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Chapter 2

SPREAD OF TREE DISEASES AND BARK BEETLES

CHAPTER CONTENTS
Increased occurrence of tree diseases: Logging, disturbance, and roads promote the spread of damaging and lethal tree diseases.

Attack by bark beetles: Diseased trees are more susceptible to bark beetle attack.

Problems with mitigation: Forest management spreads root diseases despite the use of mitigation techniques, with methods like stump removal causing additional problems such as soil compaction.



Increased occurrence of tree diseases: Logging, disturbance, and roads promote the spread of damaging and lethal tree diseases.


Key Finding:Multiple studies have shown that annosus root disease, often fatal or damaging for a number of conifer species, has increased in western forests as a result of logging.

Source: Smith, R. S., Jr. 1989. History of Heterobasidion annosum in Western United States. pp. 10-16 in Proceedings of the Symposium on Research and Management of Annosus Root Disease (Heterobasidion annosum) in Western North America. W. J. Otrosina and R. F. Scharpf, tech. coord. GTR-PSW-116. USDA Forest Service. Pacific Southwest Forest and Range Experiment Station.

This study reviewed research on Heterobasidion annosum (formerly named Fomes annosus), a fungal root pathogen of pine, fir, and hemlock in western forests. With the 1950s logging boom, forest pathologists observed that the disease characteristically appeared in pine stands several years after logging and was associated with stumps as well as with logging wounds in remnant trees. The disease spread from stumps to adjacent living trees via root contacts. Infection of western hemlock through logging wounds was also reported. In the 1970s, it was noted that true fir understories, released after high-grade logging of pines, were also suffering extensive mortality from annosus root disease. The Forest Service instituted a policy requiring treatment of stump surfaces with borax in recreation areas. However, there is continuing concern that annosus can infect stumps via the roots rather than just through the stump surface, and that borax treatment may not be fully successful in preventing the disease.


Key Finding: The incidence of annosus root disease in true fir and ponderosa pine stands increased with the number of logging entries.

Source: Goheen, E. M. and D. J. Goheen. 1989. Losses caused by annosus root disease in Pacific Northwest forests. pp. 66-69 in Proceedings of the Symposium on Research and Management of Annosus Root Disease (Heterobasidion annosum) in Western North America. W. J. Otrosina and R. F. Scharpf, tech. coord. GTR-PSW-116. USDA Forest Service. Pacific Southwest Forest and Range Experiment Station.

The authors reviewed research on the occurrence of annosus root disease (Heterobasidion annosum) in the Pacific Northwest. Studies east of the Cascades indicated that it was a significant disease of true firs. The authors presented results of stand surveys in the Fremont and Ochoco National Forests in Oregon. Logged stands had a higher incidence of annosus root disease than unentered stands, with the highest incidence in stands with a history of multiple entries. Stumps 18 inches or larger served as infection foci for the stand. True firs of all size classes died due to the disease. The Fremont National Forest, where logging began earlier than in Ochoco National Forest, had a higher incidence of the disease.

Pine species, in particular ponderosa pine, were also infected by annosus root disease. The disease reportedly caused more mortality on dry sites. Mortality was centered around old ponderosa pine stumps, which were usually greater than 18 inches in diameter. Trees of all size classes died, although significant mortality was not obvious until 10 to 15 years after logging.


Key Finding: The proportion of western hemlock trees infected by annosus root disease increased after thinning, due to infection of stumps and logging equipment wounds.

Source: Edmonds, R. L., D. C. Shaw, T. Hsiang and C. H. Driver. 1989. Impact of precommercial thinning on development of Heterobasidion annosum in western hemlock. pp. 85-94 in Proceedings of the Symposium on Research and Management of Annosus Root Disease (Heterobasidion annosum) in Western North America. W. J. Otrosina and R. F. Scharpf, tech. coord. GTR-PSW-116. USDA Forest Service. Pacific Southwest Forest and Range Experiment Station.

