Impact of vector dispersal and host-plant fidelity on the dissemination of an emerging plant pathogen

Parasitic Characteristics of Ticks (Acari: Ixodidae) Collected from Water Deer (Hydropotes inermis argyropus) and Spatiotemporal Distribution Prediction within Host-Influenced Cultivated Areas

Tick-borne diseases result from human-ixodid tick encounters, making it crucial to understand host-ixodid tick interactions and host-dependent distribution for epidemiology and prevention. This study examines water deer patterns and ixodid tick interactions in designated croplands of South Korea over two years, finding that the highest deer presence occurred in July and the lowest in May, during crop harvesting. Four tick species were identified, with Haemaphysalis longicornis being predominant (92.2%). Tick life stage analysis revealed peak nymphs and adults in July and larvae in October. Nymph abundance correlated positively with nearby water deer. MaxEnt biodiversity prediction results indicated wider water deer distribution in summer, reflecting their tendency to use multiple croplands. Areas with nymphs and adults aligned with predicted deer presence in summer, while larval areas aligned in autumn. Increased agroforestry expanded water deer habitats, enhancing tick dispersion. Prevention involved minimizing human-deer encounters by strategic land use in tick-prone areas. This comprehensive study provides insights into preventing severe fever with thrombocytopenia syndrome in agricultural workers, emphasizing the need for targeted interventions based on host behavior and tick life stages in different seasons.

Effects of insecticides and repellents on the spread of 'Candidatus Phytoplasma solani' under laboratory and field conditions

Recent outbreaks of 'Candidatus Phytoplasma solani' resulted in severe losses in potatoes, vegetable crops and grapevines in certain regions of Austria and constituted a major challenge for seed potato production. Therefore, the effects of various insecticides and insect deterrents on pathogen spread were studied both in laboratory and field experiments from 2018 to 2021. In laboratory transmission experiments, field captured Hyalesthes obsoletus were caged on differently treated Catharanthus roseus for five days. The insecticides lambda-cyhalothrin, deltamethrin, esfenvalerate, acetamiprid and chlorpyriphos showed the most rapid impact on insect survival and fully prevented phytoplasma transmission. The particle film forming products kaolin and diatomaceous earth had some effect. A transfer of the promising laboratory results to potato fields, however, was achieved to a limited extent only. Treatments with pyrethroids and acetamiprid every 8-10 days over the flight period of H. obsoletus roughly halved the number of symptomatic plants and tubers in case of moderately susceptible varieties and moderate infection pressure. In the event of susceptible varieties and high disease pressure, treatment effects were hardy discernible. In practical terms, the experiments indicate that insecticide applications alone are not sufficient to mitigate the disease. Spraying of diatomaceous earth and mineral oil did not affect disease incidence in the field.

Supplementary Information

The online version contains supplementary material available at 10.1007/s41348-023-00768-y.

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.

Detection and Multigene Typing of 'Candidatus Phytoplasma solani'-Related Strains Infecting Tomato and Potato Plants in Different Regions of Turkey

‘Candidatus Phytoplasma solani’ (‘Ca. P. solani’) is a crop pathogen that is a member of the 16SrXII-A ribosomal subgroup. It is also known as stolbur phytoplasma and causes yield losses in several important crops, especially in Solanaceous crops. Different strains of the pathogen are regularly reported all over the world, particularly in the Mediterranean region. In this study, the determination of genetic diversity for the pathogen infecting tomatoes and potatoes was carried out by using multilocus sequence typing analysis for the Tuf, SecY, and Vmp1 genes to gain insight into the epidemiology of ‘Ca. P. solani’ in Turkey. Genetic diversity of the phytoplasmas was investigated by sequence-based phylogenetic analyses and in silico RFLP analysis of related genes. It was determined that all ‘Ca. P. solani’-related strains infecting tomatoes and potatoes were tuf-b, which is linked to field bindweed (Convolvulus arvensis L.). Tomato or potato-infecting ‘Ca. P. solani’-related strains showed similarities with each other; however, the isolates collected from different plants showed genetic differences in terms of the SecY gene. This study indicates that the highest genetic variability of collected samples was found in the Vmp1 gene. RsaI-RFLP analysis of TYPH10F/R amplicons showed that potato-infecting ‘Ca. P. solani’-related strains were found to be similar to some existing V types. However, the V-type of tomato-infecting isolates is not similar to any previously reported V-type. The results indicate that there could be an important genetic diversity of ‘Ca. P. solani’-related phytoplasmas in Turkey. This could indicate various ways in which the pathogen has adapted to the two host plants as a consequence of the various Vmp1 gene rearrangements seen in these two plant hosts. Obtained results also indicate that the epidemiology of ‘Ca. P. solani’-related phytoplasmas in the tomato and potato agroecosystem may be better understood with the use of molecular data on the complex of vmp-types.

