Clonal Expansion and Migration of a Highly Virulent, Defoliating Lineage of Verticillium dahliae

Polyethyleneimine-coated MXene quantum dots improve cotton tolerance to Verticillium dahliae by maintaining ROS homeostasis

Verticillium dahliae is a soil-borne hemibiotrophic fungal pathogen that threatens cotton production worldwide. In this study, we assemble the genomes of two V. dahliae isolates: the more virulence and defoliating isolate V991 and nondefoliating isolate 1cd3-2. Transcriptome and comparative genomics analyses show that genes associated with pathogen virulence are mostly induced at the late stage of infection (Stage II), accompanied by a burst of reactive oxygen species (ROS), with upregulation of more genes involved in defense response in cotton. We identify the V991-specific virulence gene SP3 that is highly expressed during the infection Stage II. V. dahliae SP3 knock-out strain shows attenuated virulence and triggers less ROS production in cotton plants. To control the disease, we employ polyethyleneimine-coated MXene quantum dots (PEI-MQDs) that possess the ability to remove ROS. Cotton seedlings treated with PEI-MQDs are capable of maintaining ROS homeostasis with enhanced peroxidase, catalase, and glutathione peroxidase activities and exhibit improved tolerance to V. dahliae. These results suggest that V. dahliae trigger ROS production to promote infection and scavenging ROS is an effective way to manage this disease. This study reveals a virulence mechanism of V. dahliae and provides a means for V. dahliae resistance that benefits cotton production.

Population genomic analysis reveals geographic structure and climatic diversification for Macrophomina phaseolina isolated from soybean and dry bean across the United States, Puerto Rico, and Colombia

Macrophomina phaseolina causes charcoal rot, which can significantly reduce yield and seed quality of soybean and dry bean resulting from primarily environmental stressors. Although charcoal rot has been recognized as a warm climate-driven disease of increasing concern under global climate change, knowledge regarding population genetics and climatic variables contributing to the genetic diversity of M. phaseolina is limited. This study conducted genome sequencing for 95 M. phaseolina isolates from soybean and dry bean across the continental United States, Puerto Rico, and Colombia. Inference on the population structure using 76,981 single nucleotide polymorphisms (SNPs) revealed that the isolates exhibited a discrete genetic clustering at the continental level and a continuous genetic differentiation regionally. A majority of isolates from the United States (96%) grouped in a clade with a predominantly clonal genetic structure, while 88% of Puerto Rican and Colombian isolates from dry bean were assigned to a separate clade with higher genetic diversity. A redundancy analysis (RDA) was used to estimate the contributions of climate and spatial structure to genomic variation (11,421 unlinked SNPs). Climate significantly contributed to genomic variation at a continental level with temperature seasonality explaining the most variation while precipitation of warmest quarter explaining the most when spatial structure was accounted for. The loci significantly associated with multivariate climate were found closely to the genes related to fungal stress responses, including transmembrane transport, glycoside hydrolase activity and a heat-shock protein, which may mediate climatic adaptation for M. phaseolina. On the contrary, limited genome-wide differentiation among populations by hosts was observed. These findings highlight the importance of population genetics and identify candidate genes of M. phaseolina that can be used to elucidate the molecular mechanisms that underly climatic adaptation to the changing climate.

Attack of the clones: Population genetics reveals clonality of Colletotrichum lupini, the causal agent of lupin anthracnose

