Harper, Scott

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.

In sweet cherry (Prunus avium), infection by ‘Candidatus Phytoplasma pruni’ results in small fruit with poor color and taste, rendering the fruit unmarketable. Yet the disease pathology is poorly understood, particularly at the cultivar level. Therefore, in this study we examined the physiological effects of Ca. P. pruni infection across a range of cultivars and locations in eastern Washington. We found that infection could be separated into early and established stages based on pathogen titer, which correlated with disease severity, including fruit size, color, and sugar and metabolite content. Furthermore, we observed that the effects of early-stage infections were largely indistinguishable from healthy, uninfected plants. Cultivar- and location-specific disease outcomes were observed with regard to size, color, sugar content, and citric acid content. This study presents the first in-depth assessment of X-disease symptoms and biochemical content of fruit from commercially grown sweet cherry cultivars known to be infected with Ca. P. pruni.