The authors reviewed research on annosus root disease (Heterobasidion annosum) in western hemlock forests, focusing particularly on studies of precommercial thinning. The disease has been reported to cause root and butt-rot of western hemlock in coastal Oregon, Washington, British Columbia, and Alaska. Thinning provided fresh stump sources, which became infection courts for airborne spores of annosus. Summer and winter temperatures were rarely extreme enough to inhibit the fungus, and stumps of all sizes were infected. Wounds to live trees from logging equipment also contributed to higher rates of infection in thinned stands.

The authors noted that there was great variability in virulence among strains of Heterobasidion annosum. A strain at a study site near the Olympic Peninsula, Washington, was less virulent than strains collected from other sites in Washington. In the latter stands, thinning activities could result in more severe problems.

The efficacy of stump treatment, particularly with borax, was reviewed. The authors reported that borax may be ineffective because it washes off stumps and that high stump densities in precommercial thinnings make it difficult to apply. Evaluation 20 years after precommercial thinning revealed that plots with borax-treated stumps did not have significantly lower annosus infection than did untreated stands.


Key Finding: The percentage of western hemlock trees infected by annosus root disease greatly increased after thinning, with infected stumps being the primary source of infection.

Source: Chavez, T. D., R. L. Edmonds and C. H. Driver. 1980. Young-growth western hemlock stand infection by Heterobasidion annosum 11 years after precommercial thinning. Canadian Journal of Forest Research 10: 389-394.

The authors examined the extent to which infection rates of western hemlock trees by annosus root disease (Heterobasidion annosum) increased after precommercial thinning. The two study plots were on the Olympic Peninsula, Washington, in young, 26-year-old stands, composed primarily of western hemlock. The stands had been precommercially thinned 11 years earlier. Stumps on each plot were examined for the presence of the fungus. Twenty trees on each plot were also felled and examined for infection.

Of the stumps sampled, 92 and 56% in the two plots, respectively, were colonized by the fungus. Stumps were determined to be the primary source of standing tree infection, accounting for 61% of infections. Excavation of stumps and roots indicated that the fungal pathogen was transferred to standing trees via root grafts from infected stumps. Infection levels in the stand increased from an average of 8% of trees before thinning to an average of 90% of sampled trees 11 years after thinning (17 out of 20, and 19 out of 20 trees were infected in the two plots, respectively).


Key Finding: Annosus root disease was found on 89% of true fir stumps in stands that had been logged five to 10 years earlier.

Source: Filip, G. M., C. L. Schmitt and K. P. Hosman. 1992. Effects of harvesting season and stump size on incidence of annosus root disease of true fir. Western Journal of Applied Forestry 7: 54-56.

Three hundred grand fir stumps were investigated for the presence of annosus root disease (Heterobasidion annosum). The study was conducted in Umatilla National Forest, northeastern Oregon, in stands logged five to 10 years earlier. A high frequency of true fir stumps (89%) of all size classes (12 inches and greater) had annosus root disease. Stump size or the season in which logging had occurred did not affect the percentage of decay.

Mortality rates for regenerating trees surrounding infected stumps did not appear to be higher, but the authors note that more time may be needed for effects of this disease on mortality rates to become apparent.


Key Finding: Annosus root disease and Armillaria infected freshly cut stumps of young western hemlock and Sitka spruce in southeastern Alaska.

Source: Shaw III, C. G. 1981. Infection of western hemlock and Sitka spruce thinning stumps by Fomes annosus and Armillaria mellea in southeast Alaska. Plant Disease 65: 967-971.

The susceptibility of western hemlock and Sitka spruce cut stumps to two root diseases (Fomes annosus and Armillaria mellea) was investigated in seven naturally regenerated, 10- to 20-year-old stands. A total of 182 trees of each species were cut, and stumps (7-15 cm in diameter) were examined six to15 months later. Infection through deposition of natural, airborne spores was measured, as well as infection due to artificial inoculation by choosing half the stumps at random and inoculating them with Fomes annosus.

Twelve percent of the western hemlock stumps and 16% of Sitka spruce stumps became naturally infected with Armillaria mellea. Annosus root disease was found on 3% of the western hemlock stumps and 20% of Sitka spruce stumps. Of the stumps inoculated, annosus root disease infected 11% of western hemlock and 15% of Sitka spruce stumps.


