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Technological advances in genome sequencing have improved our ability to catalog genomic variation and have led to an expansion of the scope and scale of genetic studies over the past decade. Yet, for agronomically important plant pathogens such as the downy mildews (Peronosporaceae), the scale of genetic studies remains limited. This is, in part, due to the difficulties associated with maintaining obligate pathogens and the logistical constraints involved in the genotyping of these species (e.g., obtaining DNA of sufficient quantity and quality). To gain an evolutionary and ecological perspective of downy mildews, adaptable methods for the genotyping of their populations are required. Here, we describe a targeted enrichment (TE) protocol to genotype isolates from two Pseudoperonospora species (P. cubensis and P. humuli), using less than 50 ng of mixed pathogen and plant DNA for library preparation. We were able to enrich 830 target genes across 128 samples and identified 2,514 high-quality single nucleotide polymorphism (SNP) variants. Using these SNPs, we detected significant genetic differentiation (analysis of molecular variance [AMOVA], P = 0.01) between P. cubensis subpopulations from Cucurbita moschata (clade I) and Cucumis sativus (clade II) in the state of Michigan. No evidence of location-based differentiation was detected within the P. cubensis (clade II) subpopulation in Michigan. However, a significant effect of location on the genetic variation of the P. humuli subpopulation was detected in the state (AMOVA, P = 0.01). Mantel tests found evidence that the genetic distance among P. humuli samples was associated with the physical distance of the hop yards from which the samples were collected (P = 0.005). The differences in the distribution of genetic variation of the Michigan P. humuli and P. cubensis subpopulations suggest differences in the dispersal of these two species. The TE protocol described here provides an additional tool for genotyping obligate biotrophic plant pathogens and the execution of new genetic studies. [Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license .

The ability to detect and quantify aerially dispersed plant pathogens is essential for developing effective disease control measures and epidemiological models that optimize the timing for control. There is an acute need for managing the downy mildew pathogens infecting cucurbits and hop incited by members of the genus Pseudoperonospora (Pseudoperonospora cubensis clade 1 and 2 isolates and Pseudoperonospora humuli, respectively). A highly specific multiplex TaqMan quantitative polymerase chain reaction (PCR) assay targeting unique sequences in the pathogens’ mitochondrial genomes was developed that enables detection of all three taxa in a single multiplexed amplification. An internal control included in the reaction evaluated whether results were influenced by PCR inhibitors that can make it through the DNA extraction process. Reliable quantification of inoculum as low as three sporangia in a sample was observed. The multiplexed assay was tested with DNA extracted from purified sporangia, infected plant tissue, and environmental samples collected on impaction spore traps samplers. The ability to accurately detect and simultaneously quantify all three pathogens in a single multiplexed amplification should improve management options for controlling the diseases they cause.

Hops have expanded as a niche crop in Michigan and other production areas in the eastern United States, but growers in these regions face annual downy mildew outbreaks incited by Pseudoperonospora humuli, exacerbated by frequent rainfall and high relative humidity. We evaluated the efficacy of foliar- and drench-applied fungicides against downy mildew and examined Michigan isolates for point mutations linked to carboxylic acid amide (CAA) resistance. Disease severity and density were assessed weekly in 2016 and 2017 in nontrellised research hop yards in Michigan. Area under the disease progress curve values for disease severity were significantly lower for plants treated with oxathiapiprolin, ametoctradin/dimethomorph, fluopicolide, cyazofamid, or mandipropamid (90.6 to 100% control) compared with those treated with fosetyl-Al (64.3 to 93.0% control) at both locations for both years. Drench treatments of fluopicolide and oxathiapiprolin/mefenoxam reduced disease density and severity at both locations but were only moderately effective (76.4 to 91.5% control). To assess CAA resistance, the cellulose synthase CesA3 gene was aligned using reference downy mildew species and primers designed to amplify the 1105 and 1109 amino acids. Point mutations conferring CAA resistance were not detected at these loci for sporangia from 42 symptomatic shoots collected from 11 commercial hop yards. These efficacy results for hop downy mildew are needed to guide disease recommendations in this expanding Michigan industry. The absence of resistant genotypes indicates that Michigan growers can continue to utilize CAA-containing commercial fungicides as part of an overall downy mildew management program.

