Root systems of western white pine, Pinus montieola Douglas, were excavated with explosives and examined for pathogens. Data were also recorded on portions of the crown killed by blister rust caused by Cronartium ribieola Fisch. Statistical tests revealed a significant association between the bark beetles Dendroetonus ponderosae Hopkins and Pityogenes fossifrons (LeConte), and the root pathogen Armillariella mellea. (Vah!. ex Fr.) Karst and between beetles and all root diseases. Ninety-two percent of the trees attacked by bark beetles had root diseases, and 97% had either root diseases or blister rust. A discriminant analysis correctly classified 88% of the sample trees into two categories, trees infested with D. ponderosae or trees not infested with D. ponderosae, using the variables age (stump), diameter at 1.3 m in height, and percentage of the primary roots infected with pathogens. Major pathogenic organisms isolated from the roots included A. mellea, Phaeolus sehweinitzii (Fr.) Pat., Resinicium bieolor (Fr.) Parm., Vertieicladiella spp., and a Europhium strain. A hypothetical sequence of host tree invasion by blister rust, followed by infection by root diseases, and finally attacks by bark beetles, is postulated.
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