FROST, CANKER, AND DIEBACK OF DOUGLAS-FIR IN THE CENTRAL INTERIOR OF BRITISH-COLUMBIA

The relationships between dieback severity of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco), growing-season frosts, and invasion by canker pathogens in plantations >10 years old in central British Columbia are described. The coincidence of frosts and dieback events was determined by dissection and by mapping dieback damage in and around frost pockets. Putative canker pathogens were isolated from bark and fruiting bodies. The timing of significant dieback events coincided with major growing-season frosts, as determined by the formation of frost rings. Damage was most severe in shallow depressions and valley bottoms. Early flushing trees were more severely affected than late flushing trees in two of three plantations. Seven weak canker parasites were isolated from dead Douglas-fir bark. Among these, Sclerophoma semenospora Funk was restricted to tissues killed by frost or other factors, while Leucocytospora kunzei (Fr.:Fr.) Munk and a Sirodothis sp. accounted for about 65% of isolates from recently killed bark. Outside frost prone areas, these fungi were common as small latent branch cankers, but caused no further discernable damage. Areas of high dieback hazard could be identified from topographic features seen on aerial photographs. Of the area of Douglas-fir plantations >10 years old, 4.7% was severely damaged by dieback. It is recommended that lodgepole pine (Pinus contorta Dougl.) be planted in frost-prone areas.