Pseudoperonospora humuli is an obligate biotrophic oomycete that causes downy mildew, one of the most devastating diseases of cultivated hop, Humulus lupulus. Downy mildew occurs in all production areas of the crop in the Northern Hemisphere and Argentina. The pathogen overwinters in hop crowns and roots, and causes considerable crop loss. Downy mildew is managed by sanitation practices, planting of resistant cultivars, and fungicide applications. However, the scarcity of sources of host resistance and fungicide resistance in pathogen populations complicates disease management. This review summarizes the current knowledge on the symptoms of the disease, life cycle, virulence factors, and management of hop downy mildew, including various forecasting systems available in the world. Additionally, recent developments in genomics and effector discovery, and the future prospects of using such resources in successful disease management are also discussed. Taxonomy Class: Oomycota; Order: Peronosporales; Family: Peronosporaceae; Genus: Pseudoperonospora; Species: Pseudoperonospora humuli. Disease symptoms The disease is characterized by systemically infected chlorotic shoots called “spikes". Leaf symptoms and signs include angular chlorotic lesions and profuse sporulation on the abaxial side of the leaf. Under severe disease pressure, dark brown discolouration or lesions are observed on cones. Infected crowns have brown to black streaks when cut open. Cultivars highly susceptible to crown rot may die at this phase of the disease cycle without producing shoots. However, foliar symptoms may not be present on plants with systemically infected root systems. Infection process Pathogen mycelium overwinters in buds and crowns, and emerges on infected shoots in spring. Profuse sporulation occurs on infected tissues and sporangia are released and dispersed by air currents. Under favourable conditions, sporangia germinate and produce biflagellate zoospores that infect healthy tissue, thus perpetuating the infection cycle. Though oospores are produced in infected tissues, their role in the infection cycle is not defined. Control Downy mildew on hop is managed by a combination of sanitation practices and timely fungicide applications. Forecasting systems are used to time fungicide applications for successful management of the disease. Useful Websites https://content.ces.ncsu.edu/hop‐downy‐mildew (North Carolina State University disease factsheet), https://www.canr.msu.edu/resources/michigan‐hop‐management‐guide (Michigan Hop Management Guide), http://uspest.org/risk/models (Oregon State University Integrated Plant Protection Center degree‐day model for hop downy mildew), https://www.usahops.org/cabinet/data/Field‐Guide.pdf (Field Guide for Integrated Pest Management in Hops)., Pseudoperonospora humuli is an obligate biotrophic oomycete that causes downy mildew, one of the most devastating diseases of cultivated hop.

The genus Pseudoperonospora, an obligate biotrophic group of Oomycota, causes the most destructive foliar downy mildew disease on many economically important crops and wild plants. A previously unreported disease by Pseudoperonospora was found on oriental pickling melon (Cucumis melo var. conomon) in Korea, which is a minor crop cultivated in the temperate climate zone of East Asia, including China, Korea, and Japan. Based on molecular phylogenetic and morphological analyses, the causal agent was identified as Pseudoperonospora cubensis, and its pathogenicity has been proven. Importantly, two phylogenetic clades of P. cubensis, harboring probably two distinct species, were detected within the same plots, suggesting simultaneous coexistence of the two clades. This is the first report of P. cubensis causing downy mildew on oriental pickling melon in Korea, and the confirmation of presence of two phylogenetic clades of this pathogen in Korea. Given the high incidence of P. cubensis and high susceptibility of oriental pickling melon to this disease, phytosanitary measures, including rapid diagnosis and effective control management, are urgently required.

In the present study, the effect of potassium phosphite on Pseudoperonospora cubensis-inoculated cucumber plants was investigated. Different defense-related enzymes including laccase, polyphenoloxidase and glucanase as well as total protein and lignin contents were analyzed. Anatomical alterations in plant tissues were analyzed using a light microscope. Expression changes in major pathogenesis-related genes were studied at different time courses. The highest expression of glucanase was observed in pre-inoculated plants (97% higher than control) at 24 h with 97% increase compared to the control plants, while Chitinase transcripts were accumulated at a maximum level in potassium phosphite-treated plants 96 h after inoculation with 93% increase over control plants. Analysis of polygalacturonase inhibitor proteins gene expression revealed a transcription peak (96% increase over control plants) 48 h after inoculation. The potassium phosphite-treated plants exhibited an increase in β-1,3-glucanase (82%) enzymatic activity as well as total protein (53%), polyphenoloxidase (21%), laccase (11%) and lignin contents (15%) in comparison to the control. The results of the anatomical assay showed an increase in the vascular bundle diameter in potassium phosphite-treated plants (174 μm) and a decrease in pathogen-treated leaves (66 μm) compared with the control (100 μm). It can be suggested that potassium phosphite treatment induced higher expression of plant defense genes and increased laccase and polyphenoloxidase activities which in turn enhanced lignin deposition in plant tissues. The findings of the present study would be implemented for designing a controlling program to decrease the adverse effect of Pseudoperonospora cubensis on cucumber plants.

Potassium phosphite (KPhi) is widely used as a resistance inducer to protect plants against fungal pathogens. In the present study, the effect of KPhi on the activation of defence-related enzymes and biochemicals in Pseudoperonospora cubensis-challenged cucumber plants was investigated. Cucumber plants were treated with KPhi before or after inoculation with P. cubensis and leaf samples were collected at different time courses for physiological and biochemical assessments. Results revealed that the activity of reactive oxygen species (ROS)-scavenging enzymes like catalase, guaiacol peroxidase, superoxide dismutase and ascorbate peroxidase as well as proline and total carbohydrates contents were significantly increased by KPhi application, while hydrogen peroxide (H2O2) concentration, as a disease damage indicator was reduced. The maximum activity of ROS-scavenging system was achieved 3–4 days after KPhi application. These findings suggest that KPhi application prior to pathogen infection efficientl...