Key Finding: Armillaria is a primary, aggressive root pathogen in western interior forests, where it spreads into healthy stands from the stumps and roots of cut trees.

Source: Wargo, P. M. and C. G. Shaw, III. 1985. Armillaria root rot: the puzzle is being solved. Plant Disease 69: 826-832.

This review article describes the occurrence of Armillaria root rot throughout the United States. In interior western forests, Armillaria is a primary, aggressive root pathogen of pines, true firs, and Douglas-fir. The fungus colonizes stumps and roots of cut trees, then spreads to adjacent healthy trees. Roots of large trees in particular can support the fungus for many years because they are moist and large enough for the fungus to survive. Disease centers can expand to several hectares in size, with greater than 25% of the trees affected in a stand.

In western coastal forests, Armillaria occurs as a root disease primarily in plantations and in stands less than 25 years old. In eastern deciduous forests, Armillaria is a secondary pathogen that primarily attacks trees weakened by stress factors, such as soil compaction, waterlogging, drought, or insect defoliation.

The authors indicate that it is not yet well understood why some stressed stands become infected, while others do not. They also point out that losses due to Armillaria alone are hard to estimate since in many cases other root pathogens or bark beetles may also be involved.


Key Finding: Armillaria root disease was present in stumps of old-growth ponderosa pine logged up to 35 years earlier.

Key Finding: The oldest stumps of ponderosa pine had the highest rate of infection by Armillaria.

Source: Roth, L. F., L. Rolph and S. Cooley. 1980. Identifying infected ponderosa pine stumps to reduce costs of controlling Armillaria root rot. Journal of Forestry 78: 145-151.

Root rot of ponderosa pine by Armillaria mellea was studied in southcentral Washington on 100 old-growth stumps, ranging in age since cutting (two to 35 years). Infectious stumps were identified by the condition of surrounding trees and by locating pockets of infected trees in the area. Lab cultures were used to confirm the diagnosis when uncertain.

Eighty-two percent of the ponderosa pine stumps that had dead saplings within the area occupied by roots of the stump were infected. The oldest stumps showed the highest incidence of infection. The authors recommend concentrating disease treatment on the oldest stumps in a stand.


Key Finding: Mortality of saplings was significantly correlated to the number of Douglas-fir stumps infected with Armillaria and laminated root rot.

Source: Filip, G. M. 1979. Root disease in Douglas-fir plantations is associated with infected stumps. Plant Disease Reporter 63: 580-583.

Thirty-nine of 43 10- to 27-year-old Douglas-fir plantations examined near Quilcene, Washington, had tree mortality caused by either Armillaria mellea or laminated root rot (Phellinus weirii). Mortality averaged 0.5 trees/ha, was clustered within plantations, and resulted in understocked openings of 0.04 to 0.1 ha. Mortality due to root diseases was significantly correlated with the number of infected stumps.


Key Finding: The pathogenic fungus Armillaria had a threefold higher occurrence on disturbed plots compared to pristine plots at high productivity sites in the Northern Rockies.

Source: McDonald, G. I., N. E. Martin and A. E. Harvey. 1987. Armillaria in the Northern Rockies: Pathogenicity and Host Susceptibility on Pristine and Disturbed Sites. USDA Forest Service. Research Note INT-371. 5 p.

Root systems of living and dead trees in disturbed and pristine sites were inspected for evidence of Armillaria. Pristine sites were at least 75 m from a road and had no evidence of logging activity. The authors also investigated the relationship between the relative productivity of a site (site index) and the incidence of Armillaria. In high productivity sites, a threefold greater incidence of pathogenic Armillaria was recorded in disturbed as opposed to undisturbed plots in forests of the grand fir, western red cedar, and western hemlock series. In low productivity sites, the effect of disturbance on Armillaria infection rates was not clear as results were not statistically significant. The authors indicate that incidence of pathogenic Armillaria was higher in low productivity sites compared to high productivity sites, but the data were significant only for undisturbed sites, not for disturbed sites.

The authors also review past studies on Armillaria, noting a clear link between management and the severity of Armillaria-caused disease. They also report that significant Armillaria-related mortality has occurred in conifer plantations.


