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Variation in Disease Severity in the Lodgepole Pine-Western Gall Rust Pathosystem. B. J. van der Kamp, Departments of Forest Sciences, University of British Columbia, Vancouver, B.C. Canada, V6T 1W5; D. E. N. Tait, Departments of Harvesting and Wood Science, University of British Columbia, Vancouver, B.C. Canada, V6T 1W5. Phytopathology 80:1269-1277. Accepted for publication 18 May 1990. Copyright 1990 The American Phytopathological Society. DOI: 10.1094/Phyto-80-1269.

The number of western gall rust (Endocronartium harknessii) infections per tree was measured in a lodgepole pine (Pinus contorta) population consisting of 3,215 trees in 20 blocks, established from a single, large seed lot collected and planted near Prince George, B.C. Canada. Average disease incidence in these blocks varied from 0.58 to 6.07 infections per tree. A method is presented that allows the prediction of tree frequencies by disease severity classes for each block from a single distribution of relative susceptibility. Resistance genes appear to interact multiplicatively. The most susceptible 10% of the population was estimated to have an infection rate about 800? as high as the least susceptible 10%. The implications of this wide range in susceptibility for stability of the pathosystem are discussed.

Additional keywords: additive resistance, horizontal resistance, incidence severity relationship, multiplicative resistance, natural pathosystem, nonspecific resistance, Peridermium harknessii, pine stem rust, stability.