The downy mildew pathogen, Pseudoperonospora humuli, forms oospores abundantly in diseased hop tissue. Diverse monosporangial isolates of P. humuli derived from samples collected in Japan, Germany and the USA readily formed oospores within hop leaves when inoculated singly, suggesting homothallism. Single zoospore isolates also readily formed oospores within hop leaves, further supporting the homothallic nature of this oomycete. The majority of oospores were deemed viable based on cytoplasm characteristics and plasmolysis assays. However, disease symptoms failed to develop when hop leaves were inoculated with newly formed oospores, even when oospore conditioning was attempted with treatment with potassium permanganate or β-glucuronidase/arylsulphatase, brief exposure to freezing temperature, or passage through an earthworm. Oospores derived from a monosporangial isolate of P. humuli that overwintered outdoors in infested leaves buried in soil also failed to cause downy mildew. Pseudoperonospora humuli is homothallic and oospores of the organism appear to require as yet unknown conditions to stimulate their germination and/or infection.

In 2017 in Poland, cucumber was one of the most important field vegetables cultivated on 13 892 ha and with 2 490 988 tons of production

Hop downy mildew (DM) is an obligate parasite causing severe losses in hop if not controlled. Resistance to this pathogen is a primary goal for hop breeding programs. The objective of this study was to identify QTLs linked to DM resistance. Next-genera- tion-sequencing was performed on a mapping popu- lation segregating for DM resistance levels. Cloned plants were grown in a RCBD with three replicates under three environments: greenhouse (GH), field plots in Oregon (OR), Corvallis field plots in Wash- ington (WA), Yakima). The linkage map of 3,341 SNP markers was determined with a four-stage process using Rqtl, TMAP, Joinmap v 4.0 and MERGEMAP. QTL analysis was performed using JMP Genomics and TASSEL 5.0. SNP markers were distributed across 11 linkage groups (LGs) with an average distance between markers of 0.2 cM and total distance of 745.9 cM. QTLs for all three environments were identified using multiple interval mapping. Overall heritability across the three environments varied from h 2 = 0.38 (GH) to 0.57 (OR). A total of 22 QTLs across 8 LG were identified for DM resistance: 5 identified from OR field data, 12 using WA data and five from GH DM data. No epistasis was observed. This study points out the complexity of genetic control of DM resistance in hop and identifies several markers that can be potentially be used to select for DM resistance in hop. It also provides the first linkage map suitable for genome sequencing due to the high density of SNP markers.

An isolate of Bacillus amyloliquefaciens subsp. plantarum CC110 was selected as an effectiveantagonist for biological control of cucumber downy mildew caused by Pseudoperonospora cubensis.Temperature range for growth of CC110 isolate was 7.5~55.0 o C, and its optimal temperature at 36.6 o C. pHrange for growth of CC110 isolate was 4.5~9.5, and its optimal pH at 7.0. In this study, the most effectivesources of carbon and nitrogen for growth of CC110 isolate were fructose and yeast extract, respectively. Thevolatile of CC110 isolate was found to be effective to control downy mildew on cucumber showing nodiseased area whereas that of control was 13.2% using the I plate bioassay. The culture broth and cells ofisolate CC110 cultured in TSB media for 48 hours at 28 o C inhibited occurrence of cucumber downy mildew.The cells and culture broth were transformed into sporangia of P. cubensis by in observation under lightmicroscope and scanning electron microscope. Key words Biological control, Cucumber, Cultivation, Downy mildew, Bacillus amyloliquefaciens subsp.plantarum CC110, Volatile compounds

Pseudoperonospora cubensis is one of the most devastating diseases of cucurbitaceous crops. The pathogen has a worldwide distribution and occurs in all major cucurbit growing areas. It had been noticed for the first time at the end of the 19th century, but it became a globally severe disease as recently as 1984 in Europe and 2004 in North America. Despite its economic importance, species concepts in Pseudoperonospora are debated. Here, we report that the genus Pseudoperonospora contains cryptic species distinct from the currently accepted ones. Pseudoperonospora on Celtis is split into two phylogenetic lineages and Pseudoperonospora humuli is confirmed as a species distinct from the Cucurbitaceae-infecting lineages. A cryptic species occupying a basal position within the Pseudoperonospora cubensis complex is revealed to be present on Humulus japonicus, thus providing evidence that the host jump that gave rise to Pseudoperonospora cubensis likely occurred from hops. Notably, Cucurbitaceae infecting pathogens are present in two cryptic sister species or subspecies. Clade 1 contains primarily specimens from North America and likely represents Pseudoperonospora cubensis s.str.. Pre-epidemic isolates in clade 2 originate from Japan and Korea, suggesting this cryptic species or subspecies is indigenous to East Asia. Recent samples of this lineage from epidemics in Europe and the United States cluster together with clade 2. It thus seems possible that this lineage is associated with the recent severe epidemics of cucurbit downy mildew and is now naturalised in North America and Europe.

