Association of seven ‘Candidatus Phytoplasma’ species to an almond disease complex in Jordan, and preliminary information on their putative insect vectors

Abstract

During field surveys carried out from June to October 2020 and in January 2021 in orchards of northern Jordan, phytoplasma-like symptoms (early flowering along with evergreen pattern; witches’-broom, yellowing, and dieback; slim leaf and leaf rolling; stem fasciation) were observed in almond trees. In 23 investigated orchards, symptomatic almond trees ranged from 20 to 85%. PCR-based amplification of 16S rRNA gene detected phytoplasmas in 21% of 140 collected symptomatic almond trees. Sequence analyses allowed attributing the detected phytoplasmas to ‘Candidatus Phytoplasma asteris’ (taxonomic subgroups 16SrI-B and -R), ‘Ca. P. aurantifolia’ (16SrII-B and -C), ‘Ca. P. omanense’ (16SrXXIX-A and -B), ‘Ca. P. phoenicium’ (16SrIX-B), ‘Ca. P. pyri’ (16SrX-C), ‘Ca. P. solani’ (16SrXII-A), and ‘Ca. P. ulmi’ (16SrV-A). Such phytoplasmas were found associated with specific symptoms and differentially distributed in the considered locations. Moreover, further investigation identified ‘Ca. P. asteris’ (subgroup 16SrI-R) in putative insect vectors such as Empoasca sp., Reptalus sp., and Hyalesthes obsoletus, ‘Ca. P. pyri’ in Cacopsylla bidens, and ‘Ca. P. omanense’ (subgroup 16SrXXIX-B) in the non-crop plant Amaranthus sp. In conclusion, this study described an almond disease complex associated with multiple phytoplasmas, including ‘Ca. P. pyri’, ‘Ca. P. omanense’, and ‘Ca. P. ulmi’ that were never reported before in this crop. Further studies are needed to survey the diffusion of this almond disease complex in the region, demonstrate the transmission capability of the identified putative vectors, and in-depth investigate the ecologies of all phytoplasmas associated with the disease.