Enhancing identification accuracy for powdery mildews using previously underexploited DNA loci

An unexpected diversity of powdery mildew species infecting the Fabaceae in Australia

The Fabaceae family has been reported to host more than fifty species of powdery mildew worldwide. Despite being commonly found on fabaceous hosts throughout Australia, the accurate identification of many powdery mildew species remains uncertain. The objective of this study was to identify powdery mildew species that naturally occur on fabaceous hosts in Australia and provide insight into those native and weedy species that may host crop pathogens and contribute to disease in cropping systems. The ribosomal DNA internal transcribed spacer (ITS) sequences and morphology of 34 fresh and 40 herbarium powdery mildew specimens infecting diverse Fabaceae species in Australia were characterised in this study. Altogether, a total of eleven powdery mildew species were identified from 51 Fabaceae species. Podosphaera xanthii was the most common powdery mildew in this study and was detected on 18 host species across ten genera. Ten species of Erysiphe were confirmed on 37 host species covering 17 host genera, with E. diffusa and E. cf. trifoliorum the most prevalent. This work provides the most comprehensive catalogue of powdery mildew species infecting legume hosts throughout Australia.

Mapping of resistance genes to powdery mildew based on DNA re-sequencing and bulk segregant analysis in Capsicum

Powdery mildew caused by Leveillula taurica adversely affects the development and growth of pepper plants. However, there have been few reports on the fine mapping and quantitative trait locus (QTLs) gene cloning of resistance genes to powdery mildew in pepper. Herein, an F2 segregating population was constructed using the high resistance material "NuMex Suave Red" and the extremely susceptible material "c89" for bulked segregant analysis and DNA re-sequencing (BSA-seq). Molecular markers were used to achieve fine mapping, followed by expression verification. A major QTL located on chromosome 5 (Chr5, 7.20-11.75 Mb) that is associated with resistance to powdery mildew in pepper was mapped using BSA-seq. A narrow interval of 64.86 kb encompassing five genes was refined using InDel and KSAP molecular markers developed from the QTL region. Among them, the expression of the ubiquitin-conjugating enzyme E2 gene, Capana05g000392, was significantly upregulated in multiple resistant materials. In addition, there was a single nucleotide polymorphism (SNP) of A/G in the 241st position of the CDS sequence of Capana05g000392, which in turn leads to an amino acid polymorphism of M/V between susceptible parent and resistant parent. Overall, these results indicate that the Capana05g000392 gene may serve as a robust potential factor against powdery mildew in pepper. These findings further elucidate the genetic mechanism of resistance to powdery mildew in pepper and facilitate molecular marker-assisted breeding.

An emerging fungal disease is spreading across the globe and affecting the blueberry industry

Powdery mildew is an economically important disease caused by c. 1000 different fungal species. Erysiphe vaccinii is an emerging powdery mildew species that is impacting the blueberry industry. Once confined to North America, E. vaccinii is now spreading rapidly across major blueberry-growing regions, including China, Morocco, Mexico, and the USA, threatening millions in losses. This study documents its recent global spread by analyzing both herbarium specimens, some over 150-yr-old, and fresh samples collected world-wide. Our findings were integrated into a 'living phylogeny' via T-BAS to simplify pathogen identification and enable rapid responses to new outbreaks. We identified 50 haplotypes, two primary introductions world-wide, and revealed a shift from a generalist to a specialist pathogen. This research provides insights into the complexities of host specialization and highlights the need to address this emerging global threat to blueberry production.

Phylogeny and taxonomy of the genera of Erysiphaceae, part 6: Erysiphe (the "Microsphaera lineage" part 2)

This is the sixth contribution in a series devoted to the phylogeny and taxonomy of powdery mildews. This part includes our third treatment of the species of the genus Erysiphe. It continues the previous contribution on the phylogenetic-taxonomic assessment of the species belonging to the "Microsphaera lineage." Since this is a large lineage, we have split the treatment of the "Microsphaera lineage" into two parts. Phylogenetic trees based on rDNA are supplemented by sequences of additional markers (CAM, GAPDH, GS, RPB2, and TUB). The "Erysiphe trifoliorum complex" is a challenging group that belongs to the "Microsphaera lineage." Adequate clarification of this complex will be possible when additional worldwide multilocus sequence analyses are performed. The new species Erysiphe acetosae, E. acmisponis, E. lathyrina, E. salmoniana, and E. santalicola are described, and the new combinations E. biuncinata and E. pavoniae are introduced. Specimens of several species have been sequenced for the first time, particularly North American species, such as Erysiphe caryae, E. ceanothi, E. juglandis-nigrae, and E. ravenelii. Erysiphe syringae is lectotypified and 15 species names are epitypified in order to provide ex-epitype reference sequences. For other species, non-ex-type reference sequences are proposed for phylogenetic-taxonomic purposes. Ex-type sequences for Erysiphe baptisiicola, E. sesbaniae, Microsphaera sydowiana, M. umbilici, and Oidium pavoniae have been retrieved.

