Necrophylactic periderm formation and compartmentalization of infected tissue was studied in tissue samples collected from the roots of 20-30 year old western red cedar (Thuja plicata), western hemlock (Tsuga heterophylla) and Douglas-fir (Pseudotsuga menziesii) trees infected with Armillaria ostoyae. Following penetration by the fungus, a higher frequency of resistance reactions were induced in western red cedar trees than in to western hemlock and Douglas-fir. Resistance reactions in cedar involved necrophylactic periderm formation around a site of penetration. Subsequently, a hypersensitive response was induced involving rhytidome formation which extended for some distance proximally and distally from a recently formed periderm. Effective compartmentalization following cambial invasion also limited the extent of girdling by the fungus. In a survey of juvenile mixed species plantations throughout the southern interior of B.C., only 2% of the western red cedar trees were killed by A. ostoyae compared to 25% of the Douglas-fir trees. The frequency of western red cedar trees showing effective compartmentalization at the root collar was significantly higher than in Douglas-fir trees. Although the risk of mortality decreased with increasing tree size for both western red cedar and Douglas-fir trees, the rate of decrease was noticeably greater in cedar than in Douglas-fir. Results indicate that western red cedar is more resistant to A. ostoyae than other conifers and that the inclusion of cedar in higher proportions when planting infested sites may reduce the overall impact of Armillaria root disease.
Bart J. van der Kamp.
Cleary, Michelle R., Van Der Kamp, Bart J.. 2005. Reducing the impact of Armillaria root disease via mixed species plantations including western redcedar. Forest Investment Account (FIA) - Forest Science Program. Forest Investment Account Report