Black-Stain Root Disease

Black-stain root disease is a vascular wilt that infects roots and can move some distance up the base of the stem.

Description

Various western conifers are susceptible. 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.

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

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 diseases 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:

  • Ponderosa pine and relatives — L. wageneri var. ponderosum
  • Piñon — L. wageneri var. wageneri
  • Douglas-fir — L. wageneri var. pseudotsugae

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