Fuentes, Javiera

Pear decline, caused by 'Candidatus Phytoplasma pyri', has emerged in Chilean pear orchards in recent years. While several Cacopsylla species are potential vectors of 'Ca. P. pyri', the disease's full epidemiological cycle remains uncertain. Cacopsylla bidens, present in Chilean orchards, has recently been reported as a vector. This study conducted year-long surveys in two commercial pear orchards across different Chilean regions, capturing C. bidens in zones with 'Ca. P. pyri'-infected pear trees. All developmental stages were collected, with peak abundances occurring in March and April. Outside the study zones, C. bidens were found in pine trees but not in adjacent cultivated areas. Two seasonal morphotypes, summer and winter forms, were identified. Molecular analysis detected 'Ca. P. pyri' in a high proportion of insects, with maximum infection rates in March and April. These findings advance our understanding of 'Ca. P. pyri' spatial and temporal dynamics and its potential role in 'Ca. P. pyri' spreading under Chilean field conditions.

‘ Candidatus Phytoplasma pyri’, the pathogen associated with pear decline, affects pear trees across both the old and new worlds. However, research on this phytoplasma has been limited by the lack of genomic data. This study presents the first draft genome of ‘ Ca. P. pyri’ using a strain from Chile, with its genomic features analyzed in comparison with the closely related ‘ Ca. Phytoplasma’ species ‘ Ca. P. mali’ and ‘ Ca. P. prunorum’. The draft genome spans 456,478 bp with a GC content of 20.4%. Key genes possibly associated with pathogenicity and potential pathogenic effectors were identified, and they are notably lacking orthologs of known effectors. A single potential mobile unit similar to that of ‘ Ca. P. mali’ was identified. It is characterized by the absence of the transposase tra5 and the presence of the IS3 family transposase iSErh1. Multilocus sequence analysis of six genetic markers (16S rRNA gene, LSU36p, tuf, aceF, secA, and secY) from 10 Chilean and 10 Italian samples revealed high genetic uniformity among the Chilean strains, collected from five geographically distant orchards over a span of 13 months; by contrast, strains with greater diversity were detected among those from Italy, collected from a few localities over approximately 30 years. These findings suggest limited evolutionary divergence of this phytoplasma in Chile. This study provides a foundational framework for investigations into the pathogenic mechanisms and evolutionary dynamics of ‘ Ca. P. pyri’.

In Graneros, O’Higgins Region, Chile, the mallow psyllid (Russelliana solanicola Tuthill, 1959) from Malva nicaeensis L. was identified as a potential vector of ‘Candidatus Phytoplasma pruni’. Over an 8-month period, 2089 specimens of a species of Psylloidea, including immatures and adults, were captured. We only selected the adults used for transmission trials in Catharanthus roseus (L.) G. Don (periwinkle) plants. By nested PCR, using primer pairs for phytoplasma detection in 16S rRNA and IdpA genes, 7 out of 113 (6.2%) periwinkle plants used in transmission trials were found to be infected by phytoplasmas. Insects that fed on these plants also tested positive for the same phytoplasmas. Periwinkle plants never showed virescence and phyllody, as commonly occurs with phytoplasma 16SrIII-J infection due to the effector SAP54. In this case, using primer pairs for the SAP54 gene, an amplification product was never obtained. Virtual restriction fragment length polymorphism (RFLP) analysis of F2nR2 fragments indicated that the phytoplasma, found in both periwinkle plants and insects used in transmission trials, belongs to the 16SrIII-J ribosomal subgroup. The COI gene of the psyllids samples was amplified and sequenced, showing a similarity ranging from 84.84% to 85.02% with R. solanicola from Solanum tuberosum L. The mitochondrial genome of the psyllid was also sequenced, revealing a 14,835 bp circular DNA molecule with 37 genes. The mallow psyllid transmitted the phytoplasma 16SrIII-J to periwinkle plants. The molecular identification of the insect does not match the morphological one, indicating that the mallow psyllid may constitute a cryptic species within the polyphagous R. solanicola species. This is the first report of a psyllid as a vector of the phytoplasma 16SrIII-J.