Laminated Root Rot in a Western Washington Plantation: 8-Year Mortality and Growth of Douglas- Fir as Related to Infected Stumps, Tree Density, and Fertilization

Miller, Richard E.; Harrington, Timothy B.; Thies, Walter G.; Madsen, Jeff. 2006. Laminated root rot in a western Washington plantation: 8-year mortality and growth of Douglas-fir as related to infected stumps, tree density, and fertilization. Res. Pap. PNW-RP-569. Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest Research Station. 37 p. A 4-year-old Douglas-fir plantation in the western Washington Cascades was monitored for 8 years after fertilization with potassium (K), nitrogen (N), and K+N to determine fertilizer effects on rates of mortality from laminated root rot (LRR) and other causes relative to a nonfertilized control. Each element was applied at a rate of 300 lb/acre on and around 0.2-acre plots replicated seven times in a randomized complete block design. Cumulative mortality from LRR did not differ significantly among fertilizer treatments, and losses were strongly related to density of infected stumps from the previous stand (r = 0.74). Mortality from disease and other sources accelerated during the 8 years of monitoring. Average tree growth and stand volume were greatest in treatment N and were reduced where N was combined with K. Continued monitoring is needed to identify potential longer term effects of the fertilizer treatments on susceptibility of Douglas-fir to LRR and Armillaria spp.