Multilocus Genotyping of ‘Candidatus Phytoplasma Solani’ Associated with Grapevine Bois Noir in Iran

Simple Summary

Understanding the epidemiological cycle of Bois noir (BN) associated with ‘Candidatus Phytoplasma solani’ is vital to managing the disease effectively. The genotyping of ‘Ca. P. solani’ strains, according to tuf, vmp1, and stamp genes, contributes to a better knowledge of the geographical distribution of BN. In this study, we provide information on the molecular variants of ‘Ca. P. solani’ in Iranian vineyards. We observed six known vmp1 variants and discovered two new ones, V24 and V27. This information is useful for future investigations to more accurately understand the epidemiological cycle(s) of BN in Iranian vineyards, contributing to the management of the disease.

Abstract

Grapevine Bois noir (BN) is associated with ‘Candidatus Phytoplasma solani’. It has been recorded in vineyards throughout Europe as well as in different countries in Asia, where it now constitutes a threat to Iranian viticulture. BN is strictly dependent on ‘Ca. P. solani’ strains, wild host plants, and insect vectors. The molecular typing of ‘Ca. P. solani’, based on the nonribosomal gene tuf and the two hypervariable markers vmp1 and stamp, is valuable for the reconstruction and clarification of the pathways of BN spread. In this study, an RFLP analysis was performed on the vmp1 gene, and a single-nucleotide polymorphism analysis confirmed new vmp types in ‘Ca. P. solani’. A stamp gene phylogenetic analysis allowed us to distinguish between the new genotype infections in the grapevines and the ‘weeds’ Convolvulus arvensis and Erigeron bonariensis in Iranian vineyards, highlighting the close genetic relatedness of the strains of ‘Ca. P. solani’ found in Iran and Azerbaijan. The most common genotype in the grapevines was tuf b/V24/stamp III, which was associated with C. arvensis. This information contributes toward the identification of further routes of introduction of ‘Ca. P. solani’ in Iran to sustain the control measures for the management of BN.

Geographical and Temporal Diversity of 'Candidatus Phytoplasma solani' in Wine-Growing Regions in Slovenia and Austria

As the causal agent of the grapevine yellows disease Bois noir, 'Candidatus Phytoplasma solani' has a major economic impact on grapevines. To improve the control of Bois noir, it is critical to understand the very complex epidemiological cycles that involve the multiple "Ca. P. solani" host plants and insect vectors, of which Hyalesthes obsoletus is the most important. In the present study, multiple genotyping of the tuf, secY, stamp, and vmp1 genes was performed. This involved archived grapevine samples that were collected during an official survey of grapevine yellows throughout the wine-growing regions of Slovenia (from 2003 to 2016), plus samples from Austrian grapevines, stinging nettle, field bindweed, and insect samples (collected from 2012 to 2019). The data show that the tuf-b2 type of the tuf gene has been present in eastern Slovenia since at least 2003. The hypotheses that the occurrence of the haplotypes varies due to the geographical position of Slovenia on the Italian-Slovenian Karst divide and that the haplotypes are similar between Slovenian and Austrian Styria were confirmed. The data also show haplotype changes for host plants and H. obsoletus associated with 'Ca. P. solani,' which might be linked to new epidemiological cycles of this phytoplasma that involve not just new plant sources and new insect vectors, but also climate and land-use changes.

Molecular Variability and Host Distribution of 'Candidatus Phytoplasma solani' Strains from Different Geographic Origins

The knowledge of phytoplasma genetic variability is a tool to study their epidemiology and to implement an effective monitoring and management of their associated diseases. ‘Candidatus Phytoplasma solani’ is associated with “bois noir” disease in grapevines, and yellowing and decline symptoms in many plant species, causing serious damages during the epidemic outbreaks. The epidemiology of the diseases associated with this phytoplasma is complex and related to numerous factors, such as interactions of the host plant and insect vectors and spreading through infected plant propagation material. The genetic variability of ‘Ca. P. solani’ strains in different host species and in different geographic areas during the last two decades was studied by RFLP analyses coupled with sequencing on vmp1, stamp, and tuf genes. A total of 119 strains were examined, 25 molecular variants were identified, and the variability of the studied genes was linked to both geographic distribution and year of infection. The crucial question in ‘Ca. P. solani’ epidemiology is to trace back the epidemic cycle of the infections. This study presents some relevant features about differential strain distribution useful for disease monitoring and forecasting, illustrating and comparing the phytoplasma molecular variants identified in various regions, host species, and time periods.