Morpho-cultural and molecular variability of Stemphylium vesicarium causing Stemphylium leaf blight in tropical onions

The cultural, morphological, and molecular variations among 22 isolates of Stemphylium vesicarium (Wallr.) E. Simmons, collected from different locations, was carried out. Significant variations were observed among the Stemphylium isolates regarding colony diameter, mean radial growth rate and sporulation on PDA media. The colonies of S. vesicarium isolates exhibited diverse mycelial growth characteristics, including velvety, cottony, or fluffy textures and a range of colours from whitish to dark grey, olivaceous with a greenish tinge or brownish. The margins of the colonies were observed to be filiform, displaying a filamentous appearance, with entire and undulate shapes and a whitish colouration. The conidiophores of Stemphylium isolates displayed a wide range of dimensions, with average length ranging from 36.81 μm to 66.44 μm and average breadth from 3.05 μm to 6.96 μm. Similarly, the conidia exhibited variations in size, colour, shape, average length (23.31 μm-43.18 μm) and average breadth (12.84 μm-23.13 μm). The conidia were mainly light brown or brown and displayed ovoid, oblong, or ovoid-to-oblong shapes. The number and presence of transverse and longitudinal septa also varied among the isolates. Fifteen RAPD primers generated 192 banding patterns. SV4, OPL5, and SV5 were identified as the most polymorphic primers, while OPA3, OPF10, OPN7, OPS7, and OPS10 produced the fewest polymorphic bands. The average polymorphic information content (PIC) value was 0.37, with OPA5 and OPC8 showing the highest PIC values. Cluster analysis based on genetic similarity revealed five distinct clusters, but no clear correlation between isolates and their collection sites was observed. In the phylogenetic analysis, based on Internal transcribed spacer (ITS) region and Glycerol-3-Phosphate Dehydrogenase 1(gpd1) gene sequences, 20 isolates obtained from diseased onion leaves formed a distinct cluster and exhibited sequence similarity with ex-type sequence of Stemphylium vesicarium. Additionally, two isolates from diseased garlic samples showed similarity with ex-type sequence of Stemphylium eturmiunum. This is the first-time report of S. eturmiunum on Indian garlic under field conditions.

Detection of Podosphaera macularis in Air Samples by Quantitative PCR

Detection and quantification of pathogen propagules in the air or other environmental samples is facilitated by culture-independent assays. We developed a quantitative PCR assay for the hop powdery mildew fungus, Podosphaera macularis, for detection of the organism from air samples. The assay uses primers and a TaqMan probe designed to target species-specific sequences in the 28S large subunit of the nuclear ribosomal DNA. Analytical sensitivity was not affected by the presence of an exogenous internal control or potential PCR inhibitors associated with DNA extracted from soil. The level of quantification of the assay was between 200 and 350 conidia when DNA was extracted from a fixed number of conidia. The assay amplified all isolates of P. macularis tested and had minimal cross-reactivity with other Podosphaera species when assayed with biologically relevant quantities of DNA. Standard curves generated independently in two other laboratories indicated that assay sensitivity was qualitatively similar and reproducible. All laboratories successfully detected eight unknown isolates of P. macularis and correctly discriminated Pseudoperonospora humuli and a water control. The usefulness of the assay for air sampling for late-season inoculum of P. macularis was demonstrated in field studies in 2019 and 2020. In both years, airborne populations of P. macularis in hop yards were detected consistently and increased during bloom and cone development.

Powdery mildew caused by Erysiphe corylacearum: An emerging problem on hazelnut in Italy

