Armillaria Root Disease

Environmental conditions were conducive to copious fruiting body development this fall in Southcentral Alaska. Many observations of black rhizomorphs or mushrooms were recorded in rotted cottonwood, birch, and spruce tree snags after cutting, from Denali and the Alaska Range, south to the Kenai Peninsula, and near Cordova. Hemlock tree failures on Forest Service trails by Girdwood had signs of both Armillaria white rot and the red ring rot pathogen Porodaedalea pini sensu lato. Armillaria is commonly observed in stressed and recently killed trees in Alaska.

2024 Ground Detection Survey Observations: 23 observations, with broad distribution from Denali south to the Kenai Peninsula, east to Cordova, and then along coastal Southeast Alaska. Host trees affected were equally as varied including birch, cottonwood, spruce, yellow- cedar, and hemlock.  

2024 iNaturalist Observations: 1 observation of Armillaria sinapina near Skagway. 

Armillaria species are common in Alaska, though generally considered weak pathogens that kill stressed host trees or infect recently killed trees. On Zarembo Island, Armillaria has been particularly active on 40- to 50-year-old dead and dying yellow-cedar trees affected by root freezing injury of yellow-cedar decline in forests managed for timber. Infection appears to hasten mortality and contribute to more rapid snag failure of yellow-cedar, a tree known for long-term snag retention. Genetic sequencing allowed us to identify one such sample as A. nabsnona. While the mushrooms can be difficult to identify, rhizomorphs and mycelial fans (vegetative structures) are reliable keys for the genus. Distinguishing among species of Armillaria is generally not possible without specialized experience and equipment.

In general, Armillaria species in Alaska are thought to be unaggressive wood decomposers or parasites of stressed trees, but more information is needed to understand its ecological role and impacts in Alaskan forests. In the 1950s, James Kimmey conducted a study in Southeast Alaska to measure the volume and incidence of stem decay and determined that Armillaria was an important heart rot of conifers, especially western hemlock. Kimmey used the appearance of decay to identify the causal fungus (for more information on this study, see our Conifer Stem Decays page). Identification of Armillaria to species requires advanced molecular tools. Since the 1990s, these tools have revealed significantly more Armillaria species diversity in North America and Europe than was previously known. Different Armillaria species vary greatly in their aggressiveness. Work is underway in Alaska and throughout the western USA in partnership with forest pathologists at the Rocky Mountain Research Station to assess Armillaria diversity.

Armillaria spp. have been mapped on paper birch and white spruce in several locations in Interior and Southcentral Alaska and on nearly all the native tree species in Southeast Alaska (see Detection Map). Drs. John Hanna and Ned Klopfenstein (Rocky Mountain Research Station) led a west-wide project on determining the identity and distribution of Armillaria species and found A. sinapina and A. nabsnona in Southeast Alaska. More recently, they used climate envelope modeling to evaluate whether future climate scenarios favor our native Armillaria species, as well as assessing climate suitability for more aggressive Armillaria species presently found in the Pacific Northwest but not Alaska. Their findings suggest that relative climate suitability (i.e., land area with climate associated with a higher probability of disease presence) becomes greater for native species and non-native A. solidipes (the primary pathogen in the PNW) and A. cepistipes under future climate scenarios. They posit that based on these predictions, both species present potential invasive threats to Southeast Alaska under conditions of predicted climate change. These results were based on 32 A. sinapina isolates collected by our group from 12 locations in Southeast Alaska from 2016 to 2018. A single isolate of A. nabsnona was identified from a dying yellow-cedar crop tree on Prince of Wales Island, but this was insufficient for bioclimatic modeling.

Collections from hardwood and conifer hosts from the Kenai Peninsula to the Arctic Circle in 2007 were all identified as A. sinapina.Collections from hardwood and conifer hosts on the Kenai Peninsula to the Arctic Circle were all identified as A. sinapina. It is still unknown where those host and species ranges overlap; therefore, more Armillaria samples from new and unique locations in Alaska will continue to be collected in future years. This year, rhizomorph samples on Sitka spruce in Cordova were cultured and submitted to the Rocky Mountain Research Station for molecular analyses.  

Armillaria is most easily identified by signs of the fungus between the bark and the wood. These structures, which are visible throughout the year, include black shoestring-like rhizomorphs and white fan-shaped mycelium. In Alaska, these structures are frequently found on snags or trees dying from other primary causes, hastening wood decomposition. In conifers, there may be an abnormal resin flow from the root collar of infected trees. Harwoods may have longitudinal cracks that appear at the root collar and extend up the stem, a condition known as collar crack.

