White-Pine Blister Rust

(also blister rust of pine), a disease of pines caused by the rust fungi Cronartium flaccidum and Peridermium pini. White-pine blister rust is characterized by the formation on the trunks and branches of irregular spindle-shaped swellings, upon whose surface numerous dry sulfur-yellow formations (aecidia) appear in the spring. Upon rupture of the casing, a gray-yellow dusty mass of spores pours out of each aecidium. The mycelium of the fungus spreads throughout the bark, bast fiber, cambium, and wood; it causes necrosis in the wood. The dead bark peels off, exposing the affected wood and causing abundant resin flow.

White-pine blister rust is a very harmful disease. The overall growth rate of the tree is decreased, and the crown becomes sparse. The tree weakens and becomes infested with bark beetles and other trunk pests, which hasten its demise. The uredinial and teleutosporic stages of Cronartium flaccidum occur on white swallowwort (Vincetoxicum officinale), rattle (Pedicularis palustris), touch-me-not (Impatiens), and other herbaceous plants. The disease most often is chronic, lasting up to 30 years on the average. Although pines of any age may become diseased, older trees are most often affected. Control measures include removal of diseased trees and destruction of intermediate hosts.