Oomycetes contain some of the economically most important pathogens of flowering plants. Most have a rather narrow host range, often being restricted to single host species. In downy mildews and other obligate biotrophic plant parasites, like powdery mildews and rusts, delimitating species on grounds of morphological characteristics is often hardly possible and thus often based on only subtle differences. This has led to the widespread application of a broad species concept for these organisms. Consequently, despite the fact that morphological differences were reported for Pseudoperonospora cubensis from different host species, the corresponding new pathogen species were not accepted as being independent, and the host range of Pseudoperonospora cubensis is reported to encompass more than 50 host species in the Cucurbitaceae in temperate to tropical climates. However, recent studies have reported narrow host ranges for other downy mildew genera and advocated a narrow species concept. Here, we report successful colonisation of five different tribes of the Cucurbitaceae by a strain of Pseudoperonospora cubensis and demonstrate that the host matrix has a major impact on the morphology of the pathogen. On the basis of five morphological criteria significant differences could be found for all hosts. These differences were more pronounced in phylogenetically unrelated than in related hosts. Our results provide evidence for a broad host range of Pseudoperonospora cubensis and demonstrate that species delimitation based on morphological characters is not feasible in Pseudoperonospora on Cucurbitaceae. Also in other biotrophic plant pathogens, the situation could be similar, thus necessitating thorough morphological, molecular phylogenetic and cross inoculation experiments for species recognition.

Species concepts are a notoriously difficult taxonomic problem in plant-parasitic fungal-like organisms such as downy mildews (Peronosporomycetes, Peronosporales). This is particularly evident in the largest downy mildew genus, Peronospora, which contains a number of economically important pathogens. Here, we investigate relationships of Peronospora species infecting Fabaceae (angiosperms, Rosidae) originating from various collections from different species of host plants and from different European locations by molecular phylogenetic analysis of ITS sequences. Molecular trees were inferred with ML, MP and Bayesian methods and rooted with Pseudoperonospora. As in other downy mildew groups, molecular data mainly support the use of narrow species delimitations and host range as a taxonomic marker. Fabaceae parasites appear to be subdivided into a number of lineages displaying a considerable degree of host specialization with respect to host genera, as well as host subgenera or species. The number of repeats of a repetitive part of the ITS1 is, within limits, characteristic of subgroups within the cluster of Trifolium parasites. We reveal new hosts for Peronospora found on the Iberian Peninsula.

Oomycetes are “fungal-like” protista that are phylogenetically distinct from fungi, they are diploid, and cell walls contain cellulose. Oomycetes comprise major plant pathogens within the orders Saprolegniales, Pythiales, Peronosporales (“downy mildews” including species of Plasmopara, Phytophthora, Peronospora, Pseudoperonospora, Bremia), and Sclerosporales. Control of oomycete diseases relies mainly on chemical measures using products within 16 different chemical groups, among which the phenylamides (PAs), quinone outside inhibitors (QoIs), carboxylic acid amides (CAAs), and multisite inhibitors are most widely used. However, resistance evolved against most single-site inhibitors in many oomycete pathogen species. Resistance against PAs (inhibitors of RNA polymerase I) was detected in most oomycete pathogen species; the molecular mechanism was associated recently with the presence of the Y382F mutation in the RNApolI gene. QoIs inhibit electron transport in complex III of the respiration chain; two major mutations, G143A (in Plasmopara viticola) and F129L (in Pythium spp.), have been identified in the cytochrome b, cyt b gene coding for resistance. However, no resistant isolates were observed in Phytophthora species. CAAs inhibit cellulose synthase; several mutations, G1105S/V (in P. viticola) and G1105V/W (in Pseudoperonospora cubensis), were detected in the cellulose synthase, CesA3 gene of CAA-resistant field isolates. In Phytophthora infestans and P. capsici, no CAA-resistant field isolates were discovered, whereas in artificial mutants, the mutations G1105A/V and V1109L/M were found. The L1109 and M1109 amino acid configurations are responsible for the intrinsic insensitivity to CAAs of the entire genus Pythium and all oomycetes outside Peronosporales. Resistance is inherited by one recessive nuclear gene (CesA3) for CAAs, one maternal mitochondrial gene (cytb) for QoIs, and mainly one semidominant nuclear gene for PAs. The consequences of the molecular mechanisms for evolution and stability of resistance are discussed in the light of oomycete pathogen biology.