Key Finding: Infection and mortality from the root disease Armillaria ostoyae was several times higher in forest stands with logging disturbance than in undisturbed stands.

Source: Morrison, D. and K. Mallett. 1996. Silvicultural management of armillaria root disease in western Canadian forests. Canadian Journal of Plant Pathology 18: 194-199.

The authors review the occurrence of the root disease Armillaria ostoyae in British Columbian forests and report unpublished results. Surveys showed that the severity of the disease was significantly higher in forest stands with logging disturbance than with no logging disturbance. Stumps created during logging, thinning, or brushing were colonized by Armillaria. Adjacent residual trees as well as new regeneration became infected when their roots came into contact with roots from infected stumps. Infection occurred within one to 20 days of logging, depending on the age of the trees and type of cutting. The authors report that the number of infected and killed trees was several times higher in disturbed than undisturbed stands and that losses to the disease can be significant.

The authors also review the efficacy and practicality of various strategies for protecting stumps from infection. Treatments were either too expensive or difficult to apply, or did not adequately reduce mortality. Stump removal can reduce mortality due to Armillaria, but it may not be feasible on sites where soils are sensitive or slopes are steep. In addition, not all infected roots can be removed.


Key Finding: Thinning and soil disturbance led to an increased risk of infection and mortality by black-stain root disease in Douglas-fir.

Key Finding: The majority of black-stain root disease infection centers were close to roads and skid trails.

Source: Hansen, E. M., D. J. Goheen, P. F. Hessburg, J. J. Witcosky and T. D. Schowalter. 1988. Biology and management of black-stain root disease in Douglas-fir. pp. 63-80 in Leptographium Root Diseases on Conifers. T.C. Harrington and F.W. Cobb, Jr. eds. APS Press. St. Paul, Minnesota.

The authors summarize 10 years of research on black-stain root disease (Leptographium wageneri) on Douglas-fir in California, Oregon, and Washington. Black-stain root disease was reported to be increasing in incidence and severity in Douglas-fir plantations. Originally reported just from locations west of the Cascades, it was also more recently documented east of the Cascades.

Three insect species were determined to be vectors that spread the black-stain root fungus and created new infection centers. They were attracted to stumps of precommercially thinned trees, where they introduced the fungus into stump roots. The insects also wounded roots of healthy trees, another possible route for fungus introduction.

Several studies documented significantly greater numbers of these insects in thinned versus unthinned stands. Changing the timing of thinning can decrease, although not eliminate, insect activity. Most infection centers were reported to be close to roads and on sites such as skid trails, which had compacted soils and displaced topsoil.

Black-stain disease infection centers were variable in size, some remaining small, others spreading rapidly and causing heavy losses. The authors recommend avoiding management activities that involve tree injury, including road building and maintenance, and conducting thinning treatments during the months of June and early August.


Key Finding:Black-stain root disease occurred at a greater frequency in Douglas-fir trees close to roads than in trees located 25 m or more from roads.

Source: Hansen, E. M. 1978. Incidence of Verticicladiella wagenerii and Phellinus weirii in Douglas-fir adjacent to and away from roads in western Oregon. Plant Disease Reporter 62: 179-181.

A survey was conducted comparing the incidence of root disease in Douglas-fir forests in paired 0.1 ha study strips adjacent to, and 25 m or more distant from, two roads in western Oregon. Black-stain root disease (Verticicladiella wagenerii) was found more frequently in roadside strips than in strips at a distance from roads. Distribution of laminated root rot (Phellinus weirii) was not influenced by proximity to roads. Black-stain was found in 15- to 25-year-old plantations but not in mature stands.


Key Finding: Thinned stands attracted a greater number of black-stain root disease insect vectors.

Source: Witcosky, J. J., T. D. Schowalter and E. M. Hansen. 1986. Hylastes nigrinus (Coleoptera: Scolytidae), Pissodes fasciatus, and Steremnius carinatus (Coleoptera: Curculionidae) as vectors of black-stain root disease of Douglas-fir. Environmental Entomology 15: 1090-1095.

