Epidemiology of white pine blister rust on limber pine in Colorado and Wyoming

Cronartium ribicola is a non-native, heteroecious, fungal pathogen that causes white pine blister rust (WPBR) on all white pines, including Pinus flexilis James (limber pine). The pathogen continues to move southwards and eastwards in the Rocky Mountains of Colorado and Wyoming on populations of limber pine. We examined the progression and intensification of WPBR on limber pine in recently invaded areas (<40 years) by combining historical accounts of disease occurrence with recent large-scale monitoring projects to describe the epidemic in this region. Using epidemiological information, we provide spread rate, disease incidence, disease rate increase, severity, infection density, disease intensity, frequency of infection events and mortality for this pathosystem across 27 study areas in Wyoming and Colorado. Over the past four decades, WPBR spread through Wyoming and Colorado at 7.7-10.6 km/year. Epidemiological parameters varied both within and between mountain ranges. Sites were classified into four epidemic types defined by length of time the pathogen has been present and disease incidence. Disease rate increase (increase in per cent disease incidence per year) ranged from 0% year(-1) to 3.2% year(-1) and varied depending on temporal scale. Average disease severity was 4.1 cankers per infected tree, and disease intensity (disease incidence x severity) averaged 112. Infection episodes occurred fairly consistently at 1- to 3-year intervals at most sites, and by 2012, WPBR caused mortality of >2.5% of all limber pine trees (>1.37 m tall) surveyed. Future monitoring and restoration activities may use epidemiological information to prioritize sites for management actions in conjunction with other published models and monitoring data.