Pathogenicity and virulence of A rmillaria sinapina and host response to infection in D ouglas‐fir, western hemlock and western redcedar in the southern I nterior of B ritish C olumbia

Summary The frequency of infection, lesion characteristics and anatomical changes in phloem and cambial tissues caused by A rmillaria sinapina were studied on inoculated trees of D ouglas‐fir, western hemlock and western redcedar and compared with results of A . ostoyae inoculations previously reported on the same host species. Similar percentages of inoculations resulting in infection of roots on the three hosts indicate that A . sinapina and A . ostoyae are equally pathogenic. A rmillaria ostoyae was more virulent than A . sinapina as demonstrated by fungal exudates from A . ostoyae inoculum blocks, which appeared to cause lesions on roots; the higher frequency at which lignified impervious tissue ( IT ) and necrophylactic periderm ( NP ) developed in bark and following cambial invasion, compartmentalization; the large proportion of roots that showed no visible host response; the large zones of IT formed under continuous stimulation by A . ostoyae advancing in inner bark; and the high frequency of breaching of NP barriers. Spread of A . sinapina mycelium in host species appeared slower than that of A . ostoyae , particularly in D ouglas‐fir and western hemlock. In western redcedar, A . ostoyae induced stronger host responses than those following invasion by A . sinapina , which included further expansion of the induced rhytidome response, traumatic phloem resin duct formation and higher numbers of polyphenolic parenchyma comprising its barrier zone. Where damage by A . sinapina ensued, it was always associated with high inoculum potential. The ecology of virulent and less virulent species of A rmillaria in natural forests is discussed.