The authors compared precommercially thinned plots to unthinned plots (a total of 32 plots) in two 12-year-old plantations of Douglas-fir in the coast range of Oregon. Pitfall traps and sticky traps were used to capture the three insect vectors of black-stain root disease: Hylastes nigrinus, Pissodes fasciatus, and Steremnius carinatus. Significantly more beetles of each species were captured in thinned plots than in unthinned plots. Also, significantly more trees had beetle wounds in thinned than unthinned plots. The authors also verified transmission of the disease by these insects by collecting insects in established infection foci, killing insects, incubating the fungal isolate, and later using it as an artificial inoculant.

The authors artificially inoculated root systems of both cut and uncut Douglas-fir to determine their susceptibility to black-stain root disease. Root systems of cut trees were determined to be susceptible to infection for at least seven months.


Key Finding: Mechanical wounding of grand fir and white fir by logging equipment activated dormant decay fungi, such as the Indian paint fungus.

Source: Aho, P. E., G. M. Filip and F. F. Lombard. 1987. Decay fungi and wounding in advance grand and white fir regeneration. Forest Science 33: 347-355.

Grand and white fir were studied in 24 stands on eight national forests east of the Cascades in Washington and Oregon. A total of 464 trees, ranging in age from 43 to 115 years, were dissected to detect infections by decay fungi and their proximity to wounds. Most wounds were caused by equipment (as opposed to fire, animals, or insects), and more than 45% of the trees with wounds had various decay fungi species present. Infections causing decay were located within 30 cm of an external wound. The authors believe that wounds activate dormant decay fungi, including the Indian paint fungus (Echinodontium tinctorium), if the fungus is present within 30 cm of a wound. These fungi originally enter the host through small branchlet stubs but become dormant after the stubs close.


Key Finding: Port-orford-cedar root rot, a fatal fungus, is spread by logging equipment, road maintenance equipment, and construction equipment, which transport its spores to new areas.

Source: Zobel, D. B., L. F. Roth and G. M. Hawk. 1985. Pathology and control of Port-orford-cedar root rot. Chapter 7 in Ecology, pathology, and management of Port-orford-cedar (Chamaecyparis lawsoniana). GTR-PNW-184. USDA Forest Service. Pacific Northwest Research Station. 161 pp.

This report on the ecology of Port-orford-cedar, also reviews the biology and spread of its primary pest, the fatal fungus known as Port-orford-cedar root rot (Phytophthora lateralis). This disease has spread through much of Port-orford-cedar's range since its first appearance, killing trees of all sizes. The disease is believed to have originally been introduced to the region from infested soil of nursery stock. In 1952, the root rot was reported in natural stands. Epidemic conditions are closely correlated to high human activity at a site, with the disease occurring at the lowest intensity in less accessible sites. Machines such as logging equipment or road maintenance equipment are reported to transport the fungal spores in wet mud. Subsequently, the disease can spread to other areas through surface water. Cattle and elk are also important carriers during wet weather. Hauled earth, gravel, and soil-bearing debris are carriers during dry weather.



Attack by bark beetles: Diseased trees are more susceptible to bark beetle attack.


Key Finding: Root disease fungi predispose some conifer species to bark beetle attack and/or help maintain endemic populations of bark beetles.

Source: Goheen, D. J. and E. M. Hansen. 1993. Effects of pathogens and bark beetles on forests. pp. 175-196 in Beetle-Pathogen Interactions in Conifer Forests. T.D. Schowalter and G.M. Filip, eds. Academic Press. San Diego.

The authors review the association between pathogenic fungi and bark beetles in coniferous forests. Five interactions were looked at in detail.

1) Laminated root rot (Phellinus weirii) is a widespread disease of Douglas-fir west of the Cascades. Fungi can survive for up to 50 years in stumps and snags. Live trees infected with this fungus, and therefore stressed, have a greater likelihood of attack by Douglas-fir beetles (Dendroctonus pseudotsugae). By increasing the chance of windthrow of diseased and weakened trees, the likelihood of beetle outbreaks in a stand with many fallen trees also increases. By providing stressed host trees, the disease also helps maintain small populations of bark beetles until the next major outbreak.

