Risk of white pine blister rust to limber pine in Colorado and Wyoming, USA

Abstract

Cronartium ribicola, the introduced pathogen that causes white pine blister rust (WPBR), continues to spread to additional limber pine populations in the Southern Rocky Mountains of Colorado and Wyoming. Because WPBR can severely impact ecosystems, forecasts of its potential distribution and incidence would be useful to land managers. Site and climate data from long infested study areas in Wyoming were fit with two regression models [logistic and classification and regression trees (CART)] to determine the environmental conditions associated with the distribution of WPBR. These models were then used to map limber pine stands at risk of infestation by C. ribicola throughout Wyoming (where it has long occurred) and Colorado (where it is just becoming established). Although variables representing vegetation and landform could identify infested plots, 1-km-scale climate variables for monthly temperature and moisture were better predictors of current WPBR distribution and were available for mapping expected future distribution across the region. Of 280 485 ha where limber pine was projected to occur in Colorado, 41% was forecast by the logistic model to be at risk of infestation, and 53%, by the CART model. Of an estimated 782 229 ha in Wyoming with limber pine, the logistic model projected 61% to be at risk; CART projected 79%. Additional regression models were fit with site and climate data to predict WPBR incidence (per cent of trees infected) and intensification (incidence/age of the oldest canker). Nearly one half of the plot-to-plot variation in incidence was explained using environmental variables readily available to land managers. Although mean plot incidence increased over time, mean intensification decreased 50% per decade. This work provides managers with several tools to reduce uncertainty over the expected distribution and incidence of WPBR, but surveillance and monitoring remain prudent activities for supplementing forecasts of WPBR epidemics.

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

“…Relative humidity is generally low in arid regions of the central and southern Rocky Mountains, but, based on disease incidence, humidity levels will frequently reach 98%–100% on leaf surfaces in forested areas, allowing microscale conditions that promote spore production, germination and infection (Curtis & Grimes, ; Nelson, ). Climate and weather patterns are driving forces in disease modelling efforts for a pathogen so dependent on specific weather conditions (Cleaver et al., ; Kearns et al., ; Mielke, ; Van Arsdel, ; Van Arsdel et al., ).…”

“…Spore dispersal is essential for a pathogen that completes its life cycle on more than one host, and modelling disease spread of heteroecious pathogens at landscape scales is further complicated by major landscape features such as extreme topography and mountain ranges (Holdenrieder, Pautasso, Weisberg, & Lonsdale, ; Plantegenest, Le May, & Fabre, ). White pine blister rust incidence on white pines is often correlated with the presence and density of close‐proximity alternate hosts, but long‐distance movement of spores may also occur (Buchanan & Kimmey, ; Frank, Geils, Kalkstein, & Thistle, ; Kearns et al., ; Van Arsdel, ). Most long‐distance spread of WPBR likely occurs through movement of thick‐walled aeciospores, which are produced on pines and fairly resistant to desiccation and UV radiation (Frank, Geils et al., ; Kasanen, ; Zambino, ).…”

“…in lodgepole pine ( Pinus contorta Douglas ex Loudon) (Jacobi et al., ; Smith‐McKenna, Resler, Tomback, Zhang, & Malanson, ). Proximity and densities of appropriate alternate hosts are also integral to disease progression and risk modelling efforts (Kearns et al., ). Cronartium ribicola spores from either host may be able to move moderate distances of <1–15 km within a forest stand, canyon or mountain range (Van Arsdel, ), similar to comandra blister rust in comparable mountain environments (Jacobi et al., ).…”

“…(), Geils, Conklin, and Van Arsdel (), Smith and Hoffman (), Kearns et al. () and Cleaver et al. ().…”

“…Occurrence of the disease on limber pine at the stand level was modelled with good success by Kearns et al. () and Cleaver et al. (), but accurately predicting incidence or severity (branch and/or stem cankers tree −1 ) has been more difficult.…”