In June 1999, a survey was conducted north and west of Redfeather Lakes, CO (≈64 km northwest of Fort Collins) to determine the extent of white pine blister rust, caused by Cronartium ribicola. To date the disease has not been reported in Colorado on any of the known hosts. The survey was initiated after the disease was reported on limber pine, Pinus flexilis, in 1998. A total of 65 sections were traveled by driving passable roads within three townships in Larimer County in northern Colorado. Infected limber pines were observed in nine sections. Incidence of infected trees ranged from 3 to 50% of trees sampled. A minimum of 10 trees was sampled at each location. Where trees were more abundant, 40 trees were sampled. The highest incidence of blister rust was observed near the Colorado and Wyoming state line along Cherokee Park Road. Both main stem and branch cankers were observed. Cankers appeared to be 3 to 5 years old. Mortality of entire trees was not observed. Ribes spp. were observed in the vicinity of infected limber pines. However, no infection was noted on these alternate rust hosts. Infected trees were observed 18 km south of the Colorado and Wyoming state line. The southward spread of the disease into northern Colorado from infection sites in Wyoming appears to have proceeded slowly since reports of the disease in southern Wyoming during the 1970s (1). Blister rust has the potential to spread throughout the range of white pines in Colorado, which includes scattered populations of both limber and bristlecone pines, P. aristata, located along the Continental Divide from Wyoming to the Colorado and New Mexico state line.
References: (1) D. B. Brown. Plant Dis. Rep. 62:905, 1978.
