A morphologically ill-founded powdery mildew species, Pleochaeta indica, is recognized as a phylogenetic species based on the analysis of the nuclear ribosomal DNA sequences

Long-term and rapid evolution in powdery mildew fungi

The powdery mildew fungi (Erysiphaceae) are globally distributed plant pathogens with a range of more than 10,000 plant hosts. In this review, we discuss the long- and short-term evolution of these obligate biotrophic fungi and outline their diversity with respect to morphology, lifestyle, and host range. We highlight their remarkable ability to rapidly overcome plant immunity, evolve fungicide resistance, and broaden their host range, for example, through adaptation and hybridization. Recent advances in genomics and proteomics, particularly in cereal powdery mildews (genus Blumeria), provided first insights into mechanisms of genomic adaptation in these fungi. Transposable elements play key roles in shaping their genomes, where even close relatives exhibit diversified patterns of recent and ongoing transposon activity. These transposons are ubiquitously distributed in the powdery mildew genomes, resulting in a highly adaptive genome architecture lacking obvious regions of conserved gene space. Transposons can also be neofunctionalized to encode novel virulence factors, particularly candidate secreted effector proteins, which may undermine the plant immune system. In cereals like barley and wheat, some of these effectors are recognized by plant immune receptors encoded by resistance genes with numerous allelic variants. These effectors determine incompatibility ("avirulence") and evolve rapidly through sequence diversification and copy number variation. Altogether, powdery mildew fungi possess plastic genomes that enable their fast evolutionary adaptation towards overcoming plant immunity, host barriers, and chemical stress such as fungicides, foreshadowing future outbreaks, host range shifts and expansions as well as potential pandemics by these pathogens.

First Draft Genome Assemblies of Pleochaeta shiraiana and Phyllactinia moricola, Two Tree-Parasitic Powdery Mildew Fungi with Hemiendophytic Mycelia

Powdery mildew fungi (Erysiphaceae) are widespread obligate biotrophic plant pathogens. Thus, applying genetic and omics approaches to study these fungi remains a major challenge, particularly for species with hemiendophytic mycelium. These belong to a distinct phylogenetic lineage within the family Erysiphaceae. To date, only a single draft genome assembly is available for this clade, obtained for Leveillula taurica. Here, we generated the first draft genome assemblies of Pleochaeta shiraiana and Phyllactinia moricola, two tree-parasitic powdery mildew species with hemiendophytic mycelium, representing two genera that have not yet been investigated with genomics tools. The Pleochaeta shiraiana assembly was 96,769,103 bp in length and consisted of 14,447 scaffolds, and the Phyllactinia moricola assembly was 180,382,532 bp in length on 45,569 scaffolds. Together with the draft genome of L. taurica, these resources will be pivotal for understanding the molecular basis of the lifestyle of these fungi, which is unique within the family Erysiphaceae.

War and Peas: Molecular Bases of Resistance to Powdery Mildew in Pea (Pisum sativum L.) and Other Legumes

Grain legumes, or pulses, have many beneficial properties that make them potentially attractive to agriculture. However, the large-scale cultivation of legumes faces a number of difficulties, in particular the vulnerability of the currently available cultivars to various diseases that significantly impair yields and seed quality. One of the most dangerous legume pathogens is powdery mildew (a common name for parasitic fungi of the order Erisyphales). This review examines the methods of controlling powdery mildew that are used in modern practice, including fungicides and biological agents. Special attention is paid to the plant genetic mechanisms of resistance, which are the most durable, universal and environmentally friendly. The most studied legume plant in this regard is the garden pea (Pisum sativum L.), which possesses naturally occurring resistance conferred by mutations in the gene MLO1 (Er1), for which we list here all the known resistant alleles, including er1-12 discovered by the authors of this review. Recent achievements in the genetics of resistance to powdery mildew in other legumes and prospects for the introduction of this resistance into other agriculturally important legume species are also discussed.

Geographic distribution of cryptic species of Plasmopara viticola causing downy mildew on wild and cultivated grape in eastern North America

The putative center of origin of Plasmopara viticola, the causal agent of grape downy mildew, is eastern North America, where it has been described on several members of the family Vitaceae (e.g., Vitis spp., Parthenocissus spp., and Ampelopsis spp.). We have completed the first large-scale sampling of P. viticola isolates across a range of wild and cultivated host species distributed throughout the above region. Sequencing results of four partial genes indicated the presence of a new P. viticola species on Vitis vulpina in Virginia, adding to the four cryptic species of P. viticola recently recorded. The phylogenetic analysis also indicated that the P. viticola species found on Parthenocissus quinquefolia in North America is identical to Plasmopara muralis in Europe. The geographic distribution and host range of five pathogen species was determined through analysis of the internal transcribed spacer polymorphism of 896 isolates of P. viticola. Among three P. viticola species found on cultivated grape, one was restricted to Vitis interspecific hybrids within the northern part of eastern North America. A second species was recovered from V. vinifera and V. labrusca, and was distributed across most of the sampled region. A third species, although less abundant, was distributed across a larger geographical range, including the southern part of eastern North America. P. viticola clade aestivalis predominated (83% of isolates) in vineyards of the European winegrape V. vinifera within the sampled area, indicating that a single pathogen species may represent the primary threat to the European host species within eastern North America.

