Susceptibility of Intermountain Douglas-Fir to Rhabdocline Needle Cast When Grown in the Pacific Northwest

Abstract

Douglas-fir are traditionally grown as Christmas trees in the Pacific Northwest (PNW). Intermountain (IM) provenances of Douglas-fir are less likely to be injured after harvest by exposure to subfreezing temperatures than the coastal provenances. However, planting IM Douglas-fir in the coastal areas of the PNW is currently not recommended because of their high susceptibility to Rhabdocline needle cast. During a three-year period, needle cast severity was rated (0 to 100 scale) on trees in a replicated planting of IM Douglas-fir near Puyallup, WA. In 1997, needle cast ratings ranged from 0.8 to 70. Trees from the Cibola National Forest (NF) had significantly higher needle cast ratings (70.0) than trees from Carson NF (17.3), Coconino NF (15.7), Rio Grande NF (5.0), and Upper Clearwater, ID (0.8). Trees from the Apache NF (51.4), San Isabel NF (32.3), two collections representing the Lincoln NF (25.4-30.2), and the Santa Fe NF (21.3) had intermediate levels of needle cast. This study shows that certain sources of IM Douglas-fir can be grown in the PNW with minimal Rhabdocline needle cast problems. Applications of chlorothalonil also provided effective needle cast control on all but the most susceptible provenance. Accepted for publication 15 October 2001. Published 29 October 2001.