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