Colletotrichum lupini, the causative agent of lupin anthracnose, affects lupin cultivation worldwide. Understanding its population structure and evolutionary potential is crucial to design successful disease management strategies. The objective of this study was to employ population genetics to investigate the diversity, evolutionary dynamics, and molecular basis of the interaction of this notorious lupin pathogen with its host. A collection of globally representative C. lupini isolates was genotyped through triple digest restriction site-associated DNA sequencing, resulting in a data set of unparalleled resolution. Phylogenetic and structural analysis could distinguish four independent lineages (I-IV). The strong population structure and high overall standardized index of association (r̅_d ) indicates that C. lupini reproduces clonally. Different morphologies and virulence patterns on white lupin (Lupinus albus) and Andean lupin (Lupinus mutabilis) were observed between and within clonal lineages. Isolates belonging to lineage II were shown to have a minichromosome that was also partly present in lineage III and IV, but not in lineage I isolates. Variation in the presence of this minichromosome could imply a role in host-pathogen interaction. All four lineages were present in the South American Andes region, which is suggested to be the centre of origin of this species. Only members of lineage II have been found outside South America since the 1990s, indicating it as the current pandemic population. As a seedborne pathogen, C. lupini has mainly spread through infected but symptomless seeds, stressing the importance of phytosanitary measures to prevent future outbreaks of strains that are yet confined to South America.

Genome Sequence Data of MAT1-1 and MAT1-2 Idiomorphs from Verticillium dahliae

Though Verticillium dahliae is an asexually reproducing fungus, it is considered heterothallic owing to the presence of only one of the two mating-type idiomorphs (MAT1-1 or MAT1-2) in individual isolates. But sexual reproduction has never been observed either in nature or in the laboratory. All of the genomic information in the literature thus far has therefore come from studies on isolates carrying only the MAT1-2 idiomorph. Herein, we sequenced and compared high-quality reference genomes of MAT1-1 strain S011 and MAT1-2 strain S023 obtained from the same sunflower field. The two genomic sequences displayed high synteny, and encoded similar number genes, a similarity especially notable among pathogenicity-related genes. Homolog analysis between these two genomes revealed that 80% of encoded genes are highly conserved (95% identity and coverage), but only 20% of the single copy genes were identical. These novel genome resources will support the analysis of the structure and function of the two idiomorphs and provide valuable tools to elucidate the evolution and potential mechanisms of sexual reproduction in V. dahliae.

Key Insights and Research Prospects at the Dawn of the Population Genomics Era for Verticillium dahliae

The genomics era has ushered in exciting possibilities to examine the genetic bases that undergird the characteristic features of Verticillium dahliae and other plant pathogens. In this review, we provide historical perspectives on some of the salient biological characteristics of V. dahliae, including its morphology, microsclerotia formation, host range, disease symptoms, vascular niche, reproduction, and population structure. The kaleidoscopic population structure of this pathogen is summarized, including different races of the pathogen, defoliating and nondefoliating phenotypes, vegetative compatibility groupings, and clonal populations. Where possible, we place the characteristic differences in the context of comparative and functional genomics analyses that have offered insights into population divergence within V. dahliae and the related species.Current challenges are highlighted along with some suggested future population genomics studies that will contribute to advancing our understanding of the population divergence in V. dahliae.

Phylogeographic Approaches to Characterize the Emergence of Plant Pathogens

Phylogeography combines geographic information with phylogenetic and population genomic approaches to infer the evolutionary history of a species or population in a geographic context. This approach has been instrumental in understanding the emergence, spread, and evolution of a range of plant pathogens. In particular, phylogeography can address questions about where a pathogen originated, whether it is native or introduced, and when and how often introductions occurred. We review the theory, methods, and approaches underpinning phylogeographic inference and highlight applications providing novel insights into the emergence and spread of select pathogens. We hope that this review will be useful in assessing the power, pitfalls, and opportunities presented by various phylogeographic approaches.

