Rubbery Taproot Disease of Sugar Beet in Serbia Associated with ‘Candidatus Phytoplasma solani’

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.

Molecular characterization of 'Candidatus Phytoplasma solani' in celery: Case study in Futog

'Candidates Phytoplasma solani', known by its trivial name stolbur phytoplasma, is a plant pathogen infecting numerous crops in Serbia. Celery plants with prominent leaf yellowing and chlorosis, sporadically with tissue necrosis, were observed during August 2020 in Futog, situated in Novi Sad suburbia in Vojvodina. Total of 12 sampled celery plants, 8 symptomatic and 4 asymptomatic ones, were analysed for 'Ca. P. solani' presence. All symptomatic celery plants were infected with stolbur phytoplasma according to the stoll 1 gene detection and therefore were further subjected to multigene molecular characterization on three genes: tuf, stamp and vmpl. Combining molecular tools PCR/RFLP and sequencing reviled two 'Ca. P. solani' multilocus genotypes in celery: tuf-b/Rqg31/V14 and tuf-b/ Rpm35/V14, present in 4 samples each. Obtained results of the strain genotyping are in concordance with previous data on the 'Ca. P. solani' diversity on celery, but supplemented with genotyping of the vmpl gene. Outbreak of stolbur phytoplasma in the assessed locality in Futog is linked to tuf-b epidemiological cycle correlated in Serbia mainly with weed Convolvulus arvensis which was present in the subjected celery plot, and could have been the phytoplasma inoculum source. Visual evaluation of the symptom occurrence suggests on 10-15% of 'Ca. P. solani' affected celery plants scattered throughout the plot corresponding to the pathogen dispersal in crop by cixiid planthoper Hyalesthes obsoletus (Hemiptera: Auchenorrhyncha) associated with C. arvensis, main vector of stolbur phytoplasma in Serbia.