2) Black-stain root disease (Leptographium wageneri var. pseudotsugae) is another disease of Douglas-fir. Black-stain root disease is closely associated with disturbance and has been documented to be greatest along roads, in tractor trails and landings where soils have been compacted, and areas where drainage patterns have been changed. Mortality due to black-stain is a particularly severe problem in 10- to 30-year-old plantations and young natural stands on disturbed sites in northwestern California and southwestern Oregon. Although the disease is primarily documented in younger trees, the disease has also been observed in 60- to 80-year-old trees in northwestern California. Disease-weakened trees are attacked and killed by a variety of bark beetle species, including the Douglas-fir bark beetle (D. pseudotsugae) and the Douglas-fir engraver (Scolytus unispinosis).

3) The root disease Leptographium wageneri var. ponderosum predisposes ponderosa pine to several bark beetle species, including the mountain pine beetle (D. ponderosae) and the western pine beetle (D. brevicomis). This disease and beetle association results in rapidly expanding mortality centers, particularly in ponderosa stands 60 to 100 years old, although mortality can occur in stands of any age. The disease is most common in northeastern California and the central Sierra Nevada, in pure, heavily stocked stands. Many of these stands are in areas that have had heavy logging disturbance.

4) A variety of root diseases, including black-stain, Armillaria, and brown cubical butt rot (Phaeolus schweinitzii), predispose lodgepole pine to attack by mountain pine beetles in the interior west. The diseases are also believed to provide stressed host trees that help maintain endemic populations of mountain pine beetle or trigger population increases at the start of an outbreak.

5) Grand and white fir trees in interior mixed-conifer forests have been found to have a high likelihood of attack by the fir engraver (Scolytus ventralis) when they are infected by root diseases, such as laminated root rot, Armillaria, and annosus. Diseased trees also maintain small endemic populations of the fir engraver.


Key Finding: More mountain pine beetles and western pine beetles (two species of bark beetle) were captured on ponderosa pine infected with black-stain root disease than on healthy trees.

Key Finding: Two species of beetle were more frequently attracted to wounds on trees that were also diseased than to uninfected trees.

Source: Goheen, D. J., F. W. Cobb Jr., D. L. Wood and D. L. Rowney. 1985. Visitation frequencies of some insect species on Ceratocystis wageneri infected and apparently healthy ponderosa pines. Canadian Entomologist 117: 1535-1543.

Thirty overstory ponderosa pine trees were selected for study and categorized as healthy, moderately diseased, or severely diseased with black-stain root disease (Ceratocystis wageneri). The study was conducted in El Dorado County, California. Traps were mounted on each tree to monitor insects landing on the tree. Consistently more western pine beetles (Dendroctonus breviformis) and mountain pine beetles (D. ponderosae) were captured on diseased trees than on uninfected trees.

All trees were mechanically wounded as part of the sampling methodology. The red turpentine beetle (Dendroctonus valens) attacked trees at wounds, with attack rates seven-to-eight times higher on trees infected with black-stain root disease than uninfected trees. Spondylis upiformis attacked only wounded trees, not unwounded trees.


Key Finding: Loblolly pines colonized by annosus root disease had a greater probability of being infested with southern pine bark beetle.

Key Finding: Trees infected by annosus root disease had significantly less radial growth than trees not infected.

Source: Alexander, S. A., J. M. Skelly and R. S. Webb. 1981. Effects of Heterobasidion annosum on radial growth in southern pine beetle-infested loblolly pine. Phytopathology 71: 479-481.

The study was conducted in thinned loblolly pine plantations in Virginia, Texas, and Georgia. Trees colonized by annosus root disease had significantly less radial growth (an indicator of stress) over the last 10 years than trees not colonized. Those trees with reduced growth and greater disease incidence also had a significantly higher probability of being infested with southern pine bark beetle (Dendroctonus frontalis). The authors conclude that annosus-stressed trees can play a large role in the proliferation of southern pine bark beetle in plantations.


Key Finding: A significantly higher percentage of plots attacked by the southern pine beetle were infected by blue-stain fungi.