Genetic Differentiation of Verticillium dahliae Populations Recovered from Symptomatic and Asymptomatic Hosts

Verticillium dahliae is a soilborne fungal pathogen affecting many economically important crops that can also infect weeds and rotational crops with no apparent disease symptoms. The main research goal was to test the hypothesis that V. dahliae populations recovered from asymptomatic rotational crops and weed species are evolutionarily and genetically distinct from symptomatic hosts. We collected V. dahliae isolates from symptomatic and asymptomatic hosts growing in fields with histories of Verticillium wilt of potato in Israel and Pennsylvania (United States), and used genotyping-by-sequencing to analyze the evolutionary history and genetic differentiation between populations of different hosts. A phylogeny inferred from 26,934 single-nucleotide polymorphisms (SNPs) in 126 V. dahliae isolates displayed a highly clonal structure correlated with vegetative compatibility groups, and isolates grouped in lineages 2A, 2B⁸²⁴, 4A, and 4B, with 77% of the isolates in lineage 4B. The lineages identified in this study were differentiated by host of origin; we found 2A, 2B⁸²⁴, and 4A only in symptomatic hosts but isolates from asymptomatic hosts (weeds, oat, and sorghum) grouped exclusively within lineage 4B, and were genetically indistinguishable from 4B isolates sampled from symptomatic hosts (potato, eggplant, and avocado). Using coalescent analysis of 158 SNPs of lineage 4B, we inferred a genealogy with clades that correlated with geographic origin. In contrast, isolates from asymptomatic and symptomatic hosts shared some of the same haplotypes and were not differentiated. We conclude that asymptomatic weeds and rotational hosts may be potential reservoirs for V. dahliae populations of lineage 4B, which are pathogenic to many cultivated hosts.

Host-specificity factors in plant pathogenic fungi

Fortunately, no fungus can cause disease on all plant species, and although some plant-pathogenic fungi have quite a broad host range, most are highly limited in the range of plant species or even cultivars that they cause disease in. The mechanisms of host specificity have been extensively studied in many plant-pathogenic fungi, especially in fungal pathogens causing disease on economically important crops. Specifically, genes involved in host specificity have been identified during the last few decades. In this overview, we describe and discuss these host-specificity genes. These genes encode avirulence (Avr) proteins, proteinaceous host-specific toxins or secondary metabolites. We discuss the genomic context of these genes, their expression, polymorphism, horizontal transfer and involvement in pathogenesis.

Epigenetic Regulation of Verticillium dahliae Virulence: Does DNA Methylation Level Play A Role?

Verticillium dahliae is the etiological agent of Verticillium wilt of olive. The virulence of Defoliating V. dahliae isolates usually displays differences and high plasticity. This work studied whether an epigenetic mechanism was involved in this plasticity. An inverse correlation between virulence and DNA methylation of protein-coding genes was found. A set of 831 genes was selected for their highly consistent inverse methylation profile and virulence in the five studied isolates. Of these genes, ATP-synthesis was highly represented, which indicates that the more virulent D isolates are, the more energy requirements they may have. Furthermore, there were numerous genes in the protein biosynthesis process: genes coding for the chromatin structure, which suggests that epigenetic changes may also affect chromatin condensation; many transmembrane transporter genes, which is consistent with denser compounds, traffic through membranes in more virulent isolates; a fucose-specific lectin that may play a role in the attachment to plant cell walls during the host infection process; and pathogenic cutinases that facilitate plant invasion and sporulation genes for rapid spreading alongside plants. Our findings support the notion that differences in the virulence of the Defoliating V. dahliae isolates may be controlled, at least to some extent, by an epigenetic mechanism.

Verticillium Wilt of Olive and its Control: What Did We Learn during the Last Decade?

Verticillium (Verticillium dahliae Kleb.) wilt is one of the most devastating diseases affecting olive (Olea europaea L. subsp. europaea var. europaea) cultivation. Its effective control strongly relies on integrated management strategies. Olive cultivation systems are experiencing important changes (e.g., high-density orchards, etc.) aiming at improving productivity. The impact of these changes on soil biology and the incidence/severity of olive pests and diseases has not yet been sufficiently evaluated. A comprehensive understanding of the biology of the pathogen and its populations, the epidemiological factors contributing to exacerbating the disease, the underlying mechanisms of tolerance/resistance, and the involvement of the olive-associated microbiota in the tree's health is needed. This knowledge will be instrumental to developing more effective control measures to confront the disease in regions where the pathogen is present, or to exclude it from V. dahliae-free areas. This review compiles the most recent advances achieved to understand the olive-V. dahliae interaction as well as measures to control the disease. Aspects such as the molecular basis of the host-pathogen interaction, the identification of new biocontrol agents, the implementation of "-omics" approaches to unravel the basis of disease tolerance, and the utilization of remote sensing technology for the early detection of pathogen attacks are highlighted.

