Distribution, Incidence, and Severity of White Pine Blister Rust on the Medicine Bow National Forest

Kearns, Holly; Burns, Kelly

Limber pine (Pinus flexilis) is well distributed within the central Rocky Mountains and is especially important and revered because of its unique ecological and cultural characteristics. White pine blister rust (WPBR), caused by Cronartium ribicola, is an exotic, invasive disease of limber pine and other white pines that was accidentally introduced into North America in the early 1900s. Ecological impacts of WPBR may be severe. Our native white pines did not coevolve with the pathogen and thus have little or no residence, and there are few natural enemies of the pathogen. The purpose of this study was to determine the geographic distribution, incidence, and severity of WPBR in central and southeastern Wyoming and nothern Colorado. A comprehensive ground survey was conducted from 2002 to 2004 in which 504 limber pine survey plots were established in 13 study areas. The incidence of WPBR is currently low along the southern boudary of its distribution, but the disease was identified in several previously uninfested areas. The disease continues to intensify, causing severe impacts and mortality, in areas where it has been present for several decades. WPBR was present on 55% of all plots. Incidence, the proportion of trees infected, ranged from 0-100% and averaged 15.5% over all plots and 28% on infested plots. Specific results for the Medicine Bow National Forest are presented in this report. Monitoring of permanent plots and large-scale resurveys will be necessary to determine the long-term impacts.