British Columbia (BC) is the lead producer of sweet cherries in Canada with more than 2,000 ha in production and a farm gate value of over CAD$100 million annually. Since 2010, an outbreak of little cherry disease caused by Little cherry virus 1 (LChV1) and Little cherry virus 2 (LChV2), as well as X-disease (XD) caused by ‘Candidatus Phytoplasma pruni’ has caused significant economic losses in neighboring Washington State (WA), USA. LChV1 and LChV2 have long been known to occur in BC (Theilmann et al. 2002); however, ‘Ca. P. pruni’ has not yet been reported in BC. Due to its geographical proximity to WA State, the BC cherry industry expressed significant concerns about the possible presence of the phytoplasma in cherry orchards. Accordingly, the main objective of this study was to survey cherry orchards to determine whether ‘Ca. P. pruni’ was present in symptomatic trees in BC. A total of 118 samples of leaves and fruit stems from individual symptomatic trees were collected prior to harvest from nine cherry orchards and one nectarine orchard in the Okanagan and Similkameen Valleys in BC. Characteristic symptoms included small and misshapen fruit with poor color development. Samples were submitted to AGNEMA, LLC (Pasco, WA) for testing using qPCR TaqMan assays for LChV1 (Katsiani et al. 2018), LChV2 (Shires et al. 2022) and ‘Ca. P. pruni’ (Kogej et al. 2020). Test results showed 21 samples (17.8%) from three cherry orchards positive for LChV2 and 2 samples (1.7%) from one cherry orchard positive for ‘Ca. P. pruni’. In order to confirm the identification of ‘Ca. P. pruni’, part of the 16S ribosomal RNA gene was amplified by nested PCR using the P1/P7 followed by R16F2n/R2 primer sets (Gundersen and Lee 1996) and Sanger sequenced. BC-XD-Pa-1 (GenBank Acc. No. OR539920) and BC-XD-Pa-2 (OR537699) were identical to one another and showed 99.92% identity to the ‘Ca. P. pruni’ reference strain CX-95 (JQ044397). Analysis using iPhyClassifier (Zhou et al. 2009) indicated that they were 16SrIII-A strains. Interestingly, the two partial 16S sequences showed 100% nucleotide identity to strain 10324 (MH810016) and others from WA. For additional confirmation, partial secA (Hodgetts et al. 2008) and secY (Lee et al. 2010) translocases were amplified and sequenced. As with the 16S sequences, secY sequences (OR542980, OR542981) showed 99.92% nucleotide identity to strain CX-95 (JQ268249), and 100% to strain 10324 (MH810035). The secA sequences (OR542978, OR542979) had nucleotide identities of 99.77% to strain CX (MW547067), and 100% to the Green Valley strain from California (EU168733). Accordingly, ‘Ca. P. Pruni’ was confirmed to be present in sweet cherry samples from BC. ‘Ca. P. Pruni’-related strains have been previously reported to occur in Canada in commercial poinsettias (Euphorbia pulcherrima) (Arocha-Rosete et al. 2021). To our knowledge, this is the first report of ‘Ca. P. Pruni’ in sweet cherry in Canada. Due to the important economic value of sweet cherries in BC, these findings are highly significant and represent the first steps towards the development of a surveillance system for early detection of XD, and consequent implementation of management strategies, including vector control. As required by federal and provincial regulations, cherry trees infected with LChV2 and ‘Ca. P. Pruni’ found in the survey were removed by the growers.
Stone fruits are a multibillion-dollar industry for the United States and Canada, one that has repeatedly suffered significant economic losses due to outbreaks of the X-disease phytoplasma (‘ Candidatus Phytoplasma pruni’) over the last century. Orchards and entire production areas have been abandoned, with corresponding losses to growers, fruit packers, and consumers. The most recent outbreak, in the U.S. Pacific Northwest, resulted in an estimated $65 million (USD) loss in revenue between 2015 and 2020 and is only increasing in incidence. Already present across much of the continental United States and Canada, the phytoplasma has a broad host range beyond stone fruit and is transmitted by at least eight leafhopper species, therefore stone fruit production in every state is at significant risk. This recovery plan was produced as part of the National Plant Disease Recovery System and is intended to provide a review of pathogen biology, assess the status of critical recovery components, and identify disease management research, extension, and education needs.
‘Candidatus Phytoplasma pruni’ infection in cherries causes small, misshapen fruit with poor color and taste, rendering the fruit unmarketable. However, this is a disease with a long development cycle and a scattered, nonuniform symptom distribution in the early stages. To better understand the biology as well as the relationship between pathogen titer and disease expression, we carried out seasonal, spatial, and temporal examinations of ‘Ca. P. pruni’ titer and distribution in infected orchard-grown trees. Sequential sampling of heavily infected trees revealed marked seasonal patterns, with differential accumulation in woody stem and leaf tissues and, most notably, within fruit in the early stages of development from bloom to pit hardening. Furthermore, mapping phytoplasma distribution and titer in trees at different stages of infection indicated that infection proceeds through a series of stages. Initially, infection spreads basipetally and accumulates in the roots before populating aerial parts of the trees from the trunk upward, with infection of specific tissues and limbs followed by an increasing phytoplasma titer. Finally, we observed a correlation between phytoplasma titer and symptom severity, with severe symptom onset associated with three to four orders of magnitude more phytoplasma than mild symptoms. Cumulatively, these data aid in accurate sampling and management decision-making and furthers our understanding of disease development.
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.