The downy mildew diseases are caused by different fungus species in several genera in the class Oomycetes. They comprise a major group of diseases affecting a large number of crops. In India, downy mildews have been reported on several economically important crops, including maize, sorghum, pearl millet, onion, soybean, cucurbits, and grapes, causing severe economic losses in some regions and seasons. Pathogens reported to cause downy mildews in India include species in the genera, Perenosclerospora, Perenospora, Pseudoperonospora, Plasmopara, Sclerophthora and Sclerospora. In view of the economic importance of the crops, and prevalence and severity of the disease, the downy mildews have been classified into high, moderate and low research priority problems. In this article we present a brief review of the work done in India related to pathogen biology, epidemiology and management methods, and provide an outlook for future research.

Downy mildew pathogens affect several economically important crops worldwide but, due to their obligate nature, few genetic resources are available for genomic and population analyses. Draft genomes for emergent downy mildew pathogens such as the oomycetePseudoperonospora cubensis, causal agent of cucurbit downy mildew, have been published and can be used to perform comparative genomic analysis and develop tools such as microsatellites to characterize pathogen population structure. We used bioinformatics to identify 2,738 microsatellites in theP. cubensispredicted transcriptome and evaluate them for transferability to the hop downy mildew pathogen,Pseudoperonospora humuli, since no draft genome is available for this species. We also compared the microsatellite repertoire ofP. cubensisto that of the model organismHyaloperonospora arabidopsidis, which causes downy mildew in Arabidopsis. Although trends in frequency of motif-type were similar, the percentage of SSRs identified fromP. cubensistranscripts differed significantly fromH. arabidopsidis. The majority of a subset of microsatellites selected for laboratory validation (92%) produced a product inP. cubensisisolates, and 83 microsatellites demonstrated transferability toP. humuli. Eleven microsatellites were found to be polymorphic and consistently amplified inP. cubensisisolates. Analysis ofPseudoperonosporaisolates from diverse hosts and locations revealed higher diversity inP. cubensiscompared toP. humuliisolates. These microsatellites will be useful in efforts to better understand relationships withinPseudoperonosporaspecies andP. cubensison a population level.

The ultrastructure of the sporangium, particularly the dehiscence apparatus in Pseudoperonospora humuli is similar to the one described in P. cubensis. This similarity is considered an indication of the homogeneity of the genus Pseudoperonospora. The dehiscence apparatus in P. humuli is also similar to those occurring in other closely related, and even more distant genera of Oomycota. The importance of the poroid condition in distinguishing Pseudoperonospora from Peronospora is emphasized. The dehiscence apparatus is considered as the most appropriate term for the structures present in genera of Oomycota having poroid dissemination organs.

Pseudoperonospora cubensis is a species of water mould known for causing downy mildew on cucurbits. 454 GS FLX Titanium sequencing data was used to obtain its complete mitochondrial genome (38 553 bp). The mitogenome contains 60 genes, including two ribosomal RNA, 25 transfer RNA, 15 ribosomal proteins, five open reading frames (ORFs). The rps3 and rpl16 overlapped each other by 14 bp. The gene order and composition of P. cubensis was similar to that of most other oomycetes, and its GC content was 22.4%. It is the first report of the complete mitochondrial genome in the genus Pseudoperonospora. Phylogeny analysis indicates that P. cubensis has a close genetic relationship with genus Phytophthora.

Mitchell, M. N., Ocamb, C. M., Grunwald, N. J., Mancino, L. E., and Gent, D. H. 2011. Genetic and pathogenic relatedness of Pseudoperonospora cubensis and P. humuli . Phytopathology 101:805-818. The most economically important plant pathogens in the genus Pseudoperonospora (family Peronosporaceae) are Pseudoperonospora cubensis and P. humuli , causal agents of downy mildew on cucurbits and hop, respectively. Recently, P. humuli was reduced to a taxonomic synonym of P. cubensis based on internal transcribed spacer (ITS) sequence data and morphological characteristics. Nomenclature has many practical implications for pathogen identification and regulatory considerations; therefore, further clarification of the genetic and pathogenic relatedness of these organisms is needed. Phylogenetic analyses were conducted considering two nuclear and three mitochondrial loci for 21 isolates of P. cubensis and 14 isolates of P. humuli, and all published ITS sequences of the pathogens in GenBank. There was a consistent separation of the majority of the P. humuli isolates and the P. cubensis isolates in nuclear, mitochondrial, and ITS phylogenetic analyses, with the exception of isolates of P. humuli from Humulus japonicus from Korea. The P. cubensis isolates appeared to contain the P. humuli cluster, which may indicate that P. humuli descended from P. cubensis. Host-specificity experiments were conducted with two reportedly universally susceptible hosts of P. cubensis and two hop cultivars highly susceptible to P. humuli. P. cubensis consistently infected the hop cultivars at very low rates, and sporangiophores invariably emerged from necrotic or chlorotic hypersensitive-like lesions. Only a single sporangiophore of P. humuli was observed on a cucurbit plant during the course of the studies. Together, molecular data and host specificity indicate that there are biologically relevant characteristics that differentiate P. cubensis and P. humuli that may be obfuscated if P. humuli were reduced to a taxonomic synonym of P. cubensis . Thus, we recommend retaining the two species names P. cubensis and P. humuli until the species boundaries can be resolved unambiguously.

