The Effects of Pruning Young White Pine for Blister Rust Control

Abstract

White pine blister rust is the most serious disease causing mortality of 5-needled pines in North America. Control of this fungus disease has for many years involved the eradication of the alternate host Ribes plants from selected white pine stands. Alternative or supplemental control has also involved physical removal of infections since the fungus mycelium is localized rather than systemic within the tree. The mycelium also grows quite slowly, often taking ma~ years to grow from a branch to the bole where damage occurs through killing of the phloem and eventual girdling. Previous pathological pruning studies have reported only the curative effects of removing blister rust-infected and other lower branches from young white pines. This paired tree study shows that removing all lower branches in addition to the visibly cankered ones results in more benefits than determinable by infection surveys. It removes visible cankers, undetected non-symptomatic infections, and prevents new infections. Three to five years after pruning, 75% of the controls and 95% of the pruned trees remain unaffected by blister rust. New lethal infection is occurring on 3.03 times as ma~ controls as treated trees. Pruning of all branches from the lower six feet of sapling white pines as soon as the trees reach 12 to 14 feet in height will adequately protect them from blister rust in low and medium climatic hazard zones. Pruning in two steps, beginning when trees are smaller and to a greater height will give better protection and is recommended for the higher hazard areas.