Erysiphe corylacearum has recently been reported in northern Italy (Piedmont) and other European countries as the causal agent of a new emerging powdery mildew on hazelnut. This disease is much more dangerous than the common hazelnut powdery mildew caused by Phyllactinia guttata as it significantly reduces yield and quality of hazelnuts. This study aimed to perform morphological and molecular characterization of the fungal isolates from powdery mildew-infected plants in the Piedmont Italian region. Additionally, genetic diversity studies and pathogenicity tests were conducted. Thirty-six fungal isolates originating from symptomatic hazelnut plants exhibiting specific powdery mildew symptoms on the superior leaf side were identified morphologically as E. corylacearum. Single- and multilocus sequence typing of five loci (ITS, rpb2, CaM, GAPDH and GS) assigned all isolates as E. corylacearum. Multilocus and GAPDH phylogenetic studies resulted in the most efficient characterization of E. corylacearum. Studied fungal isolates were able to cause new emerging powdery mildew disease by fulfilling Koch's postulates. The emergence of powdery mildew disease in Italy revealed the E. corylacearum subgrouping, population expansion, and high nucleotide similarity with other recently identified E. corylacearum hazelnut isolates. To contain this harmful disease and inhibit the fungus spread into new geographical zones, it will be necessary to implement more rigorous monitoring in neighboring hazelnut plantations near infected hazelnuts, use sustainable fungicides and search for new biocontrol agents.

Phylogeny and taxonomy of the genera of Erysiphaceae, part 5: Erysiphe (the "Microsphaera lineage" part 1)

In this contribution, we offer the fifth installment of a series focusing on the phylogeny and taxonomy of powdery mildews. This paper is the second segment evaluating the genus Erysiphe. The first treatment of Erysiphe focused on phylogenetically basal species in the "Uncinula lineage." This research presents a phylogenetic-taxonomic assessment of species that form the group previously referred to as the "Microsphaera lineage." Given the size of the group, we split the treatment of this lineage of Erysiphe species into two parts based on their phylogenetic placement. Phylogenetic trees based on ITS+28S data are supplemented by sequences of additional markers (CAM, GADPH, GS, RPB2, and TUB). Included in the analysis of the Microsphaera lineage is the "Erysiphe aquilegiae complex" (group, clade, cluster), which encompasses sequences obtained from an assemblage of Erysiphe species with insufficient resolution in rDNA analyses. Attempts have been made to resolve this group at the species level by applying a multilocus approach. A detailed discussion of the "Erysiphe aquilegiae complex" is provided. Sequences are provided for the first time for several species, particularly North American species, such as Erysiphe aggregata, E. erineophila, E. parnassiae, and E. semitosta. Ex-type sequences for Microsphaera benzoin and M. magnusii have been retrieved. Alphitomorpha penicillata, Microsphaera vanbruntiana, and M. symphoricarpi are epitypified with ex-epitype sequences. The new species Erysiphe alnicola, E. deutziana, E. cornigena, E. lentaginis, and E. sambucina are described, the new combinations E. lauracearum, E. passiflorae, and E. sambucicola are introduced, and the new name E. santali is proposed.

Phylogeny and taxonomy of the genera of Erysiphaceae, part 4: Erysiphe (the "Uncinula lineage")

This is the fourth contribution within an ongoing series dedicated to the phylogeny and taxonomy of powdery mildews. This particular installment undertakes a comprehensive evaluation of a group previously referred to as the "Uncinula lineage" within Erysiphe. The genus Erysiphe is too large to be assessed in a single paper; thus, the treatment of Erysiphe is split into three parts, according to phylogenetic lineages. The first paper, presented here, discusses the most basal lineage of Erysiphe and its relationship to allied basal genera within tribe Erysipheae (i.e., Brasiliomyces and Salmonomyces). ITS+28S analyses are insufficient to resolve the basal assemblage of taxa within the Erysipheae. Therefore, phylogenetic multilocus examinations have been carried out to better understand the evolution of these taxa. The results of our analyses favor maintaining Brasiliomyces, Bulbomicroidium, and Salmonomyces as separate genera, at least for the interim, until further phylogenetic multilocus data are available for additional basal taxa within the Erysipheae. The current analyses also confirmed previous results that showed that the "Uncinula lineage" is not exclusively composed of Erysiphe species of sect. Uncinula but also includes some species that morphologically align with sect. Erysiphe, as well as species that had previously been assigned to Californiomyces and Typhulochaeta. Numerous sequences of Erysiphe species from the "Uncinula lineage" have been included in the present phylogenetic analyses and were confirmed by their position in well-supported species clades. Several species have been sequenced for the first time, including Erysiphe clintonii, E. couchii, E. geniculata, E. macrospora, and E. parvula. Ex-type sequences are provided for 16 taxa including E. nothofagi, E. trinae, and E. variabilis. Epitypes are designated and ex-epitype sequences are added for 18 taxa including Erysiphe carpophila, E. densa, and U. geniculata var. carpinicola. The new species Erysiphe canariensis is described, and the new names E. hosagoudarii and E. pseudoprunastri and the new combination E. ampelopsidis are introduced.

