Initial Characterization of a New Strain of Cronartium ribicola from the Cascade Mountains of Oregon. G. I. McDonald, Principal Plant Pathologist, Forestry Sciences Laboratory, Intermountain Forest and Range Experiment Station, Forest Service, U.S. Department of Agriculture, Moscow, ID 83843. E. M. Hansen, Associate Professor, Department of Botany and Plant Pathology, Oregon State University, Corvallis 97331; C. A. Osterhaus, District Geneticist, Eugene District, Bureau of Land Management, U.S. Department of the Interior, Eugene, OR 97440; and S. Samman, Program Geneticist, Dorena Tree Improvement Center, Pacific Northwest Region, Forest Service, U.S. Department of Agriculture, Cottage Grove, OR 97424. Plant Dis. 68:800-804. Accepted for publication 18 March 1984. 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, 1984. DOI: 10.1094/PD-68-800.

Inoculation of 20 families of Pinus monticola with two inoculum types of Cronartium ribicola demonstrated significant variation in virulence. Compared with wild-type inoculum from Oregon, a new strain (Champion Mine) increased the incubation period of the rust, increased rust intensity, reduced the proportion of seedlings shedding all infected needles within 1 yr of inoculation, reduced the proportion of seedlings showing retarded canker appearance, increased the proportion of seedlings with stunted leaders, reduced the period from inoculation to mortality, and reduced the proportion of trees healthy 2 yr after inoculation.

Keyword(s): resistance mechanisms.