Evidence of Asexual Overwintering of and Mapping of Stem Rust Host Resistance in .

rust is a devastating disease of shrub willow in North America. Previous work has identified as one of two identified rust species in New York State that infect and other important host species, however little is known about the population of this rust species in this region. Genotyping-by-sequencing was used to identify single nucleotide polymorphisms (SNPs) and assess population diversity of isolates collected from three breeding populations in Geneva, NY between 2015 and 2020. Statistical analyses of SNP revealed that all isolates collected were clonally derived even though they were collected across years. In 2020, isolates were collected from stem infections where uredospore pustules were observed, and these isolates were also identical to collected in previous seasons. These data suggest that sampled across multiple years overwintered and reproduced asexually and that stem infection is a possible mechanism for overwintering, both of which are novel findings for this rust species. Additionally, field disease ratings were conducted on a × F breeding population with high disease severity, enabling the discovery of QTL for resistance on chromosomes 1 and 19. Lastly, was frequently associated with stem rust and may play a role in stem infection. Together, this work is the first substantial exploration into population biology, stem infection, and host resistance in .