Resistance and tolerance of Douglas‐fir seedlings to inoculation with the fungal root pathogen Coniferiporia sulphurascens

Abstract

Two‐year‐old interior Douglas‐fir seedlings in a greenhouse were artificially inoculated with Coniferiporia sulphurascens and grown for four years. The study population included 2,261 seedlings from 94 half‐sibling families distributed across four breeding zones. There was little mortality over the 4 years, but there was advanced stain internally at the root collar from infection by the fungus indicating host resistance/susceptibility. The breeding zones had no effect on stain incidence. The tallest trees had more staining, differing up to 30% incidence over the size range, and the families had at most ±6% incidence in stain from the average. Disease tolerance, assessed by height growth relative to controls, was affected by zone and family. The tallest families were the least tolerant, by at most 5% less than control tree height. There was little association between tolerance and resistance at the family level, meaning that both disease response mechanisms were likely separate. The most common response strategy at the family level was to combine higher resistance and tolerance, being about twice as common as lower resistance and tolerance trait combinations. Interior isolates of the fungus may have lower virulence than coastal isolates, which might help to explain the low genetic response to the fungus. Inoculum reduction versus tree breeding may be a better strategy for control of the disease for interior Douglas‐fir.