Efficacy of preventive pruning juvenile Pinus monticola to reduce mortality caused by Cronartium ribicola in south coastal British Columbia

Cronartium ribicola is the causal agent of white pine blister rust, an often-fatal canker disease of five-needle pines including western white pine (Pinus monticola). Pruning is a silvicultural treatment that clears the lower canopy of branches to reduce rust infections and subsequent mortality. Studies of the efficacy of pruning in North America have mainly been conducted on five-needle pines in continental environments and often used the delayed entry technique of sanitation pruning that treats older (10 – 30 years) infected trees. In the maritime region of south coastal British Columbia, tree growth, host susceptibility and rust virulence is markedly different. This study assessed the preventative pruning of younger trees at eight sites encompassing five ecological units and four seedlots over 20 years. Two pruning lifts allowed 16.5% more trees to remain uninfected and reduced mortality by 17.2% over unpruned trees. Negative impacts of pruning were a 3.8% reduction in mean height growth and 7% lower diameter growth. Presence of the fungal alternate host (Ribes sp.) was significantly related with tree mortality but not with the gross or mean number of stem or branch cankers. Juvenile white pine can benefit from an early preventative pruning regime in south coastal British Columbia.