Phylogeny and taxonomy of Erysiphe spp. on Rhododendron, with a special emphasis on North American species

The genus Rhododendron comprises over 1000 evergreen and deciduous species. In the Pacific Northwest Coast region of North America (PNWC), powdery mildews infecting deciduous Rhododendron spp. are well documented but less so on evergreen Rhododendron spp. Infections of both groups of hosts historically have been attributed to Erysiphe azaleae or E. vaccinii. No formal characterizations of powdery mildew fungi infecting either deciduous or evergreen Rhododendron spp. in the PNWC have been completed. The objectives of this study were to identify the powdery mildew pathogens infecting evergreen Rhododendron spp. in the PNWC and to assess the phylogenetic position of these fungi within the Erysiphaceae. To ascertain valid taxonomic conclusions, and to determine whether potential introductions of exotic Rhododendron powdery mildews in North America have occurred, it was necessary to put the new North American phylogenetic data into a worldwide context. Therefore, available phylogenetic data from all Erysiphe spp. on Rhododendron have been included in our analyses.Based on analyses of numerous new internal transcribed spacer (ITS) and 28S rDNA sequences and already available sequences deposited in GenBank retrieved from evergreen and deciduous Rhododendron spp., the following Erysiphe spp. could be phylogenetically confirmed (all belonging to Erysiphe sect. Microsphaera): Erysiphe azaleae nom. cons. (Oidium ericinum could be verified as a synonym), E. digitata (holotype sequenced), E. izuensis, and E. vaccinii. Erysiphe azaleae and E. vaccinii are epitypified with sequenced specimens, and an ex-neotype sequence has been obtained for Oidium ericinum. Erysiphe rhododendri (Erysiphe sect. Erysiphe), only known from two collections in India (Himalayan region), was not available for phylogentic analyses.

Genetic Diversity Analysis based on the Virulence, Physiology and Regional Variability in Different Isolates of Powdery Mildew in Pea

Powdery mildew is an omnipresent disease that reduces the yield and quality of pea crops (Pisum sativum L.). To examine the powdery mildew pathogen’s morphological, molecular, and genetic diversity, we collected samples of powdery mildew-affected pea crops from ten distinct locations in the Nilgiris district of Tamil Nadu, India. The pathogen Erysiphe pisi was identified morphologically based on anamorphic characters. Molecular identification of E. pisi isolates was befitted by targeting the internal transcribed spacer (ITS) region of rDNA and specific primers of powdery mildew fungi. The genetic variation between ten different E. pisi isolates collected from topographically distinct mountainous areas was studied using random amplified polymorphic (RAPD). Based on its morphological characteristics, the powdery mildew fungus presented high similarities to E. pisi. Molecular characterization of the ITS rDNA of E. pisi produced 650 bp nucleotides, PMITS (powdery mildew-internal transcribed region) primers produced 700 bp nucleotides, and an Erysiphe specific ITS primer pair amplified and synthesized 560 bp nucleotides. According to the findings, the collected E. pisi strains exhibited a low level of genetic diversity and only a slight differential in virulence on the host. In the study, E. pisi isolates from Anumapuram, Emerald Valley, Indira Nagar, and Thuneri showed a greater disease incidence in the natural field conditions and shared the same genetic lineage with other isolates in UPGMA hierarchical cluster analysis based on RAPD markers. There was no evidence of a link between the occurrence of the disease and these grouped populations.

Erysiphe iranica sp. nov. on Onobrychis caput-galli in Iran

In April 2014, powdery mildew symptoms were found on Onobrychis caputgalli in Khan Ahmad, Gachsaran, Iran. Morphological characters and analysis of ITS and 28S rDNA sequences revealed that this powdery mildew does not match previously recorded species on fabaceous hosts. It is proposed as a new species, Erysiphe iranica, and is described and illustrated, and compared with previous species of powdery mildew recorded on Fabaceae.

Secondary DNA Barcodes (CAM, GAPDH, GS, and RpB2) to Characterize Species Complexes and Strengthen the Powdery Mildew Phylogeny

Powdery mildews are a group of economically and ecologically important plant pathogens. In the past 25 years the use of ribosomal DNA (rDNA) in the powdery mildews has led to major taxonomic revisions. However, the broad scale use of rDNA has also revealed multiple species complexes that cannot be differentiated based on ITS + LSU data alone. Currently, there are only two powdery mildew taxonomic studies that took a multi-locus approach to resolve a species complex. In the present study, we introduce primers to sequence four additional regions (CAM, GAPDH, GS, and RPB2) that have the potential to improve support values in both broad and fine scale phylogenetic analyses. The primers were applied to a broad set of powdery mildew genera in China and the United States, and phylogenetic analyses included some of the common complexes. In taxa with nearly identical ITS sequences the analyses revealed a great amount of diversity. In total 154 non-rDNA sequences from 11 different powdery mildew genera were deposited in NCBI’s GenBank, laying the foundation for secondary barcode databases for powdery mildews. The combined and single loci phylogenetic trees constructed generally followed the previously defined species/genus concepts for the powdery mildews. Future research can use these primers to conduct in depth phylogenetic, and taxonomic studies to elucidate the evolutionary relationships of species and genera within the powdery mildews.