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

Rubbery taproot disease of sugar beet (RTD), associated with ‘Candidatus Phytoplasma solani’, appeared in 2020 on an epidemic scale in northern Serbia and southern Slovakia, situated at opposite edges of the Pannonian Plain. In the affected locations where the disease was assessed, symptomatic sugar beets were analysed for phytoplasma infection. Additionally, multilocus sequence analyses of ‘Ca. P. solani’ strains on epidemiologically informative marker genes (tuf, stamp and vmp1) were performed. Symptomatic sugar beets from other countries of the Pannonian Plain (Croatia, Hungary and Austria), one sample from Germany, and red beets from Serbia were included in the analyses. ‘Ca. P. solani’ was detected in sugar beet in all assessed countries, as well as in red beet. Molecular analyses revealed the high genetic variability of ‘Ca. P. solani’ with the presence of all four tuf-types (a, b1, b2 and d), 14 stamp genotypes (seven new) and five vmp1 profiles (one new). The most common multilocus genotype in Serbia, Slovakia, Croatia, and Hungary was dSTOLg (tuf-d/STOL/V2-TA). It was dominant on sites with epidemic RTD outbreaks in the Pannonian Plain and in several sugar beet fields with non-epidemic RTD occurrence suggesting the prevalence of a particular epidemiological pathway during the epidemic’s phases.

New Cross-Talks between Pathways Involved in Grapevine Infection with 'Candidatus Phytoplasma solani' Revealed by Temporal Network Modelling

Understanding temporal biological phenomena is a challenging task that can be approached using network analysis. Here, we explored whether network reconstruction can be used to better understand the temporal dynamics of bois noir, which is associated with 'Candidatus Phytoplasma solani', and is one of the most widespread phytoplasma diseases of grapevine in Europe. We proposed a methodology that explores the temporal network dynamics at the community level, i.e., densely connected subnetworks. The methodology offers both insights into the functional dynamics via enrichment analysis at the community level, and analyses of the community dissipation, as a measure that accounts for community degradation. We validated this methodology with cases on experimental temporal expression data of uninfected grapevines and grapevines infected with 'Ca. P. solani'. These data confirm some known gene communities involved in this infection. They also reveal several new gene communities and their potential regulatory networks that have not been linked to 'Ca. P. solani' to date. To confirm the capabilities of the proposed method, selected predictions were empirically evaluated.

Symptomatology, (Co)occurrence and Differential Diagnostic PCR Identification of 'Ca. Phytoplasma solani' and 'Ca. Phytoplasma convolvuli' in Field Bindweed

Field bindweed (Convolvulus arvensis) is one of the major natural plant hosts and reservoirs of ‘Candidatus Phytoplasma solani’ (‘Ca. P. solani’), the causal agent of plant diseases in diverse agricultural crops, including Bois noir (BN) disease of grapevine. Phylogenetically, the most closely related phytoplasma to ‘Ca. P. solani’, the ‘Ca. P. convolvuli’, induces disease in field bindweed that is known by its symptoms as bindweed yellows (BY). The occurrence, coinfection and symptoms association of the two phytoplasmas in shared host plants were the subject of this study. Specific primers for the amplification of the elongation factor Tu gene (tuf) were developed for the identification of ‘Ca. P. convolvuli’ (by conventional nested PCR), as well as primers for simultaneous detection of ‘Ca. P. solani’ and ‘Ca. P. convolvuli’ by duplex SYBR Green real-time PCR. Among symptomatic bindweed plants, 25 and 41% were infected with a single phytoplasma species, ‘Ca. P. solani’ and ‘Ca. P. convolvuli’, respectively, while 34% were infected with both phytoplasmas. None of the non-symptomatic control plants carried phytoplasma, while non-symptomatic plants from our previous epidemiological studies in BN-affected vineyards were confirmed to be infected solely with ‘Ca. P. solani’. Stamp gene typing revealed Rqg50 and Rqg31 ‘Ca. P. solani’ genotypes in plants coinfected with ‘Ca. P. convolvuli’, while three diverse genotypes (Rqg50, GGY and Rpm35) were identified in a single locality with symptomatic bindweeds infected solely with ‘Ca. P. solani’. Variations in symptoms and their association with each of the phytoplasmas are described and documented. The symptom of bushy appearance could be single out as specific for ‘Ca. P. convolvuli’ infection, while occurrence of ‘Ca. P. solani’ could not be unequivocally associated with specific alterations in infected bindweeds. The results are discussed in the context of the epidemiological and ecological complexity of ‘Ca. P. solani’-induced diseases and the relationship between the two phytoplasma relatives in shared host plant.

Surveying Potential Vectors of Apple Proliferation Phytoplasma: Faunistic Analysis and Infection Status of Selected Auchenorrhyncha Species

Apple proliferation (AP) is one of the economically most important diseases in European apple cultivation. The disease is caused by the cell-wall-less bacterium ' Candidatus Phytoplasma mali', which is transmitted by Cacopsylla picta (Foerster) and Cacopsylla melanoneura (Foerster) (Hemiptera: Psylloidea). In South Tyrol (Italy), severe outbreaks were documented since the 1990s. Infestation rates of AP do not always correlate with the population densities of the confirmed vectors, implying the presence of other, so far unknown, hemipterian vectors. By elucidating the species community of Auchenorrhyncha (Insecta: Hemiptera) at a regional scale, more than 31,000 specimens were captured in South Tyrolean apple orchards. The occurrence of 95 species was confirmed, whereas fourteen species are new records for this territory. Based on the faunistical data, more than 3600 individuals out of 25 species were analyzed using quantitative PCR to assess the presence of AP phytoplasma. The pathogen was sporadically detected in some individuals of different species, for example in Stictocephala bisonia Kopp and Yonk (Hemiptera: Membracidae). However, the concentration of phytoplasma was much lower than in infected C. picta and C. melanoneura captured in the same region, confirming the role of the latter mentioned psyllids as the main insect vectors of AP- phytoplasma in South Tyrol.