Linking belowground microbial network changes to different tolerance level towards Verticillium wilt of olive

BACKGROUND

Verticillium wilt of olive (VWO) is caused by the soilborne fungal pathogen Verticillium dahliae. One of the best VWO management measures is the use of tolerant/resistant olive cultivars. Knowledge on the olive-associated microbiome and its potential relationship with tolerance to biotic constraints is almost null. The aims of this work are (1) to describe the structure, functionality, and co-occurrence interactions of the belowground (root endosphere and rhizosphere) microbial communities of two olive cultivars qualified as tolerant (Frantoio) and susceptible (Picual) to VWO, and (2) to assess whether these communities contribute to their differential disease susceptibility level.

RESULTS

Minor differences in alpha and beta diversities of root-associated microbiota were detected between olive cultivars regardless of whether they were inoculated or not with the defoliating pathotype of V. dahliae. Nevertheless, significant differences were found in taxonomic composition of non-inoculated plants' communities, "Frantoio" showing a higher abundance of beneficial genera in contrast to "Picual" that exhibited major abundance of potential deleterious genera. Upon inoculation with V. dahliae, significant changes at taxonomic level were found mostly in Picual plants. Relevant topological alterations were observed in microbial communities' co-occurrence interactions after inoculation, both at structural and functional level, and in the positive/negative edges ratio. In the root endosphere, Frantoio communities switched to highly connected and low modularized networks, while Picual communities showed a sharply different behavior. In the rhizosphere, V. dahliae only irrupted in the microbial networks of Picual plants.

CONCLUSIONS

The belowground microbial communities of the two olive cultivars are very similar and pathogen introduction did not provoke significant alterations in their structure and functionality. However, notable differences were found in their networks in response to the inoculation. This phenomenon was more evident in the root endosphere communities. Thus, a correlation between modifications in the microbial networks of this microhabitat and susceptibility/tolerance to a soilborne pathogen was found. Moreover, V. dahliae irruption in the Picual microbial networks suggests a stronger impact on the belowground microbial communities of this cultivar upon inoculation. Our results suggest that changes in the co-occurrence interactions may explain, at least partially, the differential VWO susceptibility of the tested olive cultivars. Video abstract.

Effect of Cultivar Resistance and Soil Management on Spatial-Temporal Development of Verticillium Wilt of Olive: A Long-Term Study

Verticillium wilt, caused by Verticillium dahliae, challenges olive cultivation and an Integrated Disease Management (IDM) approach is the best-suited tool to combat it. Since 1998, an IDM strategy in an orchard (called Granon, Spain) of the susceptible cv. Picual was conducted by increasing planting density with moderately resistant cv. Frantoio, chemical weed control, and replanting of dead olives with cv. Frantoio following soil solarization. The Verticillium wilt epidemic in Granon orchard was compared to the epidemic in a non-IDM orchard (called Ancla, Spain) with plowed soil and dead Picual olives replanted with the same cultivar. Field evaluations (2012-2013) showed an incidence and severity of the disease as Picual-Ancla > Picual-Granon > Frantoio-Granon. The spatiotemporal dynamics of the Verticillium epidemics from 1998 to 2010 were monitored with digital images using SIG. The annual tree mortalities were 5.6% for Picual olives in Ancla orchard, and 3.1 and 0.7% for Picual and Frantoio olives in Granon orchard, respectively. There was a negative relationship between the mortality of olive trees (%) by the pathogen and the height (m) above sea level. The annual mortality of cv. Picual olives was positively correlated with spring rainfalls. The Index of Dispersion and beta-binomial distribution showed aggregation of Verticillium-dead olives. In conclusion, this IDM strategy considerably reduced the disease in comparison with traditional agronomic practices.

