Black Stain Root Disease – Leptographium wageneri (Kendrick)
Black Stain Root Disease (BSRD) is caused by a fungus that infects and kills several species of western conifers. The fungus does not decay infected wood, but instead kills the host tree by blocking water conduction to the foliage. The principal hosts are ponderosa pine, Jeffrey pine, and pinyon pine. This disease has been found in relatively few locations throughout the Western United States in Idaho, Montana, Utah, Nevada and California.
Identification and Damage
Trees infected by BSRD may have foliage symptoms typical of other root diseases. Trees usually exhibit symptoms of gradual decline before they die. In the early stages of decline, height growth is reduced, and older needles become yellow. As the disease progresses older needles are shed permanently and new needles are somewhat stunted and yellow. In advanced stages new growth is sparse, yellowed and has a tufted appearance (see figure 1.). Young trees can die in 1 to 2 years following infection; older trees can take up to 8 to 10 years to die. The disease is also indicated by dark brown or black stains in roots or the outer wood which follow the annual rings. In cross section the stain forms concentric circles in the outer wood of the tree. (see figure 2.). It can be confused with blue stains which are common, but are wedge shaped and affect the inner wood of a tree.
Spread of the black stain fungus involves insects that transport the fungus. The root-feeding bark beetle Hylastis macer and Hylastis nigrinus are believed to be the primary transporters in ponderosa pine. These insects breed in roots of recently dead or dying trees. The fungus produces spores inside the galleries created by the bark beetles in infected roots and root collars. Microscopic fruiting bodies form on the gallery walls, each bearing a sticky spore. These sticky spores are well suited to insect dispersal. Adult beetles are contaminated by the fungal spores as they come in contact with them while emerging from the tree. Contaminated beetles fly or crawl from brood trees, or burrow through the duff or soil and visit roots of healthy, recently dead or dying trees and deposit spores on roots or sapwood.
Once established the fungus colonizes the outer wood of roots and stems, reducing the transportation of water and sap by clogging the water conducting vessels. Infected trees begin to dieback and usually die in the summer, when moisture is depleted because of the inability to take up water.
Black stain infections are usually found where substantial tree damage or site disturbance has occurred, especially along roads and skid trails, and where home construction has taken place in the forest landscape. Once an infection is established, it can spread to other trees through root to root contact (root grafts). The disease is relatively non-persistent in infected roots and does not remain active more than one year after the host dies.
Control and Management
There is no effective cure recognized for diseased trees, and genetically resistant hosts have not been identified. Current management strategies for BSRD are either preventative or corrective.
Preventative treatment should include minimizing site disturbance and tree injury. When working in forested areas with equipment site disturbance should be minimized, particularly avoiding soil compaction and disturbance of the soil profile. It is recommended that equipment use should occur in mid to late summer during the dry season where the risk of serious soil compaction is reduced. Special care should be taken in newly developed home sites and recreational areas to avoid this as much as possible.
Minimizing tree injury is also very important during road building and maintenance, recreation site development, and home construction in the forested landscape. Avoid tree damage to the root collar or exposing tree root systems due to these types of activities. Avoid creating flooded or poorly drained areas. Beetles that transport BSRD often visit trees with roots in flooded or poorly drained soils.
Corrective management is very difficult where BSRD is already established. Maintaining a mix of conifer species provides options for future management. If pines are the dominant species in the stand consider maintaining or planting a mix of two to four different species in in known infection centers. It is recommended that tree planting with mixed species, where possible, be implemented to prevent the spread of BSRD. Patch cutting is also an option with the creation of 50 foot buffers near known infection sites. Patch cutting in infected recreation sites or pinyon pine stands is also recommended, but does not appear to be effective unless buffers much wider than 50 feet are used.
USDA Forest Insect & Disease Leaflet 145
Hagle S.K., Gibson K. E., Tunnock S., Ffield Guide to Diseases and Insects USDA Forest Service, 2003
Forest Health Specialist
885 Eastlake Blvd.
Carson City, Nevada 89704
Phone: 775-849-2500 ext 241