A survey of limber pine (Pinus flexilis James) to determine the geographic distribution, incidence, and severity of white pine blister rust (WPBR) throughout 13 study areas in central and southeastern Wyoming and northern Colorado was conducted from 2002 to 2004. The majority (81.1%) of the 18 719 surveyed limber pines >1.37 m tall were classified as healthy, 13.5% were declining or dying from various causes, and 5.4% were dead. WPBR was present on 278 (55%) of the 504 survey plots. Incidence of the disease ranged from 0% to 100% and averaged 15.5% over all the plots and 28.0% on the infested plots. Likelihood of infection by WPBR was significantly greater for limber pines in larger diameter classes. Incidence was negatively correlated with elevation and positively correlated with geographic position, with more northerly and easterly plots having higher incidences of WPBR. Incidence varied by slope position and did not vary by aspect, slope configuration, or degree of canopy closure. The current level of infestation in central and southeastern Wyoming and northern Colorado has been attained within the past two to four decades. With time, the pathogen may spread to currently uninfested white pine populations and intensify throughout its current distribution impacting valuable ecosystems.Résumé : Un inventaire du pin flexible (Pinus flexilis James) a été réalisé de 2002 à 2004 dans 13 zones d'étude situées dans le centre et le sud-est du Wyoming et le nord du Colorado pour déterminer la répartition géographique, l'incidence et la sévérité de la rouille vésiculeuse du pin blanc (RVPB). La majorité (81,1 %) des 18 719 pins flexibles échantillonnés de plus de 1,37 m de haut ont été classés comme sains, 13,5 % dépérissaient ou étaient mourants sous l'effet de diverses causes et 5,4 % étaient morts. La RVPB était présente dans 278 (55 %) des 504 places-échantillons. L'incidence de la maladie variait de 0 % à 100 % et atteignait en moyenne 15,5 % dans l'ensemble des places-échantillons et 28,0 % dans les places-échantillons infectées. Les risques d'infection par la RVPB étaient significativement plus élevés pour les pins flexibles dans les classes de diamètre supérieures. L'incidence était négativement corrélée avec l'altitude et positivement corrélée avec la position géographique : les places-échantillons les plus au nord et les plus à l'est avaient les plus fortes incidences de RVPB. L'incidence variait selon la position de la pente mais non selon l'exposition, la configuration de la pente ou le degré de fermeture du couvert. Le niveau actuel d'infection dans le centre et le sud-est du Wyoming et le nord du Colorado a été atteint au cours des deux à quatre dernières décennies. Avec le temps, le pathogène pourrait progresser vers les populations de pin blanc non infectées et l'infection pourrait s'intensifier partout où la maladie est actuellement présente et affecter des écosystèmes de grande valeur.[Traduit par la Rédaction]

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.

Epidemiological studies of white pine blister rust on limber pine require a temporal component to explain variations in incidence of infection and mortality. Unfortunately, it is not known how long the pathogen has been present at various sites in the central Rocky Mountains of North America. Canker age, computed from canker length and average expansion rate, can be used to estimate infestation origin and infection frequency. To investigate relationships between canker lengths and canker ages for limber pine, we collected live white pine blister rust branch and stem cankers from three locations in Wyoming and two locations in Colorado. We quantified relationships between various measures of canker length and an estimate of canker age based on dendrochronological analysis. Total branch canker length was strongly, negatively correlated (r = )0.79) with the first year of incomplete, annual ring formation (canker age). Mean longitudinal canker expansion rate was 8.4 cm year )1 for branch and stem cankers where branches distal to the canker were either dead or alive. Annual longitudinal canker expansion, however, was significantly greater on a stem or branch where the portion distal to the canker was alive (11.5 cm year )1 ) rather than dead (7.1 cm year )1 ). For branches or stems, proximal expansion rate (i.e., toward or down stem) averaged 4.9 cm year )1 . The circumferential canker expansion rate (around branch or stem) was greater for stem cankers (8.3 cm year )1 ) than for branch cankers (6.2 cm year )1 ). Additional site and host tree covariates did not improve prediction of canker age. Two simple linear equations were developed to estimate a canker age from total length of a canker with the distal portion either alive or dead. An appropriate sample of canker ages can be used to determine how long a limber pine stand has been infested with white pine blister rust and how frequently infections have occurred.