Verticillium isaacii is a Pathogen and Endophyte of Potato and Sunflower in the Columbia Basin of Washington

The objective of this research was to test the hypothesis that Verticillium isaacii causes diseases of sunflower and potato plants. Two sunflower genotypes and one potato cultivar were inoculated with five V. isaacii isolates and three pathogenic V. dahliae isolates. Biomass, disease expression, and stem colonization were quantified. Overt wilt symptoms were observed on both sunflower genotypes and potato plants inoculated with a subset of the V. isaacii isolates. Biomass of both sunflower genotypes was not affected by V. isaacii infection. Tuber yields either decreased in response to infection by one V. isaacii isolate or were not affected by infection. Stems of sunflower and potato plants were infected by at least four of the five V. isaacii isolates. A new disease of sunflower and potato is documented. Evidence that V. isaacii exhibits different lifestyles including pathogenicity and endophytism is presented. Finally, this research documents variation in fungal lifestyles that can exist in samples from a single field.

Genetic Diversity of Verticillium dahliae Isolates From Mint Detected with Genotyping by Sequencing

Verticillium wilt is the most important disease threatening the commercial production of mint grown for essential oil. An important long-term goal for mint breeders is the production of cultivars with resistance to Verticillium wilt. Before that can be accomplished, a better understanding of the genetic variation within and among populations of Verticillium dahliae is needed. We characterized the extent of phenotypic and genetic diversity present in contemporary and archival populations of V. dahliae from mint fields in Oregon and other production regions of the United States using genotyping by sequencing, PCR assays for mating type and pathogenic race, vegetative compatibility group (VCG) tests, and aggressiveness assays. We report that the population in the Pacific Northwest can be described as one common genetic group and four relatively rare genetic groups. Eighty-three percent of the isolates belonged to VCG2B, and all isolates possessed the MAT1-2 idiomorph and were characterized as pathogenic race 2. These results indicate low levels of genetic diversity and a negligible risk of sexual recombination in populations of this host-adapted pathogen population. Knowledge of the genetic structure of V. dahliae in the Pacific Northwest will inform breeders about the diversity of pathogenicity factors that may need to be considered in their breeding programs.

Genetic Diversity of Verticillium dahliae Populations From Olive and Potato in Lebanon

Verticillium dahliae is widely distributed in potato and olive fields in Lebanon, causing serious economic losses. However, little is known about the inoculum source, population structure, and genetic diversity of the pathogen or the mechanisms of dissemination within Lebanon. To understand the population structure, a total of 203 isolates sampled from olive (n = 78) and potato (n = 125) were characterized for species, mating type, and race, and the genetic relationships were delineated using 13 microsatellite markers. All isolates except one from potato were V. dahliae, with 55.1 and 12.1% race 1, and 43.6 and 83.1% race 2 in olive and potato, respectively. The genetic structure of the studied population was best described by two large and two small clusters. Membership in the two large clusters was determined by the presence or absence of the effector gene Ave1. Furthermore, genetic structure was moderately associated with the host of origin but was weakly associated with the geographic origin. All but four isolates represented by three multilocus haploid genotypes were MAT1-2. This study identified a clear lack of gene flow between virulence genotypes of V. dahliae despite the proximity of these cropping systems and the wide distribution of genetic diversity among hosts and geographic regions in Lebanon.