Epidemiological Investigations and Molecular Characterization of 'Candidatus Phytoplasma solani' in Grapevines, Weeds, Vectors and Putative Vectors in Western Sicily, (Southern Italy)

Bois noir is caused by 'Candidatus Phytoplasma solani', and it is one of the most important and widespread diseases in the Euro-Mediterranean region. There are complex interactions between phytoplasma and grapevines, weeds, and vectors. These ecological relationships can be tracked according to molecular epidemiology. The aims of the 2-year study (2014-2015) were to describe incidence and spatial distribution of Bois noir in a vineyard with three grapevine varieties in Sicily, and to identify the molecular types of the tuf and vmp1 genes in these naturally infected grapevines, according to the potential reservoir plants and vectors. Disease incidence in 2015 was significantly higher in 'Chardonnay' (up to 35%) than for 'Nero d'Avola' and 'Pinot noir' (<5%). All grapevine, weed, and insect samples were infected by 'Ca. P. solani' tuf-type b. Most of the collected insects were strictly related to Vitis spp. and belonged to Neoaliturus fenestratus, Empoasca spp., and Zygina rhamni. The characterization of the vmp1 gene revealed six different vmp types in grapevines (V1, V4, V9, V11, V12, V24), three in weeds (V4, V9, V11), and four in insects (V4, V9, V11, V24). Notably, V4, V9, appear both in hosts and vectors, with V9 predominant. Virtual restriction fragment length polymorphism (RFLP) analysis based on the nucleotide sequences supported the data of the conventional RFLP. Connections between the molecular data recorded in the vineyard ecosystems and the application of innovative tools based on the geostatistical analysis will contribute to further clarification of the specific ecological and epidemiological aspects of 'Ca. P. solani' in Sicily.

Sequence Analysis of New Tuf Molecular Types of 'Candidatus Phytoplasma Solani' in Iranian Vineyards

Grapevine Bois noir (BN) is caused by ‘Candidatus Phytoplasma solani’ (‘Ca. P. solani’) and is one of the most important phytoplasma diseases in the Euro-Mediterranean viticultural areas. The epidemiology of BN can include grapevine as a plant host and is usually transmitted via sap-sucking insects that inhabit herbaceous host plants. Tracking the spread of ‘Ca. P. solani’ strains is of great help for the identification of plant reservoirs and insect vectors involved in local BN outbreaks. The molecular epidemiology of ‘Ca. P. solani’ is primarily based on sequence analysis of the tuf housekeeping gene (which encodes elongation factor Tu). In this study, molecular typing of tuf, through restriction fragment length polymorphism and sequencing, was carried out on grapevine samples from Iranian vineyards. According to the molecular characterization, three molecular types—tuf b1, tuf b5 and tuf b6—were found, with tuf b1 being the most prominent. These data provide further knowledge of tuf gene diversity and question the ecological role of such “minor” tuf types in Iranian vineyards, which have been detected only in grapevines.

Multi-scale spatial genetic structure of the vector-borne pathogen 'Candidatus Phytoplasma prunorum' in orchards and in wild habitats

Inferring the dispersal processes of vector-borne plant pathogens is a great challenge because the plausible epidemiological scenarios often involve complex spread patterns at multiple scales. The spatial genetic structure of 'Candidatus Phytoplasma prunorum', responsible for European stone fruit yellows disease, was investigated by the application of a combination of statistical approaches to genotype data of the pathogen sampled from cultivated and wild compartments in three French Prunus-growing regions. This work revealed that the prevalence of the different genotypes is highly uneven both between regions and compartments. In addition, we identified a significant clustering of similar genotypes within a radius of 50 km or less, but not between nearby wild and cultivated Prunus. We also provide evidence that infected plants are transferred between production areas, and that both species of the Cacopsylla pruni complex can spread the pathogen. Altogether, this work supports a main epidemiological scenario where 'Ca. P. prunorum' is endemic in - and generally acquired from - wild Prunus by its immature psyllid vectors. The latter then migrate to shelter plants that epidemiologically connect sites less than 50 km apart by later providing infectious mature psyllids to their "migration basins". Such multi-scale studies could be useful for other pathosystems.

