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White pine blister rust is a disease that damages and kills white pines in the Southern Rocky Mountains. Our goal was to determine whether pruning could prevent crown dieback to increase the longevity of high-value limber pines in two areas with high disease pressure. Four treatments were applied to infected trees: (1) do nothing (control); (2) preventive pruning of branches and needles up to 7 ft (2.1 m) or 60% of the crown, whichever was less; (3) preventive and sanitation pruning (removing cankers throughout the crown); or (4) sanitation pruning only. Uninfected trees were either controls or preventively pruned. Branch and stem canker counts, counts of canker removals, and branch canker length were recorded three times during the study period (2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015) and percent crown dieback in the last two assessments. Significantly less crown dieback occurred on trees that received both preventive and sanitation pruning than on controls. Sanitation pruning removed the most cankers. New cankers occurred annually on 47-52% of originally infected trees and 10% of originally uninfected trees, suggesting that a 5-year pruning rotation is needed in high disease pressure areas. Pruning could be applied on a longer rotation on sites with lower disease pressure.

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