Foreword.- Biology and Evolution.- Phylogenetic investigations in the genus Pseudoperonospora reveal overlooked species and cryptic diversity in the P. cubensis species cluster.- Host matrix has major impact on the morphology of Pseudoperonospora cubensis.- Cucurbit downy mildew (Pseudoperonospora cubensis) - biology, ecology, epidemiology, host-pathogen interaction and control.- Spinach: better management of downy mildew and white rust through genomics.- Advances in sunflower downy mildew research.- Resistance mechanisms and host-parasite interactions.- Histological responses of host and non-host plants to Hyaloperonospora parasitica.- Distribution of race-specific resistance against Bremia lactucae in natural populations of Lactuca serriola.- Site-specific field resistance of grapevine to Plasmopara viticola correlates to altered gene expression and was not modulated by the application of organic amendments.- Influence of nitric oxide and reactive oxygen species on development of lettuce downy mildew in Lactuca spp..- Identification of genes expressed during the compatible interaction of grapevine with Plasmopara viticola through suppression subtractive hybridization (SSH).- Population ecology and control.- Quantitative studies on downy mildew (Peronospora destructor Berk. Casp.) affecting onion seed production in southern Uruguay.- Diversity and fitness of Plasmopara viticola isolates resistant to QoI fungicides.- Evolution of Qol resistance in Plasmopara viticola oospores.- EMS and UV irradiation induce unstable resistance against CAA fungicides in Bremia lactucae.- Influence of dosage, storage time and temperature on efficacy of metalaxyl-treated seed for the control of pearl millet downy mildew.

Zoosporangia of Plasmopara halstedii , causal agent of sunflower downy mildew, were infective after storage in liquid nitrogen for up to 4·75 yr, using a dry spore, fast-freeze (DSFF) technique. The DSFF technique consists of vacuuming air-dried zoosporangia with a cyclone collector, placing them in a polypropylene screw-cap tube without any cryoprotectant, and immediately placing them in a liquid nitrogen storage tank. Addition of a cryoprotectant or the use of a controlled freezing rate did not enhance infectivity of zoosporangia stored in liquid nitrogen over that of the basic DSFF method. The ability of air-dried zoosporangia to withstand liquid nitrogen storage without cryoprotectants appears to be due to their lower water content compared to that of spores collected by washing and filtering. Spores stored in liquid nitrogen by the DSFF technique were infective after five freeze/thaw cycles. The technique was also successful with Peronospora parasitica and Pseudoperonospora cubensis sporangia, but not with Peronosclerospora sorghi .

2,4,6-Trihydroxy-, 2,4,6-trihydroxy-5-methyl-, and 2,4,5,6-tetrahydroxy-substituted 3-(3-phenylpropionyl)benzaldehydes were efficiently synthesized. The first two compounds showed bactericidal activity for Pseudoperonospora cubersis .

Bladder dock (Rumex vesicarius L.) is an important biennial bushy vegetable in southern India. The young leaves have a piquant flavor and sour taste, and are eaten boiled or raw in salads. The dried roots are ground into powder and used to treat diarrhoea and bleeding. To our knowledge, this is the first report of Pseudoperonospora rumicis on R. vesicarius in India. Pseudoperonospora rumicis is the second downy mildew pathogen recorded on Rumex vesicarius and is of potential concern to Rumex growers Accepted for publication 18 August 2010. Published 15 September 2010.

A rapid assay for monitoring peroxidase activity in melon as a marker for resistance to Pseudoperonospora cubensis was developed. A high correlation was demonstrated between peroxidase activity and resistance in test wells containing leaf disks of melon plants with known levels of susceptibility or resistance to P. cubensis. The possibility of using such an assay for a preliminary selection of melons resistant to P. cubensis is suggested. ZusammenfassungEin schneller Assay, um die Peroxidaseaktivitat in Melonen, die als Pseudoperonospora cufeensM-Resistenzmarker dienten, festzustellen Entwickeh wurde ein schneller Assay, um die Peroxidaseaktivitat in Melonen, die als Pseudoperonospora cubensis-Resistenzmarker dienten, festzustellen. Es gab eine gute Korrelation zwischen Peroxidaseaktivitat und Resistenz bei Blattscheibchen von Melonenpfianzen, bei denen der Grad der Anfalligkeit bzw, Resistenz gegenuber P. cubensis bekannt war. Die mogliche Anwendung dieses Assays bei der primaren Selektion von Melonenpfianzen tnit einer Resistenz gegen P. cubensis wird vorgeschlagen.