Molecular Typing of 'Candidatus Phytoplasma solani' in Iranian Vineyards

Grapevine (Vitis vinifera L.) is one of the most important horticultural crops in Iran, with >200,000 ha of cultivated area. Recently, outbreaks of the grapevine yellows Bois noir that is associated with phytoplasma strains related to 'Candidatus Phytoplasma solani' were recorded in several Iranian regions. This has resulted in severe economic losses. We carried out a survey in 2015, followed by collection of leaf samples from symptomatic grapevines and weeds. Because no information is available on the molecular epidemiology of 'Ca. P. solani' in Iran, multiple gene analyses were carried out here according to molecular characterization of the tuf and vmp1 genes. From the molecular characterization, all of the samples (i.e., grapevines, weeds) were infected with tuf b type. Detailed molecular characterization of the vmp1 gene (i.e., PCR-restriction fragment length polymorphism, sequence analysis) defined five molecular types: V1, V4, V10, V15, and V20. The abundance of Convolvulus arvensis in vineyards and detection of the same 'Ca. P. solani' molecular types in grapevines and weeds suggest that C. arvensis has a major role in Bois noir epidemiology of Iranian vineyards. Therefore, control strategies should be developed to manage these host plants to reduce inoculum sources of the phytoplasma in vineyards.

Detection of 'Candidatus Phytoplasma solani' in roots from Bois noir symptomatic and recovered grapevines

'Candidatus Phytoplasma solani' is the causal agent of Bois noir (BN) in grapevine (Vitis vinifera). It is usually detected in leaves, where typical disease symptoms are seen. However, little information is available on the presence of this phytoplasma in grapevine roots. Here, we investigated 'Ca. P. solani' in roots collected from 28 symptomatic, 27 recovered and eight asymptomatic grapevine plants. Protocols based on high-resolution melting (HRM) combined with real-time quantitative PCR (qPCR-HRM) and nested-qPCR-HRM were developed to identify 'Ca. P. solani' tuf-type variants with single nucleotide polymorphisms. In all, 21.4% of roots from symptomatic plants were positive to 'Ca. P. solani' using qPCR-HRM, and 60.7% with nested-qPCR HRM. Also, 7.4% of roots from recovered plants were positive using qPCR-HRM, which reached 44.4% using nested-qPCR HRM. These analyses identified tuf-type b1 on 88.2% of the positive samples from symptomatic grapevines, and 66.6% from recovered grapevines, with all other samples identified as tuf-type a. This study reports the presence of 'Ca. P. solani' in the roots of both symptomatic and recovered grapevines. These qPCR-HRM and nested-qPCR-HRM protocols can be applied to increase the sensitivity of detection of, and to simplify and speed up the screening for, 'Ca. P. solani' tuf-types.

Lavender Decline in France Is Associated with Chronic Infection by Lavender-Specific Strains of "Candidatus Phytoplasma solani"

The etiology and main pathways for the spread of lavender decline, an infectious disease affecting French lavender production since the 1960s, have remained unclear, hampering the development of efficient control strategies. An extensive survey of lavender fields led to the conclusion that “Candidatus Phytoplasma solani” was chronically infecting declining lavenders and was associated with large infectious populations of Hyalesthes obsoletus planthoppers living on the crop itself. Lavender appeared to be the main reservoir host for lavender-specific phytoplasma strains, an unusual feature for this phytoplasma, which usually propagates from reservoir weeds to various economically important crops. These results point out the necessity to protect young lavender fields from the initial phytoplasma inoculum coming from surrounding lavender fields or from infected nurseries and to promote agricultural practices that reduce the development of H. obsoletus vector populations.

Lavender decline compromises French lavender production, and preliminary data have suggested the involvement of “Candidatus Phytoplasma solani” in the etiology of the disease. In order to evaluate the epidemiological role of “Ca. Phytoplasma solani,” a 3-year survey was conducted in southeastern France. “Ca. Phytoplasma solani” was detected in 19 to 56% of the declining plants, depending on seasons and cultivars, and its prevalence was correlated with symptom severity. Autumn was more favorable than spring for phytoplasma detection, and “Ca. Phytoplasma solani” incidence was higher in Lavandula angustifolia than in Lavandula intermedia hybrids. Detection of the phytoplasma fluctuated over months, supporting the chronicity of infection. Three “Ca. Phytoplasma solani” secY genotypes, S17, S16, and S14, were the most prevalent in lavender fields and were also detected in nurseries, whereas strains detected in surrounding bindweed and wild carrots were mostly of the S1 and S4 genotypes. This suggests that lavender is the main pathogen reservoir of the epidemic. Adults and nymphs of the planthopper vector Hyalesthes obsoletus were commonly captured in lavender fields and were shown to harbor mainly the prevalent phytoplasma genotypes detected in lavenders. The “Ca. Phytoplasma solani” genotype S17 was transmitted to Catharanthus roseus periwinkle by naturally infected H. obsoletus. Finally, the inventory of the bacterial community of declining lavenders that tested negative for “Ca. Phytoplasma solani” by 16S rRNA deep sequencing ruled out the involvement of other phloem-limited bacterial pathogens.

