Mitrovic, Jelena

Two phloem-limited pathogens, ‘Candidatus Arsenophonus phytopathogenicus’ and ‘Candidatus Phytoplasma solani’, threaten sugar beet production in France, Switzerland, and Germany. Previous studies of these pathogens in Germany had focused on its western and southern regions, leaving a knowledge gap about eastern Germany. Despite their importance, this study is the first to investigate phytoplasmas in sugar beet in Saxony-Anhalt, Germany. A phytoplasma strain related to ‘Ca. P. solani’ is found predominant in Saxony-Anhalt, unlike in France, where ‘Ca. P. solani’ has a minor role compared with ‘Ca. A. phytopathogenicus’. The phytoplasma strain infecting sugar beet in Saxony-Anhalt was classified into a new subgroup designated as 16SrXII-P. The multilocus sequence analysis (MLSA) of nonribosomal genes of the novel phytoplasma strain showed that it is significantly different from the reference and all previously reported ‘Ca. P. solani’ strains including the strain from western Germany. Analyses of sugar beet samples from previous years confirmed the presence of the 16SrXII-P strain in sugar beet as early as 2020 and also in Bavaria in southern Germany. Based on 16S rDNA analysis, ‘Ca. A. phytopathogenicus’ in Saxony-Anhalt is identical to strains in sugar beet in other parts of Germany and France, as well as to a strain in potato from Germany. The presence and prevalence of two phytoplasmas in sugar beet in Germany suggest that more attention should be directed toward understanding phytoplasma infection in sugar beet in this country.

Rubbery taproot disease (RTD) of sugar beet was observed in Serbia for the first time in the 1960s. The disease was already described in neighboring Bulgaria and Romania at the time but it was associated with abiotic factors. In this study on RTD of sugar beet in its main growing area of Serbia, we provide evidence of the association between ‘Candidatus Phytoplasma solani’ (stolbur phytoplasma) infection and the occurrence of typical RTD symptomatology. ‘Ca. P. solani’ was identified by PCR and the sequence analyses of 16S ribosomal RNA, tuf, secY, and stamp genes. In contrast, the causative agent of the syndrome “basses richesses” of sugar beet—namely, ‘Ca. Arsenophonus phytopathogenicus’—was not detected. Sequence analysis of the stolbur strain’s tuf gene confirmed a previously reported and a new, distinct tuf stolbur genotype (named ‘tuf d’) that is prevalent in sugar beet. The sequence signatures of the tuf gene as well as the one of stamp both correlate with the epidemiological cycle and reservoir plant host. This study provides knowledge that, for the first time, enables the differentiation of stolbur strains associated with RTD of sugar beet from closely related strains, thereby providing necessary information for further epidemiological work seeking to identify insect vectors and reservoir plant hosts. The results of this study indicate that there are differences in hybrid susceptibility. Clarifying the etiology of RTD as a long-known and economically important disease is certainly the first step toward disease management in Serbia and neighboring countries.