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Host responses in Douglas-fir, western hemlock and western redcedar to infection by Armillaria ostoyae and Armillaria sinapina Cleary, Michelle R.


Necrophylactic periderm (NP) formation and compartmentalization of infected tissue were examined in roots of 20-30 year-old western redcedar (Thujaplicata), western hemlock (Tsuga heterophylla) and Douglas-fir (Pseudotsuga menziesii) trees infected by Armillaria ostoyae. Microscopic investigation of abiotically wounded roots, as well as roots naturally infected and inoculated with A. ostoyae revealed distinct differences in the types and frequency of host responses between cedar and the other two conifers. Following invasion by A. ostoyae, a higher frequency of successful resistance reactions was induced in western redcedar compared to Douglas-fir and western hemlock. Breaching of non-suberized impervious tissue (NIT) and NP was common in Douglas-fir and western hemlock trees. The barrier zone in cedar formed by the uninjured cambium was comprised of axial parenchyma with pigmented deposits and provided a permanent barrier to spread by the fungus. Unique resistance mechanisms in cedar involving induced rhytidome formation impart increased resistance to the spread of A. ostoyae in host tissue. In three inoculation trials, penetration of living bark on host roots by A. sinapina did not differ from A. ostoyae. However, the frequency of successful resistance reactions induced following invasion by A. sinapina in Douglas-fir and western hemlock was significantly higher than the same species infected with A. ostoyae. Inoculum potential and host-pathogen interactions were key determinants of pathogenicity of A. sinapina on all hosts. In a survey of twenty juvenile mixed species plantations throughout the southern Interior of B.C., cumulative mortality in Douglas-fir trees was significantly higher than in western redcedar trees (p < 0.001). The incidence o f mortality decreased with increasing tree size for both species, however the rate o f decrease was markedly greater among cedar compared to Douglas-fir trees. The proportion of trees that showed compartmentalization and callusing at the root collar increased with increasing tree size, but the increase was markedly greater for cedar than Douglas-fir and occurred much earlier even when the trees were relatively small. Results indicate that the higher degree of resistance against A. ostoyae in western redcedar may help alleviate long-term impacts of root disease when regenerated on sites infested with Armillaria root disease.

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