Vineyards of southern France and northernPhytoplasmas are phloem-restricted wall-less bacteria pathogenic to many plant species worldwide (37, 52). Phytoplasmas can be spread both by hemipteran insect vectors (63) and by vegetative multiplication of infected-plant material. Controlling phytoplasma-induced diseases in perennial crops depends on field surveys and implementation of prophylactic sanitary measures requiring sensitive and specific detection of phytoplasmas in plants. Genetically different phytoplasmas can infect the same plant species; therefore, precise identification and typing of phytoplasma strains are necessary to ascertain the causes and origin of new outbreaks and predict the route of disease spread.Vineyards in southern France, northern Italy, and Spain are affected by the flavescence dorée (FD) phytoplasma, a quarantine pathogen of grapevine (7,8,16,24). The classification of phytoplasmas, which are uncultivable and currently described under the provisional genus "Candidatus Phytoplasma," is mainly based on 16S rRNA gene phylogeny, genomic diversity, and plant and insect host ranges (32,36,59). The FD phytoplasma belongs to the 16SrV taxonomic group (36). Members of this group share high 16S rRNA gene sequence similarity (34, 38), but the group consists of phytoplasmas with an important variety of specific biological niches restricted to woody perennial hosts. "Ca. Phytoplasma ulmi" is responsible for yellows of elm species in North America and Europe (38) and "Ca. Phytoplasma ziziphi" is the agent of jujube witches'-broom and cherry lethal yellows in Asia (34,38). In Europe, other phytoplasmas of group 16SrV are mainly infecting grapevine (23,43), alder (46,51), blackberry (26,50), Spartium, and eucalyptus (44,45). Most of the insect vectors naturally disseminating group 16SrV phytoplasmas have been identified. The elm yellows phytoplasmas are transmitted in North America by Scaphoideus luteolus (Van Duzee) (5) and in Europe by Macropsis mendax (Fieber) (15), whereas FD phytoplasmas are specifically transmitted by Scaphoideus titanus (Ball) (53, 58) and rubus stunt phytoplasma by Macropsis fuscula (Zetterstedt) (26). Phytoplasmas associated with Palatinate grapevine yellows (PGY) and alder yellows (AldY) are both transmitted by the alder leafhopper Oncopsis alni (Schrank) (41, 42) and were classified as members of the group 16SrV on the basis of their high 16S rRNA gene and secY sequence similarity to the corresponding genes of FD phytoplasmas (2, 3).The genomic diversity in this phytoplasma group was recently examined. Sequence and restriction fragment length polymorphism (RFLP) analysis of the 16S rRNA genes and the 16S-23S intergenic spacer allowed differentiation of two differ-* Corresponding author. Mailing
Flavescence doré e (FD) is a European quarantine grapevine disease transmitted by the Deltocephalinae leafhopper Scaphoideus titanus. Whereas this vector had been introduced from North America, the possible European origin of FD phytoplasma needed to be challenged and correlated with ecological and genetic drivers of FD emergence. For that purpose, a survey of genetic diversity of these phytoplasmas in grapevines, S. titanus, black alders, alder leafhoppers and clematis were conducted in five European countries. Out of 132 map genotypes, only 11 were associated to FD outbreaks, three were detected in clematis, whereas 127 were detected in alder trees, alder leafhoppers or in grapevines out of FD outbreaks. Most of the alder trees were found infected, including 8% with FD genotypes M6, M38 and M50, also present in alders neighboring FD-free vineyards and vineyard-free areas. The Macropsinae Oncopsis alni could transmit genotypes unable to achieve transmission by S. titanus, while the Deltocephalinae Allygus spp. and Orientus ishidae transmitted M38 and M50 that proved to be compatible with S. titanus. Variability of vmpA and vmpB adhesin-like genes clearly discriminated 3 genetic clusters. Cluster Vmp-I grouped genotypes only transmitted by O. alni, while clusters Vmp-II and-III grouped genotypes transmitted by Deltocephalinae leafhoppers. Interestingly, adhesin repeated domains evolved independently in cluster Vmp-I, whereas in clusters Vmp-II and-III showed recent duplications. Latex beads coated with various ratio of VmpA of clusters II and I, showed that cluster
A polymerase chain reaction procedure was developed which enables specific amplification of a ribosomal sequence from the mycoplasmalike organism (MLO) associated with German grapevine yellows (Vergilbungskrankheit, VK) and stolbur-related diseases of solanaceous plants. Successful amplification from all samples prepared from various cultivars collected in different viticultural areas indicates that the causal agent is a relatively homogeneous organism. Amplification was also achieved with template DNA prepared from naturally infected weeds in vineyards such as Convolvolus arvensis and Solanum nigrum, and from the planthopper Hyalesthes obsoletus that was collected in the vineyards. Feeding of insects of this species on grapevine seedlings resulted in the development of typical yellows symptoms by the grapes. H. obsoletus could therefore be identified as a vector of Vergilbungskrankheit.Abbreviations: FD = Flavescence dor6e; GY --Grapevine yellows; MLO --Mycoplasmalike organism; PCR --Polymerase chain reaction; RFLP -restriction fragment length polymorphism; VK --Vergilbungskrankheit (German grapevine yellows).