Beyond Nuclear Ribosomal DNA Sequences: Evolution, Taxonomy, and Closest Known Saprobic Relatives of Powdery Mildew Fungi (Erysiphaceae) Inferred From Their First Comprehensive Genome-Scale Phylogenetic Analyses

Powdery mildew fungi (Erysiphaceae), common obligate biotrophic pathogens of many plants, including important agricultural and horticultural crops, represent a monophyletic lineage within the Ascomycota. Within the Erysiphaceae, molecular phylogenetic relationships and DNA-based species and genera delimitations were up to now mostly based on nuclear ribosomal DNA (nrDNA) phylogenies. This is the first comprehensive genome-scale phylogenetic analysis of this group using 751 single-copy orthologous sequences extracted from 24 selected powdery mildew genomes and 14 additional genomes from Helotiales, the fungal order that includes the Erysiphaceae. Representative genomes of all powdery mildew species with publicly available whole-genome sequencing (WGS) data that were of sufficient quality were included in the analyses. The 24 powdery mildew genomes included in the analysis represented 17 species belonging to eight out of 19 genera recognized within the Erysiphaceae. The epiphytic genera, all but one represented by multiple genomes, belonged each to distinct, well-supported lineages. Three hemiendophytic genera, each represented by a single genome, together formed the hemiendophytic lineage. Out of the 14 other taxa from the Helotiales, Arachnopeziza araneosa, a saprobic species, was the only taxon that grouped together with the 24 genome-sequenced powdery mildew fungi in a monophyletic clade. The close phylogenetic relationship between the Erysiphaceae and Arachnopeziza was revealed earlier by a phylogenomic study of the Leotiomycetes. Further analyses of powdery mildew and Arachnopeziza genomes may discover signatures of the evolutionary processes that have led to obligate biotrophy from a saprobic way of life. A separate phylogeny was produced using the 18S, 5.8S, and 28S nrDNA sequences of the same set of powdery mildew specimens and compared to the genome-scale phylogeny. The nrDNA phylogeny was largely congruent to the phylogeny produced using 751 orthologs. This part of the study has revealed multiple contamination and other quality issues in some powdery mildew genomes. We recommend that the presence of 28S, internal transcribed spacer (ITS), and 18S nrDNA sequences in powdery mildew WGS datasets that are identical to those determined by Sanger sequencing should be used to assess the quality of assemblies, in addition to the commonly used Benchmarking Universal Single-Copy Orthologs (BUSCO) values.

New Records of Powdery Mildews from Taiwan: Erysiphe ipomoeae comb. nov., E. aff. betae on Buckwheat, and E. neolycopersici comb. nov. on Cardiospermum halicacabum

Erysiphe is the largest genus of powdery mildews (PMs), a group of obligate plant pathogenic fungi. Exploration of biodiversity generally relies on regional surveys and our aim is to investigate the PMs in Taiwan. Collections of the fungi on five plant species around agricultural environments were subjected to morphological and molecular characterization, using both internal transcribed spacer (ITS) and β-tubulin gene (TUB2) regions for the phylogenetic analyses. Erysipheipomoeae comb. nov., a species able to infect Ipomoea obscura and I. aquatica demonstrated by pathogenicity tests, has been neotypified. The two buckwheat species, Fagopyrum esculentum and F. tataricum, are found to be hosts of E. aff. betae. These results suggest that hosts in some plant families can be infected by more than one Erysiphe pathogen, e.g., Convolvulaceae by E. ipomoeae and E. convolvuli and Polygonaceae by E. polygoni and E. aff. betae, respectively. In addition, phylogenetic analyses of PMs on Cardiospermum halicacabum and tomato belonging to the E. aquilegiae complex are allocated under E. neolycopersici comb. nov. This extends the potential host range of E. aquilegiae complex to the plant family Sapindaceae. We conclude that awareness of the host associations of PMs can potentially benefit crop disease management.

