Efficacy of delayed entry low branch sanitation pruning in preventing mortality of western white pine infected with white pine blister rust

Abstract

White pine blister rust, caused by the fungus Cronartium ribicola J. C. Fisch., is a virulent mortality agent of five-needle pines like western white pine (Pinus monticola D. Don.). For decades, low branch pruning has been recommended as a control by removing cankers that can migrate from infected branches to the tree stem and kill the host. These recommendations are mainly based on studies done on eastern white pine (Pinus strobus L.), or on western white pine growing primarily in the Interior Pacific Northwest of the United States. It is not clear whether these recommendations are transferable to geographically and climatically dissimilar Pacific coastal areas, where pruning often shows little benefit in commercial forest plantations. We summarize over 30 years of post-treatment data from coastal and interior sites in British Columbia. Sanitation pruning was done on 12–22-year-old stands using one or two pruning lifts and infection and mortality were tracked over the subsequent decades. Our results show that while pruning had little negative effect on treated tree height or diameter growth, mortality was not significantly different between treated and untreated groups across all sites. Cumulative mortality after three decades was >80% for coastal sites, and nearly 60% for the interior site, indicating a lack of treatment efficacy. Our results differ from earlier shorter duration studies and those from other jurisdictions. It is clear that delaying pruning until trees are older and carrying many existing branch and stem infections results in ineffective control of rust in coastal white pine stands.