2,4,6-Trihydroxy-, 2,4,6-trihydroxy-5-methyl-, and 2,4,5,6-tetrahydroxy-substituted 3-(3-phenylpropionyl)benzaldehydes were efficiently synthesized. The first two compounds showed bactericidal activity for Pseudoperonospora cubersis .

Genome size was determined, by nuclear Feulgen staining and image analysis, in 46 accessions of 31 species of Peronosporales (Oomycota), including important plant pathogens such as Bremia lactucae, Plasmopara viticola, Pseudoperonospora cubensis, and Pseudoperonospora humuli. The 1C DNA contents ranged from 0.046 (45. 6 Mb) to 0.163 pg (159.9 Mb). This is 0.041- to 0.144-fold that of Glycine max (soybean, 1C = 1.134 pg), which was used as an internal standard for genome size determination. The linearity of Feulgen absorbance photometry method over this range was demonstrated by calibration of Aspergillus species (1C = 31-38 Mb) against Glycine, which revealed differences of less than 6% compared to the published CHEF data. The low coefficients of variation (usually between 5 and 10%), repeatability of the results, and compatibility with CHEF data prove the resolution power of Feulgen image analysis. The applicability and limitations of Feulgen photometry are discussed in relation to other methods of genome size determination (CHEF gel electrophoresis, reassociation kinetics, genomic reconstruction) that have been previously applied to Oomycota.

Before the advent of molecular phylogenetics, species concepts in the downy mildews, an economically important group of obligate biotrophic oomycete pathogens, have mostly been based upon host range and morphology. While molecular phylogenetic studies have confirmed a narrow host range for many downy mildew species, others, like Pseudoperonospora cubensis affect even different genera. Although often morphological differences were found for new, phylogenetically distinct species, uncertainty prevails regarding their host ranges, especially regarding related plants that have been reported as downy mildew hosts, but were not included in the phylogenetic studies. In these cases, the basis for deciding if the divergence in some morphological characters can be deemed sufficient for designation as separate species is uncertain, as observed morphological divergence could be due to different host matrices colonised. The broad host range of P. cubensis (ca. 60 host species) renders this pathogen an ideal model organism for the investigation of morphological variations in relation to the host matrix and to evaluate which characteristics are best indicators for conspecificity or distinctiveness. On the basis of twelve morphological characterisitcs and a set of twelve cucurbits from five different Cucurbitaceae tribes, including the two species, Cyclanthera pedata and Thladiantha dubia, hitherto not reported as hosts of P. cubensis, a significant influence of the host matrix on pathogen morphology was found. Given the high intraspecific variation of some characteristics, also their plasticity has to be taken into account. The implications for morphological species determination and the confidence limits of morphological characteristics are discussed. For species delimitations in Pseudoperonospora it is shown that the ratio of the height of the first ramification to the sporangiophore length, ratio of the longer to the shorter ultimate branchlet, and especially the length and width of sporangia, as well as, with some reservations, their ratio, are the most suitable characteristics for species delimitation.

SUMMARY Significant differences in reflectances were found between healthy and diseased watermelon leaves in the 500–640 nm visible region and 740–900 nm near infrared regions of the electromagnetic spectrum. Spectrophotometric reflectance measurements were made to determine differences between healthy and diseased watermelon (Citrullus vulgaris) leaves over the 500–2700 nm wave length interval. Plants with Fusarium wilt, downy mildew (Pseudoperonospora), and watermelon mosaic were observed during the growing season. Early symptoms of the diseases had significantly greater reflectance values than the advanced stages of the diseases. A ratio of two wavelength bands, 500–640 nm and 660–900 nm gave an accurate index for determining stages of disease, both early and advanced. There were greater differences in reflectance between all three diseases and healthy leaves in both early and advanced stages in the 900–1400 nm, 1500–1900 nm, and 2000–2400 nm middle infra-red wavelengths of the electromagnetic spectrum than in the visible 450–740 nm range. All three diseases had significant differences in the 700–900 nm near-infra-red range.

A muskmelon (Cucumis melo L.) breeding line, PI-124111F, is a seventh-generation selection derived from PI-124111, which is resistant to downy mildew (Pseudoperonospora cubenis pathotype 3), powdery mildew (races 1 and 2 ofSphaerotheca fuliginea) and Fusarium wilt (Fusarium oxysporum f. sp.melonis, races 0, 1 and 2). This is the only breeding line known to carry multiple-race resistance to these diseases. PI-12411IF is a monoecious muskmelon with poor fruit characteristics.

Since June 1973 Pseudoperonospora cubensis (Berk & Curt) Rost., which causes downy mildew in cucumber, occurs in the Netherlands. The resistance against this disease appears to be based on one recessive gene in linkage with the dominant gene D for dull green fruit skin colour. It is demonstrated that this recessive gene is also linked with one of the genes for resistance to powdery mildew present in the variety Ashley.

