Molecular characterization of elm yellows phytoplasmas in Croatia and their impact on Ulmus spp.

Abstract

‘Candidatus (Ca.) Phytoplasma ulmi’, the causal agent of elm yellows, was found widely distributed across elm populations in Croatia, infecting Ulmus laevis and Ulmus minor. Especially high prevalence of the infection, approximately 75%, was detected in U. laevis, but more than half of the trees were symptomless. ‘Candidatus Phytoplasma solani’ and ‘Ca. Phytoplasma asteris’ were also detected. The latter could possibly represent a new 16SrI subgroup, most closely related to 16SrI‐B. Diversity of ‘Ca. Phytoplasma ulmi’ in Croatia was determined by sequencing of the 16S rRNA gene, ribosomal protein genes rpl22 and rps3, secY and secY‐map genes, in 62 phytoplasma isolates. Phylogenetic analysis indicated that Croatian isolates share a common origin and are closely related to strains of ‘Ca. Phytoplasma ulmi’ from southeastern Europe. However, comparative sequence analysis revealed mutations at positions where variability has never been detected before, including positions within sequences unique to ‘Ca. Phytoplasma ulmi’ in the 16S rRNA gene, rpl22, rps3 and secY. New genotypes were identified based on the sequenced genes. This study points to a significantly higher genetic diversity than previously reported, and a necessity to revise the formal description of this phytoplasma species and to include newly discovered characteristics.