Control of Armillaria and Phellinus root diseases: 20-year results from the Skimikin stump removal experiment.
The Skimikin experiment was established in 1968 in British Columbia to determine the efficacy of inoculum removal for control of Phellinus weirii. Half of a 2.56-ha tract in a mature Douglas fir (Pseudotsuga menziesii)/lodgepole pine (Pinus contorta) stand in which 60-70% if stems had been killed or infected by Phellinus weirii was whole-tree logged and root raked to remove most roots larger than 2 cm in diameter from the upper 60 cm of soil. The other half was logged conventionally. Seedlings of Douglas fir, lodgepole pine, western red cedar (Thuja plicata) and paper birch (Betula papyrifera) were planted alone and in all combinations of two species in 0.04 ha plots. Two plots were planted with western larch (Larix occidentalis) or Engelmann spruce (Picea engelmannii). Tree mortality was recorded over 20 years. After seedlings became established the principal causes of mortality were the root diseases caused by Armillaria ostoyae and Phellinus weirii. Cumulative percentage mortality of Douglas fir caused by A. ostoyae and P. weirii, and of lodgepole pine caused by A. ostoyae was less in the treated block than in the untreated block. No lodgepole pines were killed by P. weirii. Plots in which rows of lodgepole pine or Douglas fir alternated with rows of cedar or birch had fewer and smaller disease centres than plots of lodgepole pine and/or Douglas fir. Height and diameter growth of Douglas fir and lodgepole pine were greater in plots in the treated block than in those in the untreated block, mainly because of reduced competition. Where stump removal is impractical, larch is the preferred species on sites infested by A. ostoyae, because of the resistance which develops after age 20 yr. Similarly, on sites infested by P. weirii, lodgepole pine should be planted where hazard is high and alternate rows of Douglas fir and lodgepole pine where hazard is low. A mixture of lodgepole pine and larch is recommended for sites with both diseases.