Examination of Gene Loss in the DNA Mismatch Repair Pathway and Its Mutational Consequences in a Fungal Phylum

The DNA mismatch repair (MMR) pathway corrects mismatched bases produced during DNA replication and is highly conserved across the tree of life, reflecting its fundamental importance for genome integrity. Loss of function in one or a few MMR genes can lead to increased mutation rates and microsatellite instability, as seen in some human cancers. Although loss of MMR genes has been documented in the context of human disease and in hypermutant strains of pathogens, examples of entire species and species lineages that have experienced substantial MMR gene loss are lacking. We examined the genomes of 1,107 species in the fungal phylum Ascomycota for the presence of 52 genes known to be involved in the MMR pathway of fungi. We found that the median ascomycete genome contained 49/52 MMR genes. In contrast, four closely related species of obligate plant parasites from the powdery mildew genera Erysiphe and Blumeria, have lost between five and 21 MMR genes, including MLH3, EXO1, and DPB11. The lost genes span MMR functions, include genes that are conserved in all other ascomycetes, and loss of function of any of these genes alone has been previously linked to increased mutation rate. Consistent with the hypothesis that loss of these genes impairs MMR pathway function, we found that powdery mildew genomes with higher levels of MMR gene loss exhibit increased numbers of mononucleotide runs, longer microsatellites, accelerated sequence evolution, elevated mutational bias in the A|T direction, and decreased GC content. These results identify a striking example of macroevolutionary loss of multiple MMR pathway genes in a eukaryotic lineage, even though the mutational outcomes of these losses appear to resemble those associated with detrimental MMR dysfunction in other organisms.

One Crop Disease, How Many Pathogens? Podosphaera xanthii and Erysiphe vignae sp. nov. Identified as the Two Species that Cause Powdery Mildew of Mungbean (Vigna radiata) and Black Gram (V. mungo) in Australia

Powdery mildew is a significant threat to mungbean (Vigna radiata) and black gram (V. mungo) production across Australia and overseas. Although they have been present in Australia for at least six decades and are easily recognized in the field, the precise identification of the pathogens causing this disease has remained unclear. Our goal was to identify the powdery mildew species infecting mungbean, black gram, and wild mungbean (V. radiata ssp. sublobata) in Australia. The internal transcribed spacer (ITS) and large subunit sequences of the ribosomal DNA and/or morphology of 57 Australian specimens were examined. Mungbean and black gram were infected by two species: Podosphaera xanthii and a newly recognized taxon, Erysiphe vignae sp. nov. Wild mungbean was infected only with P. xanthii. Mungbean and black gram powdery mildew ITS sequences from China, India, and Taiwan revealed the presence of only P. xanthii on these crops despite controversial reports of an Erysiphe species on both crops in India. Sequence analyses indicated that the closest relative of E. vignae is E. diffusa, which infects soybean (Glycine max) and other plants. E. vignae did not infect soybean in cross-inoculation tests. In turn, E. diffusa from soybean infected black gram and provoked hypersensitive response in mungbean. The recognition of a second species, E. vignae, as another causal agent of mungbean and black gram powdery mildew in Australia may complicate plant breeding efforts and control of the disease with fungicide applications.

A global genetic analysis of herbarium specimens reveals the invasion dynamics of an introduced plant pathogen

The introduction, spread, and impact of fungal plant pathogens is a critical concern in ecological systems. In this study, we were motivated by the rather sudden appearance of Acermacrophyllum heavily infected with powdery mildew. We used morphological and genetic analyses to confirm the pathogen causing the epidemic was Sawadaea bicornis. In subsequent field studies, this pathogen was found in several locations in western North America, and in greenhouse studies, A. macrophyllum was found to be significantly more susceptible to S. bicornis than nine other Acer species tested. A genetic analysis of 178 specimens of powdery mildew from freshly collected and old herbarium specimens from 15 countries revealed seven different haplotypes. The high diversity of haplotypes found in Europe coupled with sequence results from a specimen from 1864 provides evidence that S. bicornis has a European origin. Furthermore, sequence data from a specimen from 1938 in Canada show that the pathogen has been present in North America for at least 82 years revealing a considerable lag time between the introduction and current epidemic. This study used old herbarium specimens to genetically hypothesize the origin, the native host, and the invasion time of a detrimental fungal plant pathogen.

