Prevalence of a 'Candidatus Phytoplasma solani'-Related Strain Designated as New 16SrXII-P Subgroup over 'Candidatus Arsenophonus phytopathogenicus' in Sugar Beet in Eastern Germany.

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.

Epidemiological role of novel and already known 'Ca. P. solani' cixiid vectors in rubbery taproot disease of sugar beet in Serbia.

Rubbery taproot disease (RTD) of sugar beet was recently associated with the plant pathogenic bacterium 'Candidatus Phytoplasma solani' (CaPsol) and reported throughout the Pannonian Plain with variations in severity. Tracing CaPsol epidemiological pathways was performed in the experimental sugar beet field in Rimski Sancevi (Serbia) in 2020-2021, where an RTD outbreak was recently recorded. A molecular epidemiology approach was applied to the study of three RTD occurrence scenarios: epidemic, non-epidemic and 'absence of RTD'. As a result, Hyalesthes obsoletus ex Convolvulus arvensis was detected as a CaPsol vector to sugar beet, while two other cixiids were identified for the first time as vectors of the CaPsol-induced plant disease in crops: Reptalus quinquecostatus and R. cuspidatus. R. quinquecostatus was proposed culpable for the 2020 RTD epidemic outbreak in Rimski Sancevi when dSTOLg CaPsol strain predominated in the RTD-affected sugar beet, whereas R. cuspidatus had a negligible role in RTD occurrence and displayed ambiguous involvement in CaPsol epidemiology on a wider scale. The temporal discrepancy of the offset of CaPsol dissemination and disease occurrence is the main obstacle in predicting CaPsol-induced diseases. Predicting disease occurrence and severity can only be achieved by gaining a better understanding of CaPsol epidemiological pathways and insect vectors involved in disease outbreaks.

Revision of the 'Candidatus Phytoplasma' species description guidelines.

The genus 'Candidatus Phytoplasma' was proposed to accommodate cell wall-less bacteria that are molecularly and biochemically incompletely characterized, and colonize plant phloem and insect vector tissues. This provisional classification is highly relevant due to its application in epidemiological and ecological studies, mainly aimed at keeping the severe phytoplasma plant diseases under control worldwide. Given the increasing discovery of molecular diversity within the genus 'Ca. Phytoplasma', the proposed guidelines were revised and clarified to accommodate those 'Ca. Phytoplasma' species strains sharing >98.65 % sequence identity of their full or nearly full 16S rRNA gene sequences, obtained with at least twofold coverage of the sequence, compared with those of the reference strain of such species. Strains sharing <98.65 % sequence identity with the reference strain but >98.65 % with other strain(s) within the same 'Ca. Phytoplasma' species should be considered related strains to that 'Ca. Phytoplasma' species. The guidelines herein, keep the original published reference strains. However, to improve 'Ca. Phytoplasma' species assignment, complementary strains are suggested as an alternative to the reference strains. This will be implemented when only a partial 16S rRNA gene and/or a few other genes have been sequenced, or the strain is no longer available for further molecular characterization. Lists of 'Ca. Phytoplasma' species and alternative reference strains described are reported. For new 'Ca. Phytoplasma' species that will be assigned with identity ≥98.65 % of their 16S rRNA gene sequences, a threshold of 95 % genome-wide average nucleotide identity is suggested. When the whole genome sequences are unavailable, two among conserved housekeeping genes could be used. There are 49 officially published 'Candidatus Phytoplasma' species, including 'Ca. P. cocostanzaniae' and 'Ca. P. palmae' described in this manuscript.

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.

Chamomile Floricolous Downy Mildew Caused by Peronospora radii.

Floricolous downy mildews (Peronospora, oomycetes) are a small, monophyletic group of mostly inconspicuous plant pathogens that induce symptoms exclusively on flowers. Characterization of this group of pathogens, and information about their biology, is particularly sparse. The recurrent presence of a disease causing flower malformation which, in turn, leads to high production losses of the medicinal herb Matricaria chamomilla in Serbia has enabled continuous experiments focusing on the pathogen and its biology. Peronospora radii was identified as the causal agent of the disease, and morphologically and molecularly characterized. Diseased chamomile flowers showed severe malformations of the disc and ray florets, including phyllody and secondary inflorescence formation, followed by the onset of downy mildew. Phylogeny, based on internal transcribed spacer and cox2, indicates clustering of the Serbian P. radii with other P. radii from chamomile although, in cox2 analyses, they formed a separate subcluster. Evidence pointing to systemic infection was provided through histological and molecular analyses, with related experiments validating the impact of soilborne and blossom infections. This study provides new findings in the biology of P. radii on chamomile, thus enabling the reconstruction of this floricolous Peronospora species' life cycle.

Impact of a native Streptomyces flavovirens from mushroom compost on green mold control and yield of Agaricus bisporus.

Thirty-five actinobacterial isolates, obtained from button mushroom (Agaricus bisporus) substrates (i.e., compost in different phases of composting, black peat or casing layer) in Serbia in 2014-2016 were tested in vitro against the causal agents of green mold in cultivated mushroom. Out of six most promising isolates, A06 induced 42.4% in vitro growth inhibition of Trichoderma harzianum T54, and 27.6% inhibition of T. aggressivum f. europaeum T77. The novel strain A06 was identified as Streptomyces flavovirens based on macroscopic and cultural characteristics and 16S rDNA sequence and used in mushroom growing room experiments. Actinobacteria had no negative influence on mycelial growth of the cultivated mushroom in compost in situ. Isolate S. flavovirens A06 enhanced mushroom yield significantly, up to 31.5%. The A06 isolate was more efficient in enhancing yield after inoculation with the compost mold T. aggressivum (26.1%), compared to casing mold T. harzianum (8%). Considering disease incidence, actinobacteria significantly prevented green mold in compost caused by T. aggressivum (6.8%). However, fungicide prochloraz-Mn had a more significant role in reducing symptoms of casing mold, T. harzianum, in comparison with actinobacteria (24.2 and 11.8%, respectively). No significant differences between efficacies of S. flavovirens A06 and the fungicide prochloraz-Mn against T. aggressivum were revealed. These results imply that S. flavovirens A06 can be used to increase mushroom yield and contribute to disease control against the aggressive compost green mold disease caused by Trichoderma aggressivum.