IMPORTANCE The etiology and main pathways for the spread of lavender decline, an infectious disease affecting French lavender production since the 1960s, have remained unclear, hampering the development of efficient control strategies. An extensive survey of lavender fields led to the conclusion that “Candidatus Phytoplasma solani” was chronically infecting declining lavenders and was associated with large infectious populations of Hyalesthes obsoletus planthoppers living on the crop itself. Lavender appeared to be the main reservoir host for lavender-specific phytoplasma strains, an unusual feature for this phytoplasma, which usually propagates from reservoir weeds to various economically important crops. These results point out the necessity to protect young lavender fields from the initial phytoplasma inoculum coming from surrounding lavender fields or from infected nurseries and to promote agricultural practices that reduce the development of H. obsoletus vector populations.

Widespread plant specialization in the polyphagous planthopper Hyalesthes obsoletus (Cixiidae), a major vector of stolbur phytoplasma: Evidence of cryptic speciation

The stolbur phytoplasma vector Hyalesthes obsoletus is generally considered as a polyphagous species associated with numerous wild and cultivated plants. However, recent research in southeastern Europe, the distribution centre of H. obsoletus and the area of most stolbur-inflicted crop diseases, points toward specific host-plant associations of the vector, indicating specific vector-based transmission routes. Here, we study the specificity of populations associated with four host-plants using mitochondrial and nuclear genetic markers, and we evaluate the evolution of host-shifts in H. obsoletus. Host-plant use was confirmed for Convolvulus arvensis, Urtica dioica, Vitex agnus-castus and Crepis foetida. Mitochondrial genetic analysis showed sympatric occurrence of three phylogenetic lineages that were ecologically delineated by host-plant preference, but were morphologically inseparable. Nuclear data supported the existence of three genetic groups (Evanno’s ΔK(3) = 803.72) with average genetic membership probabilities > 90%. While populations associated with C. arvensis and U. dioica form a homogenous group, populations affiliated with V. agnus-castus and C. foetida constitute two independent plant-associated lineages. The geographical signal permeating the surveyed populations indicated complex diversification processes associated with host-plant selection and likely derived from post-glacial refugia in the eastern Mediterranean. This study provides evidence for cryptic species diversification within H. obsoletus sensu lato: i) consistent mitochondrial differentiation (1.1–1.5%) among host-associated populations in syntopy and in geographically distant areas, ii) nuclear genetic variance supporting mitochondrial data, and iii) average mitochondrial genetic distances among host-associated meta-populations are comparable to the most closely related, morphologically distinguishable species, i.e., Hyalesthes thracicus (2.1–3.3%).

Molecular Typing of Bois Noir Phytoplasma Strains in the Chianti Classico Area (Tuscany, Central Italy) and Their Association with Symptom Severity in Vitis vinifera 'Sangiovese'

Bois noir (BN) is the most widespread disease of the grapevine yellows complex in the Euro-Mediterranean area. BN is caused by 'Candidatus Phytoplasma solani' (BNp), transmitted from herbaceous plants to grapevine by polyphagous insect vectors. In this study, genetic diversity among BNp strains and their prevalence and possible association with grapevine symptom severity were investigated in a Sangiovese clone organic vineyard in the Chianti Classico area (Tuscany). Field surveys over 2 years revealed a range of symptom severity on grapevine and an increase of BN incidence. A TaqMan allelic discrimination assay detected only tufB type b among BNp strains, suggesting the prevalence of the bindweed-related ecology. Nucleotide sequence analyses of vmp1 and stamp genes identified 12 vmp1 and 16 stamp sequence variants, showing an overall positive selection for such genes. The prevalent genotype was Vm43/St10, reported for the first time in this study and closely related to strains identified only in the French Eastern Pyrenees. BNp strains identified in the examined vineyard and mostly grouped in separate bindweed-related phylogenetic clusters showed statistically significant differences in their distribution in grapevines exhibiting distinct symptom severity. These results suggest the possible occurrence of a range of virulence within BNp strain populations in the Chianti Classico area.

Genetic Variability of Stolbur Phytoplasma in Hyalesthes obsoletus (Hemiptera: Cixiidae) and its Main Host Plants in Vineyard Agroecosystems

