usually extend radially across the growth rings toward the center of the tree when viewed in cross section. Decay is usually absent in the roots of recently dead trees unless other root disease agents are also present.
Black stain root disease usually occurs in discreet disease centers. Trees die standing and probably do not fall any more quickly than bark-beetle-killed trees. Pines killed by G. wageneri usually stand longer than trees killed by bark beetles alone because decay is slowed by a build-up of resin at the base of the stem. Disease centers typically contain a mixture of dead and symptomatic trees.
Bark beetles (particularly Douglas-fir pole beetle and Douglas-fir engraver on young Douglas-firs, and pine engraver, mountain pine beetle, western pine beetle, and red turpentine beetle on ponderosa pines) and woodborers frequently colonize trees infected with black stain root disease.
Black stain root disease is a vascular wilt disease that blocks the water conducting vessels of host trees. There are 2 races of G. wageneri; one infects primarily Douglas-fir and the other infects primarily ponderosa pine. Unlike other common root diseases in the Pacific Northwest, black stain root disease does not cause decay.
G.wageneri is carried from tree to tree by several species of root feeding bark beetles and weevils. It fruits in galleries constructed by the beetles, producing sticky spores that adhere to emerging beetles as they disperse to feed upon and colonize new host trees. After beetle feeding introduces spores into a new host tree, the spores germinate and G. wageneri grows down the host tree roots to infect new hosts nearby. G. wageneri does not persist much longer than one year in infected wood once the host tree dies.
Black stain root disease is most commonly found in young Douglas-fir stands less than 30 years old west of the Cascades crest in Oregon and Washington, and on ponderosa pine trees of all ages east of the Cascades crest in Oregon. It is especially prevalent in southwestern Oregon and on the southern edge of the Blue Mountains in northeastern Oregon. Injured trees growing on sites disturbed by heavy machines, roadside brushing, or precommercial thinning activities; or trees stressed by sub-optimal growing conditions induced by soil compaction, displacement, saturation, or drought provide important habitat for the initiation of G. wageneri infection centers due to their attractiveness to beetles that vector the fungus. Spread from initial infections is favored by a continuous forest cover of primary host species (Douglas-fir - Westside; ponderosa pine - Eastside).
The fungus spreads in several ways. It initially enters uninfected areas via the body surfaces of root-feeding beetles that are attracted to injured or stressed trees, where they feed and reproduce. Once introduced into a tree by the insects, the fungus grows down roots and can infect healthy new hosts across root contacts. It also can infect new hosts by growing short distances (up to 15 cm, or 6 in) through the soil. Tree-to-tree spread via fungal growth across root contacts or through the soil is independent of host vigor. Insect vectored spread, however, is associated with low vigor host trees.
Black stain root disease centers expand more rapidly than those of other root diseases, advancing up to 1.6 m per year. Infection centers tend to be small, usually less than 0.04 ha, but centers up to 25 ha also may occur, especially in ponderosa pine stands. In Douglas-fir stands, groups of small, closely associated centers commonly occur, and are especially likely to be concentrated along roads or skid trails, or on compacted soils.
Little opportunity exists for manipulating G. wageneri to increase wildlife habitat.
Black stain root disease may cause substantial mortality in some areas, reducing cover and stocking below desired levels. Screening, shading, aesthetic values, and public safety may be negatively affected when trees on developed sites are killed by black stain root disease.
Identify stands with known black stain root disease centers and high risk areas, which are stands within a 1.6 km radius of known black stain root disease centers. In these areas, non-host or resistant tree species should be favored during regeneration and thinning activities. If these stands are primarily comprised of highly susceptible host species, avoid thinning altogether, or restrict thinning to the period from June 1 to September 1, in order to avoid creating attractive habitat during the dispersal period of the beetles that vector black stain root disease. In addition, minimize soil disturbance and tree injury to residual trees during thinning, harvest, and road maintenance or construction activities by controlling the size and type of equipment, restricting operations to dry periods of the year, limiting skid trails and yarding corridors, and minimizing activities that cause soil displacement and compaction, such as tractor piling, or that injure large numbers of trees, such as rotary blade brush cutting.
Goheen, E.M. and E.A. Willhite. 2006. Field guide to common diseases and insect pests of Oregon and Washington conifers. USDA Forest Service, Pacific Northwest Region, Portland, OR. R6-NR-FID-PR-01-06. 335 pp. http://www.biodiversitylibrary.org/bibliography/80321#/summary
Hadfield, J.S., D.J. Goheen, G.M. Filip, C.L. Schmitt, and R.D. Harvey. 1986. Root diseases in Oregon and Washington conifers. USDA Forest Service, Pacific Northwest Region, Portland, OR. R6-FPM-250-86. 27 pp.
Hessburg, P.R., D.J. Goheen, and R.V. Bega. 1995. Black stain root disease of conifers. Forest Insect and Disease Leaflet 145 (revised). USDA Forest Service, Washington D.C. 9 pp.