Hoskins, Mason C.

Chokecherry (Prunus virginiana L.) is a native shrub that is widely found across North America and is a host of ‘Candidatus Phytoplasma pruni’ associated with X-disease of stone fruits. This pathogen is of major concern to U.S. cherry and peach growers, with the most recent and ongoing outbreak in the Pacific Northwest causing major economic harm. To investigate the role of chokecherry as a potential reservoir and source of orchard-associated strains, we surveyed wild chokecherry trees across 12 U.S. states, and compared the strains present to those in commercially cultivated Prunus species. Approximately 47% of chokecherry sampled tested positive for a ‘Ca. P. pruni’-related strain and phylogenetic analysis revealed high strain diversity, including six distinct clades and four newly identified lineages. In contrast, ‘Ca. P. pruni’ strains isolated from cultivated cherry and peach trees exhibited lower diversity and were infected by known disease outbreak-associated strains such as NWX. These data suggest limited transmission between wild and cultivated hosts in the western U.S., with patterns indicating spillover from orchards into nearby chokecherry populations rather than the reverse. However, in states east of the Rocky Mountains, chokecherry likely functions as a reservoir for orchard infections.

Recurrent epiphytotics of X-disease, caused by ‘Candidatus Phytoplasma pruni,’ have inflicted significant losses on commercial cherry and peach production across North America in the last century. During this period, there have been multiple studies reporting different disease phenotypes and, more recently, identifying different strains through sequencing core genes, but the symptoms have not, to date, been linked with genotype. Therefore, in this study we collected and assessed differing disease phenotypes from multiple U.S. states and conducted multilocus sequence analysis on these strains. We identified a total of five lineages associated with the induction of X-disease on commercial Prunus species and two lineages that were associated with wild P. virginiana. Despite a century of interstate plant movement, there were regional trends in terms of lineages present, and lineage-specific symptoms were observed on P. avium, P. cerasus, and P. virginiana, but not on P. persica. Cumulatively, these data have allowed us to define “true” X-disease–inducing strains of concern to the stone fruit industry across North America, as well as potential sources of infection that exist in the extraorchard environment.