The Transcriptome of Verticillium dahliae Responds Differentially Depending on the Disease Susceptibility Level of the Olive (Olea europaea L.) Cultivar

Among biotic constraints affecting olive trees cultivation worldwide, the soil-borne fungus Verticillium dahliae is considered one of the most serious threats. Olive cultivars display differential susceptibility to the disease, but our knowledge on the pathogen’s responses when infecting varieties differing in susceptibility is scarce. A comparative transcriptomic analysis (RNA-seq) was conducted in olive cultivars Picual (susceptible) and Frantoio (tolerant). RNA samples originated from roots during the first two weeks after inoculation with V. dahliae defoliating (D) pathotype. Verticillium dahliae mRNA amount was overwhelmingly higher in roots of the susceptible cultivar, indicating that proliferation of pathogen biomass is favored in ‘Picual’. A significant larger number of V. dahliae unigenes (11 fold) were only induced in this cultivar. Seven clusters of differentially expressed genes (DEG) were identified according to time-course expression patterns. Unigenes potentially coding for niche-adaptation, pathogenicity, virulence and microsclerotia development were induced in ‘Picual’, while in ‘Frantoio’ expression remained negligible or null. Verticillium dahliae D pathotype transcriptome responses are qualitatively and quantitatively different, and depend on cultivar susceptibility level. The much larger V. dahliae biomass found in ‘Picual’ roots is a consequence of both host and pathogen DEG explaining, to a large extent, the higher aggressiveness exerted over this cultivar.

Novel insights on colonization routes and evolutionary potential of Colletotrichum kahawae, a severe pathogen of Coffea arabica

Pathogenic fungi are emerging at an increasing rate on a wide range of host plants, leading to tremendous threats to the global economy and food safety. Several plant pathogens have been considered to be invasive species, rendering large-scale population genomic analyses crucial to better understand their demographic history and evolutionary potential. Colletotrichum kahawae (Ck) is a highly aggressive and specialized pathogen, causing coffee berry disease in Arabica coffee in Africa. This pathogen leads to severe production losses and its dissemination out of Africa is greatly feared. To address this issue, a population genomic approach using thousands of single nucleotide polymorphisms (SNPs) spaced throughout the genome was used to unveil its demographic history and evolutionary potential. The current study confirms that Ck is a true clonal pathogen, perfectly adapted to green coffee berries, with three completely differentiated populations (Angolan, Cameroonian and East African). Two independent clonal lineages were found within the Angolan population as opposed to the remaining single clonal populations. The most probable colonization scenario suggests that this pathogen emerged in Angola and immediately dispersed to East Africa, where these two populations began to differentiate, followed by the introduction in Cameroon from an Angolan population. However, the differentiation between the two Angolan clonal lineages masks the mechanism for the emergence of the Cameroonian population. Our results suggest that Ck is completely differentiated from the ancestral lineage, has a low evolutionary potential and a low dispersion ability, with human transport the most likely scenario for its potential dispersion, which makes the fulfilment of the quarantine measures and management practices implemented crucial.

Genetic Diversity and Population Structure of Pythium irregulare from Soybean and Corn Production Fields in Ohio

High levels of genetic diversity have been described within the Pythium irregulare complex from several host plants; however, little is known about the population structure in fields used for grain production. Therefore, the objective of this study was to evaluate the genetic diversity and population structure of 53 isolates baited from 28 soybean and corn production fields from 25 counties in Ohio. Genetic diversity was characterized based on sequence analysis of the internal transcribed spacer (ITS1-5.8S-ITS2) region and with 21 simple sequence repeat (SSR) markers. In addition, aggressiveness on soybean, optimum growth temperature, and sensitivity to metalaxyl fungicide were determined. ITS sequence analysis indicated that four isolates clustered with P. cryptoirregulare, whereas the remaining isolates clustered with P. irregulare that was subdivided into two groups (1 and 2). Cluster analysis of SSR data revealed a similar subdivision, which was also supported by structure analysis. The isolates from group 2 grew at a slower rate, but both groups of P. irregulare and P. cryptoirregulare recovered in this study had the same optimum growth at 27°C. Variability of aggressiveness and sensitivity toward metalaxyl fungicide was also observed among isolates within each group. The results from this study will help in the selection of isolates to be used in screening for resistance, assessment of fungicide efficacy, and disease management recommendations.

