A New Threat to Limber Pine ( Pinus flexilis ) Restoration in Alberta and Beyond: First Documentation of a Cronartium ribicola Race ( vcr4 ) Virulent to Cr4- 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.