First Report of Podosphaera xanthii Causing Powdery Mildew on Mungbean (Vigna radiata) in Taiwan

Mungbean (Vigna radiata L.) is routinely grown in the experimental fields at the headquarters of the World Vegetable Center (23°6'30.88"N, 120°17'51.31"E) for breeding, research and germplasm multiplication. In a spring 2016 mungbean trial, about 50% of the plants were affected with powdery mildew. The white, powdery-like patches first appeared on the upper leaf surfaces, and soon developed to grey patches on both sides of the leaves. Purple to brown discoloration appeared on the underside of the infected leaf. Microscopy examination revealed that the causal organism was not Erysiphe polygoni, which had previously been documented as the powdery mildew pathogen on mungbean in Taiwan (Hartman et al. 1993). The fungus produced typical structures of the powdery mildew Euoidium, anamorph of the genus Podosphaera. The mycelium consisted of septate, flexuous hyphae with indistinct appressoria. The erect conidiophores arising from superficial hyphae varied from straight or slightly curved to curled. Three to ten conidia were borne in long chains with crenate edges. Foot-cells were straight, cylindrical and measured 30 to 52 µm long. Conidia were hyaline, ellipsoid-ovoid to barrel-shaped, with fibrosin bodies, and measured 27 to 33 (mean = 30.4) × 15 to 20 (mean = 16.6) µm. Germ tubes were clavate and occasionally forked, and were produced from the lateral sites of the conidia. No chasmothecia were found in the samples. The morphological characteristics were consistent with P. xanthii (Castagne) U. Braun & Shishkoff (Braun & Cook 2012). To confirm the identity, the internal transcribed spacer (ITS) region of rDNA and partialβ-tubulin gene (TUB2) for the isolate MG3 were amplified with the primers ITS4/ITS5 (White et al. 1990) and BtuF5/BtuR7a (Ellingham et al. 2019), respectively. BLASTn analysis revealed the ITS sequence (MN833717) was 100% identical to many records of P. xanthii whereas the TUB2 sequence (MW363957) was 100% identical to a record of P. fusca (syn. P. xanthii; KC333362) in NCBI GenBank. A pathogenicity test was conducted by dusting conidia from an infected leaf onto six healthy four-week-old mungbean plants (cv 'Tainan No. 3'). Another three plants were not inoculated and were used as control. All the plants were maintained in a greenhouse at 25 to 28°C. All inoculated plants developed powdery mildew symptoms after 10 days, whereas the control plants remained symptomless. To our knowledge, this is the first report of P. xanthii causing disease on mungbean in Taiwan. P. xanthii also has been reported on mungbean in Thailand (Meeboon et al. 2016), while other records referring to E. polygoni infecting Vigna spp. are from Brazil and Fiji (Farr & Rossman 2020). Although both P. xanthii and E. polygoni have now been reported as causing powdery mildew on mungbean in Taiwan, which species predominates or is more important remains unclear. A comprehensive survey with accurate species identification is required to develop effective management of the disease, particularly for resistance breeding.

First records of powdery mildew fungi (Erysiphales) on medicinal plants in Taiwan

BACKGROUND

Production of medicinal plants in Taiwan is not only hampered by international market competition, but also lack of knowledge of their pathogens, such as powdery mildew fungi (Erysiphales, Ascomycota). Records of these fungi in Taiwan originate from few researchers for the last one hundred years and are still incomplete. Since powdery mildews in tropical/subtropical environments rarely develop the sexual stages with morphologically diagnostic characteristics, internal transcribed spacer sequences (ITS) of the ribosomal RNA genes obtained from the asexual stages have become important modern tools for species identification.

RESULTS

Powdery mildews on medicinal plants from educational and ornamental plantations in Taiwan were identified based on the anamorph morphology and ITS sequences. Four powdery mildews on medicinal plants are new records for Taiwan, Arthrocladiella mougeotii on Lycium chinense, Erysiphe glycines on Pueraria lobata, Erysiphe lespedezae on Bauhinia sp., Desmodium caudatum, and Uraria crinita, and E. lonicerae on Lonicera japonica. Eryngium foetidum is a new host for Erysiphe heraclei hitherto known on other host plants in Taiwan. Eryngium foetidum and Uraria crinita are new host plants for powdery mildews worldwide. Only specific field collection of the pathogens yielded the new records, not checking plant specimens in a phanerogam herbarium. The pathogens did not cause death of the host plants, but appeared to enhance stress by infection of mature leaves.

CONCLUSIONS

Taxonomic study of powdery mildews in Taiwan results into new host records of economically important medicinal plants in Taiwan with potential consequences for plant production and quarantine and also shows that host records are quite incomplete worldwide. Although ITS sequences were useful for species identification, the lack of data for several species on the same host genus on the one hand and the low variation between closely related species on the other indicate the need for further study.