Source: Otrosina, W. J., N. J. Hess, S. J. Zarnoch, T. J. Perry and J. P. Jones. 1997. Blue-stain fungi associated with roots of southern pine trees attacked by the southern pine beetle, Dendroctonus frontalis. Plant Disease 81: 942-945.

Forty paired plots were established in southern pine stands (loblolly pine, slash pine, mixed loblolly and slash, and longleaf pine) from Alabama to Texas. Plots with evidence of attack by the southern pine beetle (Dendroctonus frontalis) were compared to non-attacked control plots. Roots from a total of 240 trees were examined for the presence of several different blue-stain fungal species (Ophiostoma ips, Leptographium terebrantis, and L. procerum).

A significantly higher percentage of beetle-attacked plots (50%) had blue-stain fungi than unattacked plots (25%). When the three fungal species were analyzed separately, L. terebrantis had a significantly higher presence in beetle-infested plots than uninfested plots, while the other two insect species also had higher numbers, though these were not statistically significant. Plots in plantations and natural stands were also analyzed separately. Plantations had a significantly higher proportion of the fungi in attacked plots, while results for natural stands, although also higher, were not statistically significant.



Problems with mitigation: Forest management spreads root diseases despite the use of mitigation techniques, with methods like stump removal causing additional problems such as soil compaction.


Key Finding: Borax-treated plots did not have lower rates of annosus root disease infection compared to untreated plots 20 years after thinning.

Source: Edmonds, R. L., D. C. Shaw, T. Hsiang and C. H. Driver. 1989. Impact of precommercial thinning on development of Heterobasidion annosum in western hemlock. pp. 85-94 in Proceedings of the Symposium on Research and Management of Annosus Root Disease (Heterobasidion annosum) in Western North America. W. J. Otrosina and R. F. Scharpf, tech. coord. GTR-PSW-116. USDA Forest Service. Pacific Southwest Forest and Range Experiment Station.*

The authors reviewed research on annosus root disease (Heterobasidion annosum) in western hemlock forests, focusing particularly on studies of precommercial thinning. The disease has been reported to cause root and butt-rot of western hemlock in coastal Oregon, Washington, British Columbia, and Alaska. Thinning provided fresh stump sources, which became infection courts for airborne spores of annosus. Summer and winter temperatures were rarely extreme enough to inhibit the fungus, and stumps of all sizes were infected. Wounds to live trees from logging equipment also contributed to higher rates of infection in thinned stands.

The authors noted that there was great variability in virulence among strains of Heterobasidion annosum. A strain at a study site near the Olympic Peninsula, Washington, was less virulent than strains collected from other sites in Washington. In the latter stands, thinning activities could result in more severe problems.

The efficacy of stump treatment, particularly with borax, was reviewed. The authors reported that borax may be ineffective because it washes off stumps and that high stump densities in precommercial thinnings make it difficult to apply. Evaluation 20 years after precommercial thinning revealed that plots with borax-treated stumps did not have significantly lower annosus infection than did untreated stands.

* See also key finding above.


Key Finding: Mitigation measures for Armillaria root disease were problematic in several regards.

Source: Morrison, D. and K. Mallett. 1996. Silvicultural management of armillaria root disease in western Canadian forests. Canadian Journal of Plant Pathology 18: 194-199.*

The authors review the occurrence of the root disease Armillaria ostoyae in British Columbian forests and report unpublished results. Surveys showed that the severity of the disease was significantly higher in forest stands with logging disturbance than with no logging disturbance. Stumps created during logging, thinning, or brushing were colonized by Armillaria. Adjacent residual trees as well as new regeneration became infected when their roots came into contact with roots from infected stumps. Infection occurred within 1 to 20 days of logging, depending on the age of the trees and type of cutting. The authors report that the number of infected and killed trees was several times higher in disturbed than in undisturbed stands, and that losses to the disease can be significant.

The authors also review the efficacy and practicality of various strategies for protecting stumps from infection. Treatments were either too expensive or difficult to apply, or did not adequately reduce mortality. Stump removal can reduce mortality due to Armillaria, but it may not be feasible on sites where soils are sensitive or slopes are steep. In addition, not all infected roots can be removed.