SUMMARY Experiments are described in which successive groups of healthy, susceptible, potted hop-plants were exposed, each 24 h, to the weather and to sources of Pseudoperonospora kumuli inoculum in an unsprayed hop garden for 48 h periods during May and June in 1969, 1970 and 1971. The infection which arose after standardized incubation in isolation was measured and then related, both by inspection and by multiple regression analysis, to the conditions during exposure. Severe infection could be associated with certain sequences of events in which rain contributed substantially to plant wetness; relatively light or no infection developed when dew provided the wetness of a period. Infection was markedly correlated with the occurrence of infection periods, as defined from earlier growth-room studies, especially when dew-wetness was omitted. The regression analysis showed that, in all years, infection was highly correlated with variables reflecting wet conditions. It was inconsistently correlated with vapour pressure deficit (VPD) and with sunshine, not at all with temperature, and with airborne spore concentration only in the combined years. The spore catches of a funnel trap, used in 1971, which depended upon rain varied closely with infection. R2 values for multiple regression equations indicated that, in the years separately and together, well over 70% of the variation in infection could be explained by the environmental variables and, in 1971, almost 90%. Variables (temperature, VPD, airborne spores) which singly were poorly correlated with infection often significantly improved regression equations which were based on variables expressing wetness. When prediction of each year's infection levels was attempted using selected regression equations calculated from data of the other 2 years there was, in general, very good agreement between measured and estimated values. The best predictions were given by an equation utilizing rain-wetness duration, rainfall amount and airborne spore concentration while an equation based solely on relative humidity and rainfall was only slightly inferior. The results are discussed in relation to the possible value of the regression models for short-term forecasting of hop downy mildew.

Summary 1 Peronospora Urticae, founded by Berkeley in 1846 on Botrytis Urticae Libert MSS., has an oval conidium, minutely apiculate at the apex—as described by Berkeley and Broome. The present writers have previously pointed out that the conidium germinates by means of zoospores and that the species is therefore to be referred to the genus Pseudoperonospora. 2 In 1863 de Bary described a species having a broadly ovoid or subglobose conidium, very obtuse at the apex and with no apiculus and, misled through seeing a drawing sent by Berkeley to Caspary, identified it as Peronospora Urticae (Lib.). Subsequent authors (e.g. Berlese, Fischer, Migula, Gaumann) have followed de Bary in this identification. 3 The examples sent out under the name Peronospora Urticae in various Exsiccati are sometimes Berkeley's and sometimes de Bary's species. 4 The name Peronospora deBaryi nomen novum is proposed for de Bary's species. 5 Pseudoperonospora Urticae occurs on Urtica dioica and U. urens, and Peronospora deBaryi on U. urens only. The distribution of each species is given. 6 Living examples of P. deBaryi were first met with by the writers at Wye, Kent, in 1926; the study of these and the evidence obtained in an inoculation experiment have shown that P. deBaryi causes a systemic infection of the host-plant.

The somatic structure of some species of Peronospora, Pseudoperonospora, Plas-mopara, Bremia and Albugo is described. The effect of the fungi on their hosts and the nature and importance of their haustoria are considered. Haustoria are not essential to all these parasites since they are not developed if sufficient food is supplied to the invading hyphae by diffusion from the host cells. The haustoria of Peronospora and Pseudoperonospora are hyphal branches which, when not impeded in their growth by the production of haustorial sheaths, are filamentous. Alterations in form and elaboration of haustoria, whether it is the development of branches or the formation of vesicles, is a result of host reaction. The haustoria of Albugo, Plasmopara and Bremia are more specialized, and these groups may therefore be considered to be more advanced as regards this feature than Peronospora and Pseudoperonospora . Some correlation has been found in certain species between the extent of development of haustoria and the supply of food, haustoria are simple in regions of the host where it is plentiful and they are more elaborate, presenting a more extensive surface where it is in short supply. The length of life of the fungus mycelium in the host, whether it is relatively harmless and capable of perennation or whether it causes serious disease and rapid death, does not influence the form of haustoria. The vegetative structure and form of haustoria of these fungi is not correlated with their taxonomie position nor with the taxonomic position of their hosts.

Plant parasitism has independently evolved as a nutrition strategy in both true fungi and Oomycetes (stramenopiles). A large number of species within phytopathogenic Oomycetes, the so-called downy mildews, are defined as obligate biotrophs since they have not, to date, been cultured on any artificial medium. Other genera like Phytophthora and Pythium can in general be cultured on standard or non-standard agar media. Within all three groups there are many important plant pathogens responsible for severe economic losses as well as damage to natural ecosystems. Although they are important model systems to elucidate the evolution of obligate parasites, the phylogenetic relationships between these genera have not been clearly resolved. Based on the most comprehensive sampling of downy mildew genera to date and a representative sample of Phytophthora subgroups, we inferred the phylogenetic relationships from a multi-gene dataset containing both coding and non-coding nuclear and mitochondrial loci. Phylogenetic analyses were conducted under several optimality criteria