Australia: A Continent Without Native Powdery Mildews? The First Comprehensive Catalog Indicates Recent Introductions and Multiple Host Range Expansion Events, and Leads to the Re-discovery of Salmonomyces as a New Lineage of the Erysiphales

In contrast to Eurasia and North America, powdery mildews (Ascomycota, Erysiphales) are understudied in Australia. There are over 900 species known globally, with fewer than currently 60 recorded from Australia. Some of the Australian records are doubtful as the identifications were presumptive, being based on host plant-pathogen lists from overseas. The goal of this study was to provide the first comprehensive catalog of all powdery mildew species present in Australia. The project resulted in (i) an up-to-date list of all the taxa that have been identified in Australia based on published DNA barcode sequences prior to this study; (ii) the precise identification of 117 specimens freshly collected from across the country; and (iii) the precise identification of 30 herbarium specimens collected between 1975 and 2013. This study confirmed 42 species representing 10 genera, including two genera and 13 species recorded for the first time in Australia. In Eurasia and North America, the number of powdery mildew species is much higher. Phylogenetic analyses of powdery mildews collected from Acalypha spp. resulted in the transfer of Erysiphe acalyphae to Salmonomyces, a resurrected genus. Salmonomyces acalyphae comb. nov. represents a newly discovered lineage of the Erysiphales. Another taxonomic change is the transfer of Oidium ixodiae to Golovinomyces. Powdery mildew infections have been confirmed on 13 native Australian plant species in the genera Acacia, Acalypha, Cephalotus, Convolvulus, Eucalyptus, Hardenbergia, Ixodia, Jagera, Senecio, and Trema. Most of the causal agents were polyphagous species that infect many other host plants both overseas and in Australia. All powdery mildews infecting native plants in Australia were phylogenetically closely related to species known overseas. The data indicate that Australia is a continent without native powdery mildews, and most, if not all, species have been introduced since the European colonization of the continent.

Sequencing Herbarium Specimens of a Common Detrimental Plant Disease (Powdery Mildew)

Powdery mildew (Erysiphaceae) is a detrimental plant disease that occurs on a variety of economically important crops. Powdery mildew consists of over 873 species of fungal pathogens that affect over 10,000 plant species. Genetic identification of powdery mildew is accomplished using the internal transcribed spacer (ITS) and large subunit (LSU) regions of the nuclear ribosomal RNA gene cluster. The ITS and LSU regions of powdery mildews can be useful in ecological, epidemiological, phylogenetic, and taxonomic investigations. However, sequencing these regions is not without its challenges. For example, powdery mildew sequences are often contaminated with plant and/or fungal DNA. Also, there tends to be a limited amount and older specimens' DNA can fragment over time. The success of sequencing powdery mildew often depends on the primers used for running polymerase chain reaction (PCR). The primers need to be broad enough that they match the majority of powdery mildew DNA yet specific enough that they do not align with other organisms. A review of the taxonomy and phylogeny of the powdery mildews is presented with an emphasis on sequencing the ITS + LSU genomic regions. Additionally, we introduce a new nested primer protocol for sequencing powdery mildew herbarium samples that includes six new powdery mildew-specific primers. The new sequencing protocol presented allows specimens up to 130 years old to be sequenced consistently. Sequencing herbarium specimens can be extremely useful for addressing many ecological, epidemiological, phylogenetic, and taxonomic problems in multiple plant pathogenic systems including the powdery mildews.

Rediscovering an old foe: Optimised molecular methods for DNA extraction and sequencing applications for fungarium specimens of powdery mildew (Erysiphales)

The purpose of this study was to identify a reliable DNA extraction protocol to use on 25-year-old powdery mildew specimens from the reference collection VPRI in order to produce high quality sequences suitable to address taxonomic phylogenetic questions. We tested 13 extraction protocols and two library preparation kits and found the combination of the E.Z.N.A.® Forensic DNA kit for DNA extraction and the NuGen Ovation® Ultralow System library preparation kit was the most suitable for this purpose.

Morphophylogenetic analyses revealed that Podosphaera tridactyla constitutes a species complex

Podosphaera tridactyla (s. lat.) is a powdery mildew species occurring on a wide range of Prunus spp. almost worldwide. We have investigated the phylogeny of the Po. tridactyla complex, with special emphasis on potential aspects of cryptic speciation. The results suggested that Po. tridactyla represents a species complex consisting of at least 12 different species. Based on detailed morphological examinations and molecular sequence analyses, we propose dividing Po. tridactyla s. lat. into 10 species, encompassing 7 new species (Po. ampla, Po. pruni-avium, Po. pruni-cerasoidis, Po. prunigena, Po. pruni-lusitanicae, Po. prunina, and Po. pruni-japonicae) and 3 known species (Po. longiseta, Po. salatai, and Po. tridactyla s. str.). Oidium passerinii on Pr. laurocerasus is confirmed as a synonym of Po. tridactyla s. str. Epitypes are designated for Po. tridactyla and Oidium passerinii.