Black Stain Root Disease - USDA Forest Service - U.S. Department ...

Black Stain Root Disease
Mortality centers in pinyon
Pathogen—Black stain root disease is caused by the fungus Leptographium wageneri var. wageneri. This variety
infects only pinyons, including two-needle as well as singleleaf pinyon in other Regions.
Hosts—The disease causes expanding patches of
mortality in many pinyon stands of the western
slope of the southern and middle Rocky Mountains
as far north as Idaho (figs. 1-2). It has not been
found east of the Continental Divide. Two other
varieties do not occur in this Region: L. wageneri
var. ponderosum infects lodgepole, Jeffrey, and
ponderosa pine; and L. wageneri var. pseudotsugae
infects Douglas-fir. Both those varieties occur
primarily in the northern Rocky Mountains, British
Columbia, and Pacific coast states.
Signs and Symptoms—In advanced disease,
foliage is sparse and sometimes chlorotic (yellow).
A mortality center is often evident, with old snags
near the center, recent mortality farther out, and
symptomatic, live trees at the edge (fig. 1).
Figure 1. Dead and dying pinyon near the edge of a mortality center caused
by black stain root disease. The smaller trees in the foreground may not be
affected yet because their smaller root systems are not yet contacting infected
roots. Photo: Jim Worrall, USDA Forest Service.
An intense black stain can be found in
wood of the roots, root collar, and often
the lower stem. Unlike the pattern
of blue stain, which progresses radially
along the rays to the inner sapwood,
black stain progresses longitudinally
and somewhat tangentially. Longitudinally,
it forms long streaks following
the wood grain (fig. 3). In cross section,
it appears as arcs following short
segments of annual rings (fig. 4).
Disease Cycle—Unlike blue-stain
fungi, which colonize rays, this pathogen
colonizes the tracheids (waterconducting
and structural elements) of
the sapwood and causes a wilt disease.
It aggressively invades living wood of
the roots, root collar, and lower stem
until trees are killed. It does not decay
wood.
Figure 2. Distribution of black stain root disease in Colorado (from Landis and Helburg 1976)
Where roots grow closely together, the fungus can grow from tree to tree. In this way, mortality centers expand.
Rate of radial expansion of disease centers in pinyon is about 3.3-6.6 ft (1-2 m) per year, but they do not expand
indefinitely. It is not clear why expansion eventually ceases.
Forest Health Protection Rocky Mountain Region • 2011

Black Stain Root Disease - page 2
Other varieties of the pathogen are transmitted long distances by
insect vectors, primarily root weevils and bark beetles. The vectors
are attracted to dying roots, so the disease is associated with
stand disturbance, especially thinning and road construction. As
the insects burrow into the soil looking for those roots, they tend
to graze on intermingled roots of other trees, thereby inoculating
the pathogen. A vector has not been identified in pinyon, but it is
strongly suspected that there is one and that the relationship to
stand disturbance is similar to that of the disease caused by the
other varieties.
Fruiting of the fungus is almost microscopic and difficult to observe
in the field. Minute black stalks are formed in cavities under
the bark, such as insect galleries. Sticky masses of spores are
produced on the stalks. Insects that contact the fruiting structures
are readily coated with spores.
Impact—In Colorado, disease frequency is clearly associated with
soil depth, precipitation, site quality, and tree size and density.
Stands with large or dense trees may facilitate root-to-root spread
of the pathogen because of more frequent root contacts. However,
rate of disease expansion was not related to density in one study.
Because the pathogen is restricted to pinyon, mixed stands inhibit
tree-to-tree spread of the disease.
In the Rocky Mountains, the disease is most active in southwestern
Colorado, especially in the Four Corners area, but it occurs all
along the Western Slope (fig. 2). Over the long term, black stain is
an important disturbance agent in regulating structure and composition
of pinyon-juniper stands. It creates structural heterogeneity
in the forest, with openings that provide habitat diversity for other
plants and animals. In this sense, the disease promotes old-growth
characteristics of pinyon-juniper stands.
The pathogen dies quickly after host tissue dies and after a stand
replacing fire. Disease inci dence then increases with time since
regeneration. Just as fire makes the disease less likely, the disease
makes fire more likely. One of the major impacts of the disease
is an increase in dead fuels. Thus, a fire-disease feedback loop
can be envisioned that contributes to disease reduction and forest
regeneration. The pinyon engraver commonly attacks trees with
black stain. Under endemic beetle populations, black-stain centers
are probably a major source of beetles.
Figure 3. Black stain in the lower stem of diseased pinyon.
Stem with the wood exposed in tangential view. Photo: Jim
Worrall, USDA Forest Service.
Figure 4. Black stain in the lower stem of diseased pinyon.
Stem with the wood exposed in transverse view. Photo: Jim
Worrall, USDA Forest Service.
Management—No control approach has been effective in pinyon. Cutting and burning killed or symptomatic trees
does nothing to stop the disease, but because pinyon engraver often invades diseased trees before death, sanitation
may be effective in preventing beetle outbreaks. Any replanting should be with species other than pinyon. Trenching
around a disease center to sever all media through which the pathogen may grow has been attempted but
was unsuccessful, probably because the pathogen is often in at least two trees beyond the last symptomatic one.
Forest Health Protection Rocky Mountain Region • 2011

Black Stain Root Disease - page 3
Another approach is removing all pinyon from in and around disease centers, waiting four years, and replanting
pinyon. This could be effective because the pathogen does not survive long in roots once they are dead. If implemented,
at least three healthy-looking trees beyond the center should be removed.
Any discretionary pinyon cutting would best be done in the fall or winter. Vectors of the other forms of black stain
root disease, which are attracted to fresh cuts, are active in spring and early summer; presumably, the same is true
of the pinyon form.
1. Hessburg, P.F.; Goheen, D.J.; Bega, R.V. 1995. Black stain root disease of conifers. Forest Insect and Disease Leaflet 145
(revised). Washington, DC: U.S. Department of Agriculture, Forest Service. 9 p. Online: http://www.fs.fed.us/r6/nr/fid/fidls/
fidl-145.pdf.
2. James, R.L.; Lister, C.K. 1978. Insect and disease conditions of pinyon pine and Utah juniper in Mesa Verde National Park,
Colorado. Biological Evaluation R2-78-4. Lakewood, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain
Region, Forest Insect and Disease Management.
3. Kearns, H.S.J.; Jacobi, W.R. 2005. Impacts of black stain root disease in recently formed mortality centers in the piñonjuniper
woodlands of southwestern Colorado. Canadian Journal of Forest Research 35:461-471.
4. Landis, T.D.; Helburg, L.B. 1976. Black stain root disease of pinyon pine in Colorado. Plant Disease Reporter 60:713-717.
5. Witcosky, J.J.; Schowalter, T.D.; Hansen, E.M. 1986. Hylastes nigrinus (Coleoptera: Scolytidae), Pissodes fasciatus, and
Steremnius carinatus (Coleoptera: Curculionidae) as vectors of black stain root disease of Douglas-fir. Environmental
Entomology (15):1090-1095.
Forest Health Protection Rocky Mountain Region • 2011