Cowell, S. J.

The X-disease phytoplasma (‘Candidatus Phytoplasma pruni’) is an obligate pathogen that is capable of infection, persistence, and pathogenicity in both its major plant host (Prunus spp.) and leafhopper vector (Colladonus spp.) species. How the ‘Ca. P. pruni’ interacts with its plant and insect hosts, and how it alters its gene expression to do so is unknown, therefore in this study we conducted comparative RNAseq and differential gene expression analysis on ‘Ca. P. pruni’ infected P. avium and C. reductus samples. We found that the phytoplasma altered expression of approximately 32% of its annotated protein coding and pseudogenes, including intercellular transporters, proteolytic activity, its membrane structure, as well as upregulating genes associated with potential mobile units when in insect tissues vs. in plant tissues. Most notably, differential expression was observed in genes that were identified by in silico analysis as being putative secreted effectors that may play a role in allowing phytoplasma infection and survival in these two very different host systems, as well as inducing X-disease in Prunus spp., and offers targets for control by of this damaging pathogen by disrupting phytoplasma-host interactions.

The ongoing spread of X-disease, which is 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 Pacific Northwest, 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 plant 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 that alternative host removal, along with the removal of infected trees, is an effective means of slowing disease spread.