Heights to white pine blister rust cankers caused by Cronartium ribicola on young Pinus monticola in the Oregon East Cascades

In addition to breeding for genetic resistance, lower branch pruning is a management strategy to mitigate adverse effects of Cronartium ribicola on young western white pine (Pinus monticola; WWP) in portions of the Interior Northwest, USA (INW). However, only data on heights of white pine blister rust (WPBR) cankers and post-treatment effects from outside the Oregon East Cascades (OEC) have previously informed INW pruning guidelines. To evaluate the appropriateness of these guidelines for WWP in the OEC, heights to cankers on young WWP (<30 years old) were measured in 120 plots within 12 stands throughout the OEC. Canker heights were analysed for live WWP >= 2.54 cm in diameter at 1.37 m. Incidence and severity of WPBR on live WWP, in addition to heights to cankers, varied among stands. Increased severity of WPBR (number of cankers on live-infected trees and percent mortality by WPBR) was observed in stands with the alternate host Ribes more frequently occurring in plots. Of all cankers, approximately 97% were found in the lower half of the total tree height, and only 10% of cankers on young WWP were found above the first third of the total tree height. When evaluating canker heights in all stands, mean heights to branch and bole cankers were <2.2 m. Bole cankers were lower than branch cankers (p = .01), reaffirming that potentially lethal cankers in the INW often originate from infections lower in the crown. In stands, the number of plots with Ribes present was positively correlated with the number of cankers on live-infected WWP and percent mortality of WWP due to WPBR (p < .04). As the incidence of WPBR on live WWP increased in stands, heights to the highest cankers also increased (p = .002). The number of cankers on live-infected trees was positively correlated with the mean and highest heights to cankers (p < .002). Given that the vast majority of cankers were prunable and occurred in the lower crown of young WWP, pruning lower branches to increase survival of WWP is a management option in the OEC, and prioritization of stands for treatment is recommended.