Variation in Pathogenicity and Virulence of Isolates of Armillaria ostoyae on Eight Tree Species. D. W. Omdal, Former Graduate Research Assistant, Department of Plant Pathology and Weed Science, Colorado State University, Fort Collins, Colo. 80523. C. G. Shaw, III, W. R. Jacobi, T. C. Wager. Plant Dis. 79:939-944. Accepted for publication 30 May 1995. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1995. DOI: 10.1094/PD-79-0939.

Thirteen isolates representing 10 genotypes of Armillaria ostoyae, obtained in northern New Mexico from Pinus ponderosa, Abies concolor, Pseudotsuga menziesii, Pinus strobiformis, Picea pungens, and Populus tremuloides were used to inoculate seedlings of these hosts and Larix occidentalis and Pinus contorta var. latifolia. At 18 months there were no significant differences (P> 0.05) in mortality among the eight hosts, or in virulence across all isolates except for one isolated from P. pungens, which failed to infect any trees. After three growing seasons (30 months), significantly more (P < 0.05) P. contorta var. latifolia were infected than either A. concolor or P. menziesii. Pinus ponderosa, the dominant species in this region, did not differ significantly from either of the exotic species (L. occidentalis and P. contorta var. latifolia) in susceptibility to infection or mortality. Populus tremuloides was significantly (P < 0.05) more tolerant than conifers, being frequently infected but rarely killed. A fungal isolate's ability to incite disease was highly correlated with its production of rhizomorphs (r = 0.94, P < 0.01). Across all fungal isolates and hosts save P. tremuloides, the order of the isolate's ability to incite disease matched its order in killing hosts.