Cowell, Sarah J.

An ongoing outbreak of X-Disease, associated with ‘Candidatus Phytoplasma pruni’ has resulted in severe economic losses for cherry and stone fruit growers in the U.S. Pacific Northwest in the last decade. Given that this pathogen is transmitted by polyphagous leafhopper species, primarily Colladonus montanus ssp. reductus and C. geminatus in the PNW, alternative plant hosts present a significant management concern. Here we surveyed phytoplasma incidence in non-Prunus plants found in and around stone fruit orchard borders and compared these results to the leafhopper feeding patterns through gut content analysis. We confirmed ‘Ca. P. pruni’ infection in 21 plant species from 15 families. Of these, 15 species were commonly found in the diets of phytoplasma-carrying leafhoppers. Due to the abundance of common species, including Taraxacum, Malva, and Trifolium spp., alternative hosts were found across all growing seasons, although ‘Ca. P. pruni’ titer was low (<102 cells) in most of these hosts. No geographic patterns were identified, instead phytoplasma incidence and spread at individual orchards was related to the management of annual and biennial host species, thus removing preferred leafhopper feeding hosts. These data suggest alternative host removal, along with removal of infected trees, is an effective means of slowing disease spread.

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.