The mushrooms of Armillaria, sometimes called honey mushrooms, occur in clusters at the base of infected trees and stumps in late summer or autumn. Honey-colored mushrooms may extend up the bole of infected snags. Sometimes mushrooms have dark scales on the top. The white gills are attached to both the stalk and the cap, and a ring (annulus) may occur around the stem below the gills. There can be significant variation in mushroom appearance.

Armillaria root disease causes growth loss, butt and root rot, and mortality. Armillaria can quickly colonize root systems of recently dead or dying trees. In most environments in Alaska, the fungus rarely kills trees that are not under other stress. For example, Armillaria occurs on the roots and root collars of about one half of trees dying from yellow-cedar decline. It is not the primary cause of tree mortality, but probably speeds the process of tree death. Wood being decayed by Armillaria sometimes glows in the dark, a property called bioluminescense. Massive colonies of the fungus in North America (Michigan and eastern Oregon) have led Armillaria to be dubbed the Humongous Fungus (Oregon Public Broadcasting article and video from 2018 featuring USFS Forest Pathologists, available here), with colonies covering several square miles and estimated to be 2,000-8,000 years old.Georeferenced observations of Armillaria species in Alaska (as of 20120), with the modeled distribution of its hosts. Host distributions were developed by the Forest Health Technology Enterprise Team in 2011, 240m-resolution, based on dominant tree species by tree diameter.

Armillaria Root Disease in Conifers of Western North America. Forest Insect and Disease Leaflet 188. Revised July 2024. USDA-Forest Service. Available here. 

Hanna, J. W.; Mulvey, R. L.; Kim, M. -S.; Stewart, J. E.; Bright, B. C.; Warwell, M. V.; Klopfenstein, N. B. 2020. Bioclimatic modeling of Armillaria species in southeastern Alaska, including potentially invasive Armillaria species under changing climate. In: Reynolds, Gregory J.; Wilhelmi, Nicholas P.; Palacios, Patsy, compilers. Proceeding of the 66th Western International Forest Disease Work Conference; 3-7 June 2019; Estes Park, CO. WIFDWC: www.wifdwc.org. p. 171-176. Available here.

Hanna, J.; Ashiglar, S.; Case, A.; Fairweather, M. L.; Hoffman, C.; Kim, M.-S.; Maffei, H.; Mathiasen, R.; McDonald, G.; Nelson, E.; Ross-Davis, A.; Shaw, J.; Klopfenstein, N. 2012. Armillaria root disease in the western USA. Journal of Forestry. 110(8): 489. Available here.

Kim, M.-S.; Klopfenstein, N. B.; Hanna, J. W.; McDonald, G. I. 2006. Characterization of North American Armillaria species: Genetic relationships determined by ribosomal DNA sequences and AFLP markers. Forest Pathology. 36: 145-164. Available here.

Kim, M.-S.; Klopfenstein, N.B.; McDonald, G.I. 2010. Effects of forest management practices and environment on occurrence of Armillaria species. Journal of the Korean Forest Society. 99(2): 251-257. Available here.

Kim, M.-S.; Hanna, J.W.; Stewart, J.E.; Warwell, M.V.; McDonald, G.I.; Klopfenstein, N.B. 2021. Predicting Present and Future Suitable Climate Spaces (Potential Distributions) for an Armillaria Root Disease Pathogen (Armillaria solidipes) and Its Host, Douglas-fir (Pseudotsuga menziesii), Under Changing Climates. Front. For. Glob. Change 4:740994. doi: 10.3389/ffgc.2021.740994. Available here.

Lockman, I.B.; Kearns, Holly S.J., eds. 2016. Forest root diseases across the United States. Gen. Tech. Rep. RMRS-GTR-342. Ogden, UT: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station. 55 p. Available here.

Ross-Davis, A.L., Hanna, J.W.; Klopfenstein, N.B. 2011. DNA-based identification and phylogeny of North American Armillaria species. In: Fairweather, Mary Lou; Palacios, Patsy, comps. Proceedings of the 58th Annual Western International Forest Disease Work Conference; 2010 October 4-8; Valemount, BC. Flagstaff, AZ: U.S. Department of Agriculture, Forest Service, AZ Zone Forest Health. p. 129-142. Available here.

Williams, R.E., Shaw, C.G. III, Wargo, P.M, Sites, W.H. 1986. Forest Insect and Disease Leaflet 78. Armillaria Root Disease. Available here.