Multilocus Genotyping of 'Candidatus Phytoplasma solani' Associated with Rubbery Taproot Disease of Sugar Beet in the Pannonian Plain

Geographical variations, prevalence, and molecular dynamics of fastidious phloem-limited pathogens infecting sugar beet across Central Europe

In Europe, two fastidious phloem-limited pathogens, 'Candidatus Phytoplasma solani' (16SrXII-A) and 'Candidatus Arsenophonus phytopathogenicus', are associated with rubbery taproot disease (RTD) and syndrome basses richesses (SBR) of sugar beet, respectively. Both diseases can significantly reduce yield, especially when accompanied by root rot fungi. This study investigates the presence, geographic distribution and genetic traits of fastidious pathogens and the accompanying fungus, Macrophomina phaseolina, found on sugar beet across four geographically separated plains spanning seven countries in Central Europe. The survey revealed variable incidences of symptoms linked to these fastidious pathogens in the Pannonian and Wallachian Plains, sporadic occurrence in the North European Plain, and no symptomatic sugar beet in the Bohemian Plain. Molecular analyses unveiled the occurrence of both 'Ca. P. solani' and 'Ca. A. phytopathogenicus' throughout Central Europe, with a predominance of the phytoplasma. These fastidious pathogens were detected in all six countries surveyed within the Pannonian and Wallachian Plains, with only a limited presence of various phytoplasmas was found in the North European Plain, while no fastidious pathogens were detected in Bohemia, aligning with observed symptoms. While 16S rDNA sequences of 'Ca. P. solani' remained highly conserved, multi-locus characterization of two more variable loci (tuf and stamp) unveiled distinct variability patterns across the plains. Notably, the surprising lack of variability of tuf and stamp loci within Central Europe, particularly the Pannonian Plain, contrasted their high variability in Eastern and Western Europe, corresponding to epidemic and sporadic occurrence, respectively. The current study provides valuable insights into the genetic dynamics of 'Ca. P. solani' in Central Europe, and novel findings of the presence of 'Ca. A. phytopathogenicus' in five countries (Slovakia, Czech Republic, Austria, Serbia, and Romania) and M. phaseolina in sugar beet in Slovakia. These findings emphasize the need for further investigation of vector-pathogen(s)-plant host interactions and ecological drivers of disease outbreaks.

Prevalence in Potato of 'Candidatus Arsenophonus Phytopathogenicus' and 'Candidatus Phytoplasma Solani' and Their Transmission via Adult Pentastiridius leporinus

The planthopper Pentastiridius leporinus (Hempiptera: Cixiidae) is the main vector of two bacterial pathogens: the γ-proteobacterium 'Candidatus Arsenophonus phytopathogenicus' and the stolbur phytoplasma 'Candidatus Phytoplasma solani'. These pathogens cause the disease syndrome basses richesses (SBR) in sugar beet (Beta vulgaris), which reduces the yields and sugar content. In 2022, potato (Solanum tuberosum) fields were found to be colonized by P. leporinus, and the transmission of Arsenophonus was confirmed, resulting in symptoms like wilting, yellow leaves, and rubbery tubers. We monitored both pathogens in Southwest Germany in 2022 and 2023. This revealed their widespread presence in potato tubers, although there were differences in regional prevalence. The broad prevalence of Arsenophonus was maintained in 2023, whereas the prevalence of stolbur increased in most locations. We confirmed that P. leporinus adults can transmit both pathogens to potatoes, but neither pathogen reduced the germination rate of tubers, and no plants showed abnormal growth after germination. Arsenophonus was not detected in germinated shoots, but 5.4% contained stolbur, emphasizing the need for plant material testing to maintain phytosanitary conditions.

Plants under Siege: Investigating the Relevance of 'Ca. P. solani' Cixiid Vectors through a Multi-Test Study

Crop losses caused by the plant pathogenic bacterium 'Candidatus Phytoplasma solani' (CaPsol) underscore the need to better understand its perplexing epidemiological pathways. Hyalesthes obsoletus (Hemiptera, Cixiidae) is a prominent CaPsol vector with three plant associations in Serbia (ex Urtica dioica/HobsUd; ex Convolvulus arvensis/HobsCa; ex Crepis foetida/HobsCf). Another cixiid planthopper, Reptalus quinquecostatus (Dufour), has been recently confirmed as a noteworthy CaPsol vector. A multi-test study assessed the relevance of H. obsoletus associations and R. quinquecostatus populations from Crataegus monogyna and Prunus spinosa in CaPsol occurrence in sugar beet, maize, and tobacco. Molecular typing of the CaPsol strains transmitted to test plants in experimental trials provided the first evidence of HobsUd transmitting CaPsol tuf-a type to sugar beet, HobsCa infecting maize and tobacco with tuf-b type, and HobsCf transmitting CaPsol tuf-b to maize. Affiliation of R. quinquecostatus with the specific CaPsol genotype, dSTOLg, was reaffirmed in this study. The possible involvement of R. quinquecostatus in maize redness disease and tobacco stolbur was suggested, given that this cixiid was identified as a vector of CaPsol to these crops. The obtained results indicate that the tested vectors pose a threat to cultivated plants in Serbia, underscoring the need to recognize their relevance in CaPsol disease occurrences.