Bois noir is an economically important grapevine yellows that is induced by 'Candidatus Phytoplasma solani' and principally vectored by the planthopper Hyalesthes obsoletus Signoret (Hemiptera: Cixiidae). This study explores the 'Ca. P. solani' genetic variability associated to the nettle-H. obsoletus and bindweed-H. obsoletus systems in vineyard agroecosystems of the central-eastern Italy. Molecular characterization of 'Ca. P. solani' isolates was carried out using polymerase chain reaction/restriction fragment length polymorphism to investigate the nonribosomal vmp1 gene. Seven phytoplasma vmp-types were detected among the host plants- and insect-associated field-collected samples. The vmp1 gene showed the highest polymorphism in the bindweed-H. obsoletus system, according to restriction fragment length polymorphism analysis, which is in agreement with nucleotide sequence analysis. Five vmp-types were associated with H. obsoletus from bindweed, of which one was solely restricted to planthoppers, with one genotype also in planthoppers from nettle. Type V12 was the most prevalent in both planthoppers and bindweed. H. obsoletus from nettle harbored three vmp-types, of which V3 was predominant. V3 was the only type detected for nettle. Our data demonstrate that planthoppers might have acquired some 'Ca. P. solani' profiles from other plant hosts before landing on nettle or bindweed. Overall, the different vmp1 gene rearrangements observed in these two plant hosts-H. obsoletus systems might represent different adaptations of the pathogen to the two host plants. Molecular information about the complex of vmp-types provides useful data for better understanding of Bois noir epidemiology in vineyard agroecosystem.

In-vineyard population structure of 'Candidatus Phytoplasma solani' using multilocus sequence typing analysis

'Candidatus Phytoplasma solani' is a phytoplasma of the stolbur group (16SrXII subgroup A) that is associated with 'Bois noir' and causes heavy damage to the quality and quantity of grapevine yields in several European countries, and particularly in the Mediterranean area. Analysis of 'Ca. P. solani' genetic diversity was carried out for strains infecting a cv. 'Chardonnay' vineyard, through multilocus sequence typing analysis for the vmp1, stamp and secY genes. Several types per gene were detected: seven out of 20 types for vmp1, six out of 17 for stamp, and four out of 16 for secY. High correlations were seen among the vmp1, stamp and secY typing with the tuf typing. However, no correlations were seen among the tuf and vmp1 types and the Bois noir severity in the surveyed grapevines. Grouping the 'Ca. P. solani' sequences on the basis of their origins (i.e., study vineyard, Italian regions, Euro-Mediterranean countries), dN/dS ratio analysis revealed overall positive selection for stamp (3.99, P=0.019) and vmp1 (2.28, P=0.001). For secY, the dN/dS ratio was 1.02 (P=0.841), showing neutral selection across this gene. Using analysis of the nucleotide sequencing by a Bayesian approach, we determined the population structure of 'Ca. P. solani', which appears to be structured in 3, 5 and 6 subpopulations, according to the secY, stamp and vmp1 genes, respectively. The high genetic diversity of 'Ca. P. solani' from a single vineyard reflects the population structure across wider geographical scales. This information is useful to trace inoculum source and movement of pathogen strains at the local level and over long distances.

An abundant 'Candidatus Phytoplasma solani' tuf b strain is associated with grapevine, stinging nettle and Hyalesthes obsoletus

Bois noir (BN) associated with 'Candidatus Phytoplasma solani' (Stolbur) is regularly found in Austrian vine growing regions. Investigations between 2003 and 2008 indicated sporadic presence of the confirmed disease vector Hyalesthes obsoletus and frequent infections of bindweed and grapevine. Infections of nettles were rare. In contrast present investigations revealed a mass occurrence of H. obsoletus almost exclusively on stinging nettle. The high population densities of H. obsoletus on Urtica dioica were accompanied by frequent occurrence of 'Ca. P. solani' in nettles and planthoppers. Sequence analysis of the molecular markers secY, stamp, tuf and vmp1 of stolbur revealed a single genotype named CPsM4_At1 in stinging nettles and more than 64 and 90 % abundance in grapevine and H. obsoletus, respectively. Interestingly, this genotype showed tuf b type restriction pattern previously attributed to bindweed associated 'Ca. P. solani' strains, but a different sequence assigned as tuf b2 compared to reference tuf b strains. All other marker genes of CPsM4_At1 clustered with tuf a and nettle derived genotypes verifying distinct nettle phytoplasma genotypes. Transmission experiments with H. obsoletus and Anaceratagallia ribauti resulted in successful transmission of five different strains including the major genotype to Catharanthus roseus and in transmission of the major genotype to U. dioica. Altogether, five nettle and nine bindweed associated genotypes were described. Bindweed types were verified in 34 % of grapevine samples, in few positive Reptalus panzeri, rarely in bindweeds and occasionally in Catharanthus roseus infected by H. obsoletus or A. ribauti. 'Candidatus Phytoplasma convolvuli' (bindweed yellows) was ascertained in nettle and bindweed samples.

Population structure of mountain pine beetle symbiont Leptographium longiclavatum and the implication on the multipartite beetle-fungi relationships

Over 18 million ha of forests have been destroyed in the past decade in Canada by the mountain pine beetle (MPB) and its fungal symbionts. Understanding their population dynamics is critical to improving modeling of beetle epidemics and providing potential clues to predict population expansion. Leptographium longiclavatum and Grosmannia clavigera are fungal symbionts of MPB that aid the beetle to colonize and kill their pine hosts. We investigated the genetic structure and demographic expansion of L. longiclavatum in populations established within the historic distribution range and in the newly colonized regions. We identified three genetic clusters/populations that coincide with independent geographic locations. The genetic profiles of the recently established populations in northern British Columbia (BC) and Alberta suggest that they originated from central and southern BC. Approximate Bayesian Computation supports the scenario that this recent expansion represents an admixture of individuals originating from BC and the Rocky Mountains. Highly significant correlations were found among genetic distance matrices of L. longiclavatum, G. clavigera, and MPB. This highlights the concordance of demographic processes in these interacting organisms sharing a highly specialized niche and supports the hypothesis of long-term multipartite beetle-fungus co-evolutionary history and mutualistic relationships.