Dissemination of vector-transmitted pathogens depend on the survival and dispersal of the vector and the vector's ability to transmit the pathogen, while the host range of vector and pathogen determine the breath of transmission possibilities. In this study, we address how the interaction between dispersal and plant fidelities of a pathogen (stolbur phytoplasma tuf-a) and its vector (Hyalesthes obsoletus: Cixiidae) affect the emergence of the pathogen. Using genetic markers, we analysed the geographic origin and range expansion of both organisms in Western Europe and, specifically, whether the pathogen's dissemination in the northern range is caused by resident vectors widening their host-plant use from field bindweed to stinging nettle, and subsequent host specialisation. We found evidence for common origins of pathogen and vector south of the European Alps. Genetic patterns in vector populations show signals of secondary range expansion in Western Europe leading to dissemination of tuf-a pathogens, which might be newly acquired and of hybrid origin. Hence, the emergence of stolbur tuf-a in the northern range was explained by secondary immigration of vectors carrying stinging nettle-specialised tuf-a, not by widening the host-plant spectrum of resident vectors with pathogen transmission from field bindweed to stinging nettle nor by primary co-migration from the resident vector's historical area of origin. The introduction of tuf-a to stinging nettle in the northern range was therefore independent of vector's host-plant specialisation but the rapid pathogen dissemination depended on the vector's host shift, whereas the general dissemination elsewhere was linked to plant specialisation of the pathogen but not of the vector.
Within the past 10 years, the yellows disease ‘bois noir’ (BN) has become one of the commercially most important diseases of grapevine [Vitis vinifera L. (Vitaceae)] in Europe. Infection pressure is caused by phytoplasmas of the stolbur 16SrXII‐A group that are transmitted by a planthopper vector, Hyalesthes obsoletus Signoret (Homoptera: Auchenorrhyncha). Infestation happens as an accidental side‐effect of the feeding behaviour of the vector, as vector and pathogen proliferation is dependent on other plants. In Germany, the increase of BN is correlated with the use of a new host plant by the vector, increase in abundance of the vector on the new host plant, and dissemination of host plant‐specific pathogen strains. In this article, we investigate geographic and host‐associated range expansion of the vector. We test whether host‐plant utilization in Germany, hence the increase in BN, is related to genetic host races of the vector and, if so, whether these have evolved locally or have immigrated from southern populations that traditionally use the new host plant. The genetic population analysis demonstrates a recent expansion and circum‐alpine invasion of H. obsoletus into German and northern French wine‐growing regions, which coincides with the emergence of BN. No H. obsoletus mitochondrial DNA haplotype host‐plant affiliation was found, implying that the ability to use alternative host plants is genetically intrinsic to H. obsoletus. However, subtle yet significant random amplified polymorphic DNA (RAPD) genetic differentiation was found among host plant populations. When combined, these results suggest that a geographic range expansion of H. obsoletus only partly explains the increase of BN, and that interactions with host plants also occur. Further possible beneficial factors to H. obsoletus, such as temperature increase and phytoplasma interactions, are discussed.
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