Blister rust testing in British Columbia: Choosing inoculum sources and a screening site

In order to test for possible geographic variants in white pine blister rust (Cronartium ribicola J.C. Fisch.), four rust sources from British Columbia (BC), Canada were cultivated on Ribes spp. and used to inoculate sets of 2-year-old seedling progeny of 24 parents of western white pine (Pinus monticola D. Don). The six parents from Coastal United States possessed known rust defenses; six parents from Interior Washington, and Idaho, were considered somewhat resistant to rust, whereas 12 parents from BC (six from the Coastal zone and six from the Interior zone) were untested previously. All seedlings were inspected annually in transplant beds, where rust symptoms and stages were recorded. A pooled source, composed of aeciospores from six lower-Coastal BC locations, was the most virulent, judged by infection percentage, highest mean spots per seedling, lowest spots-only response, shortest latency, greatest aecial production, and greatest mortality of infected seedlings. Combined data from the two Coastal rust sources, (pooled and Cowichan) demonstrated more virulence than both Interior sources (Nakusp and Valemount). No evidence was found to suggest that a unique race exists in the Cowichan source that overcomes spots-only resistance. Screening Interior western white pine for resistance to blister rust with pooled Coastal rust sources would be superior to using single or pooled Interior rust sources: therefore, only one Coastal inoculation facility is necessary to screen seedlings for both Coastal and Interior zonesMeagher and Hunt "Blister rust testing in British Columbia: Choosing inoculum sources and a screening site." Northwest Science. 1999; 73(3): 225-23