Black-Stain Root Disease

Black-stain root disease is a vascular wilt restricted to western North America that infects roots and can move some distance up the base of the stem. Although it is now an important disease, it was first reported in 1961 [3], and wasn’t reported on Pseudotsuga menziesii until 1967 [1].

Host

Various western conifers in three distinct host groups are susceptible. Other tree species are seldom infected [2].

  1. Pseudotsuga menziesii (Douglas-fir).
  2. The hard pines Pinus ponderosa and P. jeffreyi, and P. contorta (lodgepole pine).
  3. The piñons P. edulis (piñon) and P. monophylla (singleleaf piñon).

Pathogen

The pathogen is Leptographium wageneri, an ascomycete closely related to Ophiostoma stain fungi. Thus far no sexual fruting has been found with certainty. Like blue-stain fungi, this pathogen has dark hyphae, but unlike them, it grows predominantly in tracheids rather than ray parenchyma and epithelial cells of resin canals as blue-stain fungi do.

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

Three types have been described as varieties or subspecies. They are physiologically and, to a slight degree, morphologically distinguishable. The most important point is they are host-specialized:

PathogenHostsComments
L. wageneri var. wageneriPinus edulis
P. monophylla
This is the type variety of the species; the species was described based on an isolate from piñon.
L. wageneri var. ponderosumP. ponderosa
P. jeffreyi
P. contorta
Distinguished based on morphology and host range.
L. wageneri var. pseudotsugaePseudotsuga menziesii

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

    References

    1.
    Cobb FW, Platt WD. 1967. Pathogenicity of Verticicladiella wageneri to Douglas-fir. Phytopathology 57(9):998–999.
    2.
    Hessburg PF, Goheen DJ, Bega RV. 1995. Black Stain Root Disease of Conifers. Forest Insect & Disease Leaflet 145 (revised)., Vol. 145 Washington, DC: USDA Forest Service. [PDF]
    3.
    Wagener WW, Mielke JL. 1961. A staining-fungus root disease of ponderosa, Jeffrey, and pinyon pines. Plant Dis Rep 45(11):831–835.