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.
Various western conifers in three distinct host groups are susceptible (see table). Other tree species are seldom infected .
|Pathogen||Primary Hosts||Vectors||Notes||Geographic Range|
|L. wageneri var. wageneri||The piñons, Pinus edulis (piñon) and P. monophylla (singleleaf piñon)||Vectors not identified; several Hylastes spp. (root-feeding bark beetles) are candidates||This 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. pseudotsugae||Pseudotsuga 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 disturbance||Western USA and British Columbia|
|L. wageneri var. ponderosum||The hard pines Pinus ponderosa, P. jeffreyi, and P. contorta||Hylastes macer||Distinguished based on culture morphology and host range.||Northern Rocky Mountains and Pacific coast states|
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).
Distribution and Damage
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.
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.
- 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.