* See also key finding above.


Key Finding: Stump removal, a method of Armillaria root disease control, resulted in high levels of soil compaction in ash-cap soils.

Source: Page-Dumroese, D. S., A. E. Harvey, M. F. Jurgensen and M. P. Amaranthus. 1998. Impacts of soil compaction and tree stump removal on soil properties and outplanted seedlings in northern Idaho, USA. Canadian Journal of Soil Science. 78: 29-34.

The authors examined the impact of stump removal, a method that is sometimes used to control Armillaria root disease in the western United States. Control (no compaction), stump-extraction treatments, and severe compaction treatments were investigated on a site in northern Idaho with an ash-cap soil. Soil bulk density increased by 15-20% to a depth of 30 cm in the compaction treatment. Stump removal increased bulk density at the 20-30 cm depth, though surface soil bulk density decreased. Soil strength increased at the 40-45 cm depth after stump removal and was similar to values in the compacted treatment. This increase in soil strength was believed to be due to equipment vibration because volcanic ash soils are apparently particularly susceptible to vibrational compaction. The authors reported that increased soil strength has been shown to impede root growth.

A year after planting, Douglas-fir seedlings had reduced root volume in the soil compaction treatment. Soil compaction and stump removal also resulted in decreased ectomycorrhizal development and non-ectomycorrhizal short roots on Douglas-fir seedlings. After stump removal, there was a 70% decline in numbers of ectomycorrhizal root tips and a 63% reduction in morphological types on Douglas-fir compared to the no-compaction treatments. After three years, the height of Douglas-fir seedlings in the stump-removal plot was 20% lower and root collar diameter was 30% lower than for seedlings in the other treatments.

Western white pine seedling root volumes were not affected. They had smaller root collar diameters in the stump-removal and compaction treatments, but greater height in the compaction treatment. Western white pines' numbers of non-ectomycorrhizal root tips decreased significantly, although ectomycorrhizal diversity was not affected. Seedlings' ability to capture site resources was considered to be negatively affected.


Key Finding:Restricting thinning to summer months, a recommended practice for mitigating the spread of annosus root disease in southern forests, was not a reliable form of disease control.

Source: Witcher, W. and C. L. Lane. 1980. Annosus root rot in slash pine plantations in the sandhill section of South Carolina. Plant Disease 64: 398-399.

This study addresses whether or not thinning during the summer months can be a successful method of controlling annosus root disease on freshly cut tree stumps. Annosus has been reported to be less viable during hot dry summer months in the southern U.S. The authors investigated slash pine stump infection and mortality based on cutting in each month of the year. Differences among months were not significant, and the authors indicate that disease prevention based on summer thinning may have inconsistent results from year to year because of annual variation in temperature and humidity.


Key Finding: Annosus root disease may spread via root systems from stumps to neighboring trees even following treatment of stumps with borax.

Source: Smith, R. S., Jr. 1989. History of Heterobasidion annosum in Western United States. pp. 10-16 in Proceedings of the Symposium on Research and Management of Annosus Root Disease in Western North America. W. J. Otrosina and R. F. Scharpf, tech. coord. GTR-PSW-116. USDA Forest Service. Pacific Southwest Forest and Range Experiment Station.*

This study reviews research on Heterobasidion annosum (formerly named Fomes annosus), a fungal root pathogen of pine, fir, and hemlock in western forests. With the 1950s logging boom, forest pathologists observed that the disease characteristically appeared in pine stands several years after logging and was associated with stumps as well as with logging wounds in remnant trees. The disease spread from stumps to adjacent living trees via root contacts. Infection of western hemlock through logging wounds was also reported. In the 1970s, it was noted that true fir understories, released after high-grade logging of pines, were also suffering extensive mortality from annosus root disease. The Forest Service instituted a policy requiring treatment of stump surfaces with borax in recreation areas. However, there is continuing concern that annosus can infect stumps via the roots rather than just through the stump surface, and that borax treatment may not be fully successful in preventing the disease.

* See also key finding above.

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