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 Šančevi (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 Šančevi 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.

A biotroph sets the stage for a necrotroph to play: 'Candidatus Phytoplasma solani' infection of sugar beet facilitated Macrophomina phaseolina root rot

'Candidatus Phytoplasma solani' (stolbur phytoplasma) is associated with rubbery taproot disease (RTD) of sugar beet (Beta vulgaris L.), while Macrophomina phaseolina is considered the most important root rot pathogen of this plant in Serbia. The high prevalence of M. phaseolina root rot reported on sugar beet in Serbia, unmatched elsewhere in the world, coupled with the notorious tendency of RTD-affected sugar beet to rot, has prompted research into the relationship between the two diseases. This study investigates the correlation between the occurrence of sugar beet RTD and the presence of root rot fungal pathogens in a semi-field 'Ca. P. solani' transmission experiment with the cixiid vector Reptalus quinquecostatus (Dufour), in addition to naturally infected sugar beet in the open field. Our results showed that: (i) Reptalus quinquecostatus transmitted 'Ca. P. solani' to sugar beet which induced typical RTD root symptoms; (ii) Macrophomina phaseolina root rot was exclusively present in 'Ca. P. solani'-infected sugar beet in both the semi-field experiment and naturally infected sugar beet; and that (iii) even under environmental conditions favorable to the pathogen, M. phaseolina did not infect sugar beet, unless the plants had been previously infected with phytoplasma.

Biology and Rearing of an Emerging Sugar Beet Pest: The Planthopper Pentastiridius leporinus

Simple Summary

Syndrome des Basses Richesses (SBR) is a bacterial yellowing disease of sugar beet (Beta vulgaris) that reduces the sugar content of the root tissue significantly. There is currently neither a cure for the disease nor effective control measures for the main insect vector, the Cixiid planthopper Pentastiridius leporinus. SBR is therefore spreading in Germany and has caused substantial yield losses in recent years. The development of effective control measures requires a better understanding of P. leporinus biology, including its life cycle and feeding behavior. We therefore established methods for the continuous mass rearing of P. leporinus. In host plant choice experiments, we found that the nymphs are polyphagous when offered sugar beet, winter wheat or maize. Life cycle data indicated that adult planthoppers emerge 170 days after hatching, having passed through five larval instar stages. We developed a state-of-the-art qRT-PCR protocol using TaqMan probes to study the prevalence of two bona fide SBR pathogens: Candidatus Arsenophonus phytopathogenicus (Gammaproteobacteria) and Candidatus Phytoplasma solani (stolbur phytoplasma). These were identified during field studies in newly-infected regions of Rhineland-Palatinate and south Hesse, Germany, where we also observed insect mobility patterns in two consecutive years, including the abundance of adults at four locations and soil-depth monitoring of nymphs. We documented the spread of P. leporinus northward and eastward in Germany, accompanied by an increase in the number of SBR-carrying planthoppers. Interestingly, P. leporinus does not appear to hibernate during winter. Stolbur phytoplasma has a significant impact on SBR pathology in sugar beet.

Abstract

The rapid spread of the bacterial yellowing disease Syndrome des Basses Richesses (SBR) has a major impact on sugar beet (Beta vulgaris) cultivation in Germany, resulting in significant yield losses. SBR-causing bacteria are transmitted by insects, mainly the Cixiid planthopper Pentastiridius leporinus. However, little is known about the biology of this emerging vector, including its life cycle, oviposition, developmental stages, diapauses, and feeding behavior. Continuous mass rearing is required for the comprehensive analysis of this insect. Here we describe the development of mass rearing techniques for P. leporinus, allowing us to investigate life cycle and ecological traits, such as host plant choice, in order to design agronomic measures that can interrupt the life cycle of nymphs in the soil. We also conducted field studies in recently-infected regions of Rhineland-Palatinate and south Hesse, Germany, to study insect mobility patterns and abundance at four locations during two consecutive years. The soil-depth monitoring of nymphs revealed the movement of the instars through different soil layers. Finally, we determined the prevalence of SBR-causing bacteria by designing TaqMan probes specific for two bona fide SBR pathogens: Candidatus Arsenophonus phytopathogenicus (Gammaproteobacteria) and Candidatus Phytoplasma solani (stolbur phytoplasma). Our data suggest that P. leporinus is spreading northward and eastward in Germany, additionally, the abundance of SBR-carrying planthoppers is increasing. Interestingly, P. leporinus does not appear to hibernate during winter, and is polyphagous as a nymph. Stolbur phytoplasma has a significant impact on SBR pathology in sugar beet.

