Visible versus actual incidence of Armillaria root disease in juvenile coniferous stands in the southern interior of British Columbia

The relationship between aboveground symptoms and belowground incidence of Armillaria ostoyae (Romagn.) Herink on conifers in 13- to 24-year-old stands was investigated at five sites in each of the dry, moist, and wet climatic regions in the Nelson forest region, British Columbia. All trees >1.3 m in height in 0.01-ha circular plots centred on a tree killed fewer than two or more than five years previously or located where there were no symptomatic trees were removed from the soil by an excavator. The location and host response at each A. ostoyae lesion on root systems were recorded. Significant differences in belowground incidence were seen among climatic regions and plot types, with distance from the centre of plots, and between planted and naturally regenerated trees. Belowground incidence was related to the percentage of putatively colonized stumps within and adjacent to plots. There were significant differences among climatic regions in the intensity of infection, host reaction to infection, and percentage of diseased trees showing aboveground symptoms. These results have implications for interpreting results of surveys for Armillaria root disease in juvenile stands and for tending of such stands.