Network Analysis: A Systems Framework to Address Grand Challenges in Plant Pathology

Plant pathology must address a number of challenges, most of which are characterized by complexity. Network analysis offers useful tools for addressing complex systems and an opportunity for synthesis within plant pathology and between it and relevant disciplines such as in the social sciences. We discuss applications of network analysis, which ultimately may be integrated together into more synthetic analyses of how to optimize plant disease management systems. The analysis of microbiome networks and tripartite phytobiome networks of host-vector-pathogen interactions offers promise for identifying biocontrol strategies and anticipating disease emergence. Linking epidemic network analysis with social network analysis will support strategies for sustainable agricultural development and for scaling up solutions for disease management. Statistical tools for evaluating networks, such as Bayesian network analysis and exponential random graph models, have been underused in plant pathology and are promising for informing strategies. We conclude with research priorities for network analysis applications in plant pathology.

Population Genetics of Verticillium dahliae in Iran Based on Microsatellite and Single Nucleotide Polymorphism Markers

Verticillium dahliae is a plant pathogenic fungus that reproduces asexually and its population structure is highly clonal. In the present study, 78 V. dahliae isolates from Iran were genotyped for mating type, single nucleotide polymorphisms (SNPs), and microsatellites to assign them to clonal lineages and to determine population genetic structure in Iran. The mating type of all isolates was MAT1-2. Based on neighbor-joining analysis and minimum spanning networks constructed from SNPs and microsatellite genotypes, respectively, all but four isolates were assigned to lineage 2B⁸²⁴; four isolates were assigned to lineage 4B. The inferred coalescent genealogy of isolates in lineage 2B⁸²⁴ showed a clear divergence into two clades that corresponded to geographic origin and host. Haplotypes of cotton and pistachio isolates sampled from central Iran were in one clade, and those of isolates from Prunus spp. sampled from northwestern Iran were in the other. The strong divergence in haplotypes between the two clades suggests that there were at least two separate introductions of lineage 2B⁸²⁴ to different parts of Iran. Given the history of cotton and pistachio cultivation and Verticillium wilt in Iran, these results are consistent with the hypothesis that cotton was historically a likely source inoculum causing Verticillium wilt in pistachio.

Tolerance of olive (Olea europaea) cv Frantoio to Verticillium dahliae relies on both basal and pathogen-induced differential transcriptomic responses

Verticillium wilt of olive (VWO) is one of the most serious biotic constraints for this tree crop. Our knowledge of the genetics of the tolerance/resistance to this disease is very limited. Here we show that tolerance of the cv Frantoio relies on both basal and early pathogen-induced differential transcriptomic responses. A comparative transcriptomic analysis (RNA-seq) was conducted in root tissues of cvs Frantoio (VWO-tolerant) and Picual (VWO-susceptible). RNA samples originated from roots of inoculated olive plants during the early infection stages by Verticillium dahliae (highly virulent, defoliating pathotype). A huge number of differentially expressed genes (DEGs) were found between 'Frantoio' and 'Picual' (27 312 unigenes) in the absence of the pathogen. Upon infection with V. dahliae, 'Picual' and 'Frantoio' plants responded differently too. In the early infection stages, four clusters of DEGs could be identified according to their time-course expression patterns. Among others, a pathogenesis-related protein of the Bet v I family and a dirigent-like protein involved in lignification, and several BAK1, NHL1, reactive oxygen species stress response and BAM unigenes showed noticeable differences between cultivars. Tolerance of 'Frantoio' plants to VWO is a consequence of a complex and multifaceted process which involves many plant traits.

