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Predicting the Incidence of Comandra Blister Rust on Lodgepole Pine: Site, Stand, and Alternate-Host Influences. W. R. Jacobi, Associate professor, Department of Plant Pathology and Weed Science, Colorado State University, Fort Collins 80523; B. W. Geils(2), J. E. Taylor(3), and W. R. Zentz(4). (2)Research plant pathologist, United States Department of Agriculture, Forest Service, Rocky Mountain Forest and Range Experiment Station, Fort Collins, CO 80526; (3)(4)former graduate students, Department of Plant Pathology and Weed Science, Colorado State University, Fort Collins 80523; (3)Current address: United States Department of Agriculture, Forest Service, Northern Region, P. O. Box 7669, Missoula, MT 59087; (4)Current address: City of Fort Collins, Parks and Recreation Department, 413 S. Bryan, Fort Collins, CO 80521. Phytopathology 83:630-637. Accepted for publication 5 February 1993. 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, 1993. DOI: 10.1094/Phyto-83-630.

Incidence of cankers caused by comandra blister rust (Cronartium comandrae) on lodgepole pine (Pinus contorta) and distribution of the rust’s alternate host, pale comandra (Comandra umbellata subsp. pallida), were mapped in portions of two Wyoming forests. Rust incidence in 24 stands in the Shoshone National Forest varied from 14 to 64%, and rust incidence in 190 plots in the Medicine Bow National Forest ranged from 0 to 36%. Comandra populations occurred on open, upper slopes surrounded by lodgepole pine stands in the Shoshone study area and on dry ridge tops along the eastern and western slopes of the Medicine Bow study area. Simulations of wind speed and direction during periods favorable for basidiospore dispersal were used to identify comandra populations upwind of surveyed lodgepole pine stands. Rust incidence was highest in stands older than 40 yr along forest edges adjacent to comandra but also was high in some stands 1–10 km downwind of likely inoculum sources. Rust incidence was significantly negatively correlated with distance to comandra and with stand density and was significantly positively correlated with average tree diameter, height, and age. Expected incidence of comandra blister rust across surveyed portions of the two forests can be predicted from average tree height or diameter and distance to comandra.

Additional keywords: hazard rating, risk rating, spatial analysis, wind simulation.