Conidial germination patterns in powdery mildews

Four conidial germination types namely, polygoni (syn. Pseudoidium), cichoracearum (syn. Reticuloidium), pannosa (syn. Fibroidium) and fuliginea (syn. Magnicellulatae) are commonly used as an aid in the identification of the Oidium anamorphs of powdery mildews. However, results of germination tests and a survey of the literature showed that these types did not adequately distinguish all taxa and did not reflect the range of species covered. Hence two new main types, Striatoidium and Blumeria, are proposed for the newly created genus Neoerysiphe and for the unique pattern of B. graminis. Two new names, orthotubus and brevitubus subtypes of Fibroidium, are proposed for the pannosa and fuliginea types respectively. Also proposed is a special longitubus pattern for the long, undifferentiated, negatively hydrotropic germ tubes prevalent in Erysiphe trifolii and species in Golovinomyces sect. Depressi. The recognition of the Striatoidium type of N. galeopsidis as distinct from the Pseudoidium type of E. elevata facilitated the detection of a simultaneous infection of Catalpa by these two powdery mildews. A key is provided for the identification of Oidium genera based on germination types. A review of germination patterns in the tribe Phyllactinieae found no consistent differences amongst the genera. Golovinomyces sect. Depressi is re-described to accommodate Golovinomyces spp. often having a longitubus pattern of germination. It includes G. cichoracearum var. latisporus, now considered a separate species based on its germination type, other anamorphic morphology and previous molecular sequence analyses. A new combination, Golovinomyces ambrosiae, is proposed for this species. Other anomalies within G. cichoracearum s. lat. were addressed by proposing another new combination, G. fischeri for the former G. cichoracearum var. fischeri that differs from G. cichoracearum s. str. in having larger chasmothecia and a well distinguished anamorph, and by proposing a new species, G. sonchicola, that is biologically, phylogenetically and morphologically distinct from G. cichoracearum s. str.

Oidium neolycopersici: intraspecific variability inferred from amplified fragment length polymorphism analysis and relationship with closely related powdery mildew fungi infecting various plant species

Previous works indicated a considerable variation in the pathogenicity, virulence, and host range of Oidium neolycopersici isolates causing tomato powdery mildew epidemics in many parts of the world. In this study, rDNA internal transcribed spacer (ITS) sequences, and amplified fragment length polymorphism (AFLP) patterns were analyzed in 17 O. neolycopersici samples collected in Europe, North America, and Japan, including those which overcame some of the tomato major resistance genes. The ITS sequences were identical in all 10 samples tested and were also identical to ITS sequences of eight previously studied O. neolycopersici specimens. The AFLP analysis revealed a high genetic diversity in O. neolycopersici and indicated that all 17 samples represented different genotypes. This might suggest the existence of either a yet unrevealed sexual reproduction or other genetic mechanisms that maintain a high genetic variability in O. neolycopersici. No clear correlation was found between the virulence and the AFLP patterns of the O. neolycopersici isolates studied. The relationship between O. neolycopersici and powdery mildew anamorphs infecting Aquilegia vulgaris, Chelidonium majus, Passiflora caerulea, and Sedum alboroseum was also investigated. These anamorphs are morphologically indistinguishable from and phylogenetically closely related to O. neolycopersici. The cross-inoculation tests and the analyses of ITS sequences and AFLP patterns jointly indicated that the powdery mildew anamorphs collected from the above mentioned plant species all represent distinct, but closely related species according to the phylogenetic species recognition. All these species were pathogenic only to their original host plant species, except O. neolycopersici which infected S. alboroseum, tobacco, petunia, and Arabidopsis thaliana, in addition to tomato, in cross-inoculation tests. This is the first genome-wide study that investigates the relationships among powdery mildews that are closely related based on ITS sequences and morphology. The results indicate that morphologically indistinguishable powdery mildews that differed in only one to five single nucleotide positions in their ITS region are to be considered as different taxa with distinct host ranges.

The powdery mildews: a review of the world's most familiar (yet poorly known) plant pathogens

The past decade has seen fundamental changes in our understanding of powdery mildews (Erysiphales). Research on molecular phylogeny demonstrated that Erysiphales are Leotiomycetes (inoperculate discomycetes) rather than Pyrenomycetes or Plectomycetes. Life cycles are surprisingly variable, including both sexual and asexual states, or only sexual states, or only asexual states. At least one species produces dematiaceous conidia. Analyses of rDNA sequences indicate that major lineages are more closely correlated with anamorphic features such as conidial ontogeny and morphology than with teleomorph features. Development of molecular clock models is enabling researchers to reconstruct patterns of coevolution and host-jumping, as well as ancient migration patterns. Geographic distributions of some species appear to be increasing rapidly but little is known about species diversity in many large areas, including North America. Powdery mildews may already be responding to climate change, suggesting they may be useful models for studying effects of climate change on plant diseases.