| Summary: | The purpose of this study was to assess the potential of Neonectria neomacrospora as an inundative biological control agent for hemlock dwarf mistletoe (Arceuthobium tsugense). Based on the growth characteristics of six isolates of N. neomacrospora, isolate PFC 2559 was selected as the inoculum source. A field trial was conducted on Vancouver Island to determine the requirement of wounding for N. neomacrospora infection and to measure the impact of TV. neomacrospora infection on A. tsugense vigour. Conidia were mass produced and suspended in "Stabileze" formulation, which was sprayed onto entire hemlock dwarf mistletoe infections (swellings and shoots). Treatments involved unwounded A. tsugense swellings, cut A. tsugense aerial shoots, and wounded A. tsugense swellings. The number of healthy and diseased^, tsugense aerial shoots was recorded at 2 weeks and 1, 2, 3.5, 5, 6, and 9 months after treatment. At 10 months, a sub-portion of the treated A. tsugense swellings was destructively sampled and an attempt to re-isolate N. neomacrospora was done. The greatest amount of re-isolation of N. neomacrospora occurred for the wounded, inoculated treatment (73%), followed by the cut shoots, inoculated treatment (60%), and the unwounded, inoculated treatment (55%); (35%, 20%, and 21% for controls, respectively). Based on the extent of bark necrosis, the occurrence of sporodochia, and re-isolation of N. neomacrospora, the wounded, inoculated treatment had the greatest impact on N. neomacrospora infection when compared to the unwounded, inoculated treatment and the cut shoots, inoculated treatment. There was no evidence that N. neomacrospora could infect wounded A. tsugense shoots. Once infection by N. neomacrospora was established, a 38% decline in the number of healthy A. tsugense aerial shoots was observed at 9 months. Applying inoculum to unwounded A. tsugense swellings is predicted to reduce the number of A. tsugense shoots by 14% in the field at 9 months. This is a conservative estimate. Due to the extensive bark necrosis and girdling of the A. tsugense swelling, infection by N. neomacrospora is expected to have a greater long-term impact on A. tsugense vigour than that observed in the present field trial.
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