Armillaria altimontana Is Associated with Healthy Western White Pine (Pinus monticola): Potential in Situ Biological Control of the Armillaria Root Disease Pathogen, A. solidipes

Research Highlights: Two genets of Armillaria altimontana Brazee, B. Ortiz, Banik, and D.L. Lindner and five genets of Armillaria solidipes Peck (as A. ostoyae [Romagnesi] Herink) were identified and spatially mapped within a 16-year-old western white pine (Pinus monticola Doug.) plantation, which demonstrated distinct spatial distribution and interspecific associations. Background and Objectives: A. solidipes and A. altimontana frequently co-occur within inland western regions of the contiguous USA. While A. solidipes is well-known as a virulent primary pathogen that causes root disease on diverse conifers, little has been documented on the impact of A. altimontana or its interaction with A. solidipes on growth, survival, and the Armillaria root disease of conifers. Materials and Methods: In 1971, a provenance planting of P. monticola spanning 0.8 ha was established at the Priest River Experimental Forest in northern Idaho, USA. In 1987, 2076 living or recently dead trees were measured and surveyed for Armillaria spp. to describe the demography and to assess the potential influences of Armillaria spp. on growth, survival, and the Armillaria root disease among the study trees. Results: Among the study trees, 54.9% were associated with Armillaria spp. The genets of A. altimontana and A. solidipes comprised 82.7% and 17.3% of the sampled isolates (n = 1221) from the study plot, respectively. The mapped distributions showed a wide, often noncontiguous, spatial span of individual Armillaria genets. Furthermore, A. solidipes was found to be uncommon in areas dominated by A. altimontana. The trees colonized by A. solidipes were associated with a lower tree growth/survival and a substantially higher incidence of root disease than trees colonized only by A. altimontana or trees with no colonization by Armillaria spp. Conclusions: The results demonstrate that A. altimontana was not harmful to P. monticola within the northern Idaho planting. In addition, the on-site, species-distribution patterns suggest that A. altimontana acts as a long-term, in situ biological control of A. solidipes. The interactions between these two Armillaria species appear critical to understanding the Armillaria root disease in this region.