Global genotype flow in Cercospora beticola populations confirmed through genotyping-by-sequencing

Genotyping-by-sequencing (GBS) was conducted on 333 Cercospora isolates collected from Beta vulgaris (sugar beet, table beet and swiss chard) in the USA and Europe. Cercospora beticola was confirmed as the species predominantly isolated from leaves with Cercospora leaf spot (CLS) symptoms. However, C. cf. flagellaris also was detected at a frequency of 3% in two table beet fields in New York. Resolution of the spatial structure and identification of clonal lineages in C. beticola populations using genome-wide single nucleotide polymorphisms (SNPs) obtained from GBS was compared to genotyping using microsatellites. Varying distance thresholds (bitwise distance = 0, 1.854599 × 10-4, and 1.298 × 10-3) were used for delineation of clonal lineages in C. beticola populations. Results supported previous reports of long distance dispersal of C. beticola through genotype flow. The GBS-SNP data set provided higher resolution in discriminating clonal lineages; however, genotype identification was impacted by filtering parameters and the distance threshold at which the multi-locus genotypes (MLGs) were contracted to multi-locus lineages. The type of marker or different filtering strategies did not impact estimates of population differentiation and structure. Results emphasize the importance of robust filtering strategies and designation of distance thresholds for delineating clonal lineages in population genomics analyses that depend on individual assignment and identification of clonal lineages. Detection of recurrent clonal lineages shared between the USA and Europe, even in the relaxed-filtered SNP data set and with a conservative distance threshold for contraction of MLGs, provided strong evidence for global genotype flow in C. beticola populations. The implications of intercontinental migration in C. beticola populations for CLS management are discussed.

Genetic structure of Cercospora beticola populations on Beta vulgaris in New York and Hawaii

Cercospora leaf spot (CLS), caused by Cercospora beticola, is a major disease of Beta vulgaris worldwide. No sexual stage is known for C. beticola but in its asexual form it overwinters on infected plant debris as pseudostromata, and travels short distances by rain splash-dispersed conidiospores. Cercospora beticola infects a broad range of host species and may be seedborne. The relative contribution of these inoculum sources to CLS epidemics on table beet is not well understood. Pathogen isolates collected from table beet, Swiss chard and common lambsquarters in mixed-cropping farms and monoculture fields in New York and Hawaii, USA, were genotyped (n = 600) using 12 microsatellite markers. All isolates from CLS symptoms on lambsquarters were identified as C. chenopodii. Sympatric populations of C. beticola derived from Swiss chard and table beet were not genetically differentiated. Results suggested that local (within field) inoculum sources may be responsible for the initiation of CLS epidemics in mixed-cropping farms, whereas external sources of inoculum may be contributing to CLS epidemics in the monoculture fields in New York. New multiplex PCR assays were developed for mating-type determination for C. beticola. Implications of these findings for disease management are discussed.

Population Genomics of Fungal and Oomycete Pathogens

We are entering a new era in plant pathology in which whole-genome sequences of many individuals of a pathogen species are becoming readily available. Population genomics aims to discover genetic mechanisms underlying phenotypes associated with adaptive traits such as pathogenicity, virulence, fungicide resistance, and host specialization, as genome sequences or large numbers of single nucleotide polymorphisms become readily available from multiple individuals of the same species. This emerging field encompasses detailed genetic analyses of natural populations, comparative genomic analyses of closely related species, identification of genes under selection, and linkage analyses involving association studies in natural populations or segregating populations resulting from crosses. The era of pathogen population genomics will provide new opportunities and challenges, requiring new computational and analytical tools. This review focuses on conceptual and methodological issues as well as the approaches to answering questions in population genomics. The major steps start with defining relevant biological and evolutionary questions, followed by sampling, genotyping, and phenotyping, and ending in analytical methods and interpretations. We provide examples of recent applications of population genomics to fungal and oomycete plant pathogens.