Black-Stain Root Disease

Black-stain root disease is unique among conifer diseases in being a vascular wilt. It is restricted to western North America, infects roots, and can move some distance up the base of the stem. Apparently it is more common now than when it was first discovered, likely because primary infections are often associated with stand disturbance such as thinning.

Host

Various western conifers in three distinct host groups are susceptible (see table). Other tree species are seldom infected [4].

PathogenPrimary HostsVectorsNotesGeographic Range
L. wageneri var. wageneriThe piñons, Pinus edulis (piñon) and P. monophylla (singleleaf piñon)Vectors not identified; several Hylastes spp. (root-feeding bark beetles) are candidatesThis is the type variety of the species; the species was described based on an isolate from P. monophylla.Southwestern USA, southern Idaho
L. wageneri var. pseudotsugaePseudotsuga menziesii (Douglas-fir)Hylastes nigrinus
In addition, two weevils are vectors: Steremnius carinatus, is involved in local spread and Pissodes fasciatus is less common
Vectors strongly attracted to forest disturbanceWestern USA and British Columbia
L. wageneri var. ponderosumThe hard pines Pinus ponderosa, P. jeffreyi, and P. contortaHylastes macerDistinguished based on culture morphology and host range.Northern Rocky Mountains and Pacific coast states

Pathogen

The pathogen is Leptographium wageneri, an ascomycete closely related to Ophiostoma stain fungi. Thus far no sexual fruiting has been found with certainty. Like blue-stain fungi, this pathogen has dark hyphae, but unlike them, it grows almost exclusively in tracheids [1, 5]. This clearly separates it from blue-stain fungi, which are uncommon in tracheids and grow almost exclusively in ray parenchyma and epithelial cells of resin canals.

The pathogen is almost certainly insect vectored for long-distance dispersal, but it grows root-to-root for secondary spread as many root diseases do.

Three types have been described as varieties or subspecies [2, 3]. They are physiologically and, to some degree, morphologically distinguishable. The most important point is they are host-specialized (see table).

Environment

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Disease Cycle

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Distribution and Damage
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Management

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Other Issues

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Black stain in tangential section. Note the long streaks, indicating that the fungus moves longitudinally much faster than it does tangentially.

Symptoms

Trees develop thin, chlorotic crowns and eventually die in expanding disease centers. If you cut into roots and the lower stem, you will be thrilled to find dramatic streaks of black stain.

An infection center of black stain root disease in Pinus edulis (piñon).
An infection center of black stain root disease in Pinus edulis (piñon) located in a picnic area.
The black stain in cross section. The arc-shaped pattern indicates that the fungus moves tangentially along an annual ring much more readily than it moves radially. This differs completely from blue stain.

Epidemiology

In Douglas-fir, vectors are known, root-feeding weevils. They are attracted to fresh wounds and stumps, so thinning initiates disease in some stands. In pines, damage is associated with slightly wetter conditions, flats and drainages.

Management

Current approaches to management of this disease are probably not fully effective or satisfactory.

  • Avoid undue wounding or disturbance
  • Avoid creating wet areas
  • Avoid or delay precommercial thinning
  • Conduct operations in summer to avoid peak times of vector activity
  • Manage for nonhosts of the pathogen variety on the site

Other management approaches are available.

References

1.
Cobb FW. 1988. Leptographium wageneri, cause of black-stain root disease: a review of its discovery, occurrence and biology with emphasis on pinyon and ponderosa pine. In: Leptographium Root Diseases on Conifers, Harrington TC, Cobb FW eds, pp. 41–62. St. Paul, Minnesota, USA: APS Press.
2.
Harrington TC, Cobb FW. 1986. Varieties of Verticicladiella wageneri. Mycologia 78(4):562. [Source]
3.
Harrington TC, Cobb FW. 1987. Leptographium wageneri var. pseudotsugae, var. nov., cause of black stain root disease on Douglas fir. Mycotaxon 30:501–507.
4.
Hessburg PF, Goheen DJ, Bega RV. 1995. Black Stain Root Disease of Conifers., Vol. 145 (revised) Washington, DC: USDA Forest Service. [PDF]
5.
Wagener WW, Mielke JL. 1961. A staining-fungus root disease of ponderosa, Jeffrey, and pinyon pines. Plant Dis Rep 45(11):831–835.