ALBERT

Description: 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.

Description: In selectively cut and undisturbed parts of four mature stands, five 0.04-ha plots were established, and trees were measured, mapped, and examined for aboveground symptoms of armillaria root disease. Trees were felled, and stumps and their root systems were removed by an excavator and were measured and examined for Armillaria lesions. Isolates from root lesions, rhizomorphs associated with lesions, and basidiomes collected in or adjacent to plots were of Armillaria ostoyae (Romagn.) Herink. All trees were assigned to one of five tree condition classes based on the location of lesions and host response. The merchantable volume in each class was calculated. In undisturbed plots, incidence of trees with A. ostoyae lesions on roots was about 10% in the dry climatic region compared with about 75% in the moist region and 35% in the wet region. In plots in the selectively cut parts of the stands, 50-100% of stumps were colonized by A. ostoyae. Results of a logistic regression analysis showed that selective cutting was associated with a statistically significant increase in the probability of a tree having A. ostoyae lesions, where the magnitude of the increase depended on tree diameter. The increase in the probability of a tree being diseased was accompanied by an increase in the proportion of primary roots with lesions and the average number of lesions per diseased tree; however, the increases in disease intensity were statistically significant at only two (one dry and one moist) of the four sites. The percentage of merchantable volume threatened or killed by A. ostoyae was usually higher in cutover than undisturbed plots.