Armillaria is a fungal genus that is associated with forest root disease and the degradation of organic matter. We surveyed more than 2,000 trees in a Pinus monticola (western white pine) plantation in northern Idaho to assess Armillaria species diversity and its interaction with foresttrees. We found Armillaria composed of two species, A. altimontana and A. solidipes, colonized 54.9 percent of the trees. These Armillaria species were widely distributed and were estimated to have been interacting for at least 250 years on the site. Notably, A. solidipes was uncommon in areas dominated by A. altimontana. A. solidipes was frequently (74.7 percent) associated with Armillaria root disease and trees colonized by A. solidipes exhibited the lowest growth and survival. In contrast, a large majority (98.2 percent) of A. altimontana was not associated with Armillaria root disease and trees colonized only by A. altimontana exhibited the greatest growth and survival even in comparison with trees where no Armillaria was found. These findings support the hypothesis that A. altimontana is typically nonpathogenic and may function as a beneficial symbiont, epiphyte, or both in western white pine. Furthermore, the results support the possibility of new approaches for natural control of Armillaria root disease through management that encourages A. altimontana.
A) mean height, (B) mean diameter at breast height, and (C) percent survival among western white pine (Pinus monticola) growing in association with Armillaria altimontana (A. alt.), A. solidipes (A. sol.), both A. altimontana and A. solidipes (Both), or neither Armillaria species (Neither) at 16 years post-planting at Priest River Experimental Forest, northern Idaho, USA. Height and DBH measures were not available for every live tree in the data set. Bars show standard error. Means sharing a lower case letter within each bar graph (i.e., A, B, and C) are not significantly different (p < 0.05) by Tukey-adjusted means separation (from Warwell et al. 2019).