A New Threat to Limber Pine () Restoration in Alberta and Beyond: First Documentation of a Race () Virulent to Controlled Major Gene Resistance.

The coevolution of virulence reduces the effectiveness of host resistance to pathogens, posing a direct threat to forest species and their key ecosystem functions. This is a threat to limber pine (), an endangered species in Canada due to rapid decline mainly driven by white pine blister rust caused by . We present the first report of a new, virulent race of . (designated ) that overcomes limber pine major gene () resistance (MGR). Field surveys found that three parental trees (pf-503, pf-508, and pf-2015-0070) were cankered with white pine blister rust in Alberta, but their progenies showed MGR-related phenotypic segregation postinoculation with an avirulent race of . (). Genotyping of their progenies using -linked DNA markers and a genome-wide association study provided additional support that these cankered parental trees had -controlled MGR. To confirm the presence of , aeciospores were collected from the cankered pf-503 tree to inoculate resistant seedlings that had survived prior inoculation using the race, as well as seedlings of two U.S. seed parents, one previously confirmed with MGR () and one without MGR, respectively. All inoculated seedlings showed clear stem symptoms, confirming that the virulent race is . These results provide insights into the evolution of . virulence and reinforce caution on deployment of controlled MGR. The information will be useful for designing a breeding program for durable resistance by layering both R genes with quantitative trait loci for resistance to white pine blister rust in North America.