Molecular Identification and Characterization of Two Groups of Phytoplasma and Candidatus Liberibacter Asiaticus in Single or Mixed Infection of Citrus maxima on Hainan Island of China

Simple Summary

Based on the 16S rRNA and β-operon gene fragments, two subgroups of phytoplasma—CmPII-hn belonging to 16SrII-V and CmPXXXII-hn belonging to 16SrXXXII-D—and Candidatus Liberibacter asiaticus CmLas-hn were detected separately in 12, 2 and 6 out of 54 citrus samples of Citrus maxima, an important economic crop in Hainan Island, China, infected with Huanglongbing. Among the detection results, mixed infection of 16SrII-V subgroup phytoplasma and Candidatus Liberibacter asiaticus was identified in four samples, accounting for 7.4%. The CmPII-hn strain was in a cluster belonging to the 16SrII-V subgroup, with a 99% bootstrap value. The CmPXXXII-hn strain, Trema tomentosa witches’ broom phytoplasma, belonging to 16SrXXXII-D, and the other 16SrXXXII subgroup strains were in one cluster with a 99% bootstrap value. Sixteen variable loci were detected in the 16S rRNA genes of the tested 16SrXXXII group phytoplasma strains, of which two bases had an insertion/deletion. The CmLas-hn strain and Candidatus Liberibacter asiaticus were in one independent cluster with a 99% bootstrap value. In the study, Citrus maxima, showing yellowing and mottled leaves as disease symptoms, were found, which could have been infected separately by 16SrII-V and 16SrXXXII-D subgroup phytoplasmas or could have been subjected to mixed infection by 16SrII-V phytoplasmas and Candidatus Liberibacter asiaticus in China.

Abstract

The pathogens associated with citrus Huanglongbing symptoms, including yellowing and mottled leaves in Citrus maxima, an important economic crop on Hainan Island of China, were identified and characterized. In the study, detection, genetic variation and phylogenetic relationship analysis of the pathogens were performed based on 16S rRNA and β-operon gene fragments specific to phytoplasma and Candidatus Liberibacter asiaticus. The results indicated that the pathogens—such as phytoplasma strains of CmPII-hn belonging to the 16SrII-V subgroup and CmPXXXII-hn belonging to the 16SrXXXII-D subgroup, as well as Candidatus Liberibacter asiaticus strains CmLas-hn—were identified in the diseased plant samples, with numbers of 12, 2 and 6 out of 54, respectively. Among them, mixed infection with the 16SrII-V subgroup phytoplasma and Candidatus Liberibacter asiaticus was found in the study, accounting for 7.4% (four samples). The phytoplasma strains of CmPII-hn—Tephrosia purpurea witches’ broom, Melochia corchorifolia witches’ broom and Emilia sonchifolia witches’ broom—were clustered into one clade belonging to the 16SrII-V subgroup, with a 99% bootstrap value. The phytoplasma strains of CmPXXXII-hn and Trema tomentosa witches’ broom belonging to 16SrXXXII-D, and the other 16SrXXXII subgroup strains were clustered into one clade belonging to the 16SrXXXII group with a 99% bootstrap value. There were 16 variable loci in the 16S rRNA gene sequences of the tested 16SrXXXII group phytoplasma strains, of which two bases had an insertion/deletion. The strains of Candidatus Liberibacter asiaticus, identified in the study and the strains that had been deposited in GenBank, were in one independent cluster with a 99% bootstrap value. To our knowledge, this is the first report showing that Citrus maxima can be infected by 16SrII-V and16SrXXXII-D subgroup phytoplasmas in China. Moreover, this is also the first report in which the plants are co-infected by 16SrII-V subgroup phytoplasmas and Candidatus Liberibacter asiaticus. More comprehensive and detailed identification and characterization of the pathogens associated with the diseased symptoms in Citrus maxima on the island in China would be beneficial for epidemic monitoring and for the effective prevention and control of related plant diseases.

Application of TaqMan Real-Time PCR for Detecting 'Candidatus Arsenophonus Phytopathogenicus' Infection in Sugar Beet

The γ-proteobacterium ‘Candidatus Arsenophonus phytopathogenicus’ is assigned as the major pathogen of “Syndrome des basses richesses”, a sugar beet disease characterised by a reduction in the sugar content of taproots and biomass yield. Despite the economic impact of this bacteriosis, diagnostics for this important pathogen currently rely on end-point PCR detection. Herein, we introduce a TaqMan qPCR for diagnostics of the agent targeting genes encoding a heat shock protein of the Hsp20 family and mannose-6-phosphate isomerase. Quantitation with synthetic oligonucleotides as standard showed that the developed TaqMan qPCR assays enable the detection of up to 100 target copies. A comparison between the TaqMan qPCR and end-point PCR for ‘Ca. A. phytopathogenicus’ detection was carried out on 78 sugar beet samples from different locations in southern Germany. The newly developed assays enable the fast, reliable and sensitive detection of ‘Ca. A. phytopathogenicus’ in sugar beet.