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

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 (r2 = 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.