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 exacerbates the threat to limber pine ( Pinus flexilis ), an endangered species in Canada due to rapid declines mainly driven by white pine blister rust (WPBR) as caused by Cronartium ribicola . We present the first report on a new C. ribicola virulent race (designated vcr4 ) that overcomes limber pine major gene ( Cr4 ) resistance (MGR). Field surveys found that three parental trees (pf-503, pf-508 and pf-2015-0070) were cankered with WPBR in Alberta, but their progenies showed MGR-related phenotypic segregation post-inoculation of avirulent race ( Avcr4 ). Genotyping of their progenies using Cr4 -linked DNA markers and genome-wide association study (GWAS) provided additional support that these cankered parental trees had Cr4 -controlled MGR. To confirm the presence of vcr4 , aeciospores were collected from the cankered pf-503 tree to inoculate resistant seedlings that had survived prior inoculation using Avcr4 race, as well as seedlings of two US seed parents, one previously confirmed with MGR ( Cr4 ) and one non-MGR, respectively. All inoculated seedlings showed clear stem symptoms, confirming the virulent race is vcr4 . These results provide insights into evolution of C. ribicola virulence, and reinforces caution on deployment of Cr4 -controlled MGR. The information will be useful for designing a breeding program for durable resistance by layering both R genes with quantitative trait loci (QTLs) for resistance to WPBR in North America.