Survival relative to new and ancestral host plants, phytoplasma infection, and genetic constitution in host races of a polyphagous insect disease vector

Dissemination of vectorborne diseases depends strongly on the vector's host range and the pathogen's reservoir range. Because vectors interact with pathogens, the direction and strength of a vector's host shift is vital for understanding epidemiology and is embedded in the framework of ecological specialization. This study investigates survival in host-race evolution of a polyphagous insect disease vector, Hyalesthes obsoletus, whether survival is related to the direction of the host shift (from field bindweed to stinging nettle), the interaction with plant-specific strains of obligate vectored pathogens/symbionts (stolbur phytoplasma), and whether survival is related to genetic differentiation between the host races. We used a twice repeated, identical nested experimental design to study survival of the vector on alternative hosts and relative to infection status. Survival was tested with Kaplan–Meier analyses, while genetic differentiation between vector populations was quantified with microsatellite allele frequencies. We found significant direct effects of host plant (reduced survival on wrong hosts) and sex (males survive longer than females) in both host races and relative effects of host (nettle animals more affected than bindweed animals) and sex (males more affected than females). Survival of bindweed animals was significantly higher on symptomatic than nonsymptomatic field bindweed, but in the second experiment only. Infection potentially had a positive effect on survival in nettle animals but due to low infection rates the results remain suggestive. Genetic differentiation was not related to survival. Greater negative plant-transfer effect but no negative effect of stolbur in the derived host race suggests preadaptation to the new pathogen/symbiont strain before strong diversifying selection during the specialization process. Physiological maladaptation or failure to accept the ancestral plant will have similar consequences, namely positive assortative mating within host races and a reduction in the likelihood of oviposition on the alternative plant and thus the acquisition of alternative stolbur strains.

Rust fungi and global change

Rust fungi are important components of ecological communities and in ecosystem function. Their unique life strategies as biotrophic pathogens with complicated life cycles could make them vulnerable to global environmental change. While there are gaps in our knowledge, especially in natural plant–rust systems, this review of the exposure of rust fungi to global change parameters revealed that some host–rust relationships would decline under predicted environmental change scenarios, whereas others would either remain unchanged or become more prevalent. Notably, some graminicolous rusts are negatively affected by higher temperatures and increased concentrations of atmospheric CO2. An increase of atmospheric O3 appears to favour rust diseases on trees but not those on grasses. Combined effects of CO2 and O3 are intermediary. The most important global drivers for the geographical and host plant range expansion and prevalence of rusts, however, are global plant trade, host plant genetic homogenization and the regular occurrence of conducive environmental conditions, especially the availability of moisture. However, while rusts thrive in high-humidity environments, they can also survive in desert habitats, and as a group their environmental tolerance is large, with no conclusive change in their overall prevalence predictable to date.

Sympatric diversification vs. immigration: deciphering host-plant specialization in a polyphagous insect, the stolbur phytoplasma vector Hyalesthes obsoletus (Cixiidae)

The epidemiology of vector transmitted plant diseases is highly influenced by dispersal and the host-plant range of the vector. Widening the vector's host range may increase transmission potential, whereas specialization may induce specific disease cycles. The process leading to a vector's host shift and its epidemiological outcome is therefore embedded in the frameworks of sympatric evolution vs. immigration of preadapted populations. In this study, we analyse whether a host shift of the stolbur phytoplasma vector, Hyalesthes obsoletus from field bindweed to stinging nettle in its northern distribution range evolved sympatrically or by immigration. The exploitation of stinging nettle has led to outbreaks of the grapevine disease bois noir caused by a stinging nettle-specific phytoplasma strain. Microsatellite data from populations from northern and ancestral ranges provide strong evidence for sympatric host-race evolution in the northern range: Host-plant associated populations were significantly differentiated among syntopic sites (0.054 < F(HT) < 0.098) and constant over 5 years. While gene flow was asymmetric from the old into the predicted new host race, which had significantly reduced genetic diversity, the genetic identity between syntopic host-race populations in the northern range was higher than between these populations and syntopic populations in ancestral ranges, where there was no evidence for genetic host races. Although immigration was detected in the northern field bindweed population, it cannot explain host-race diversification but suggests the introduction of a stinging nettle-specific phytoplasma strain by plant-unspecific vectors. The evolution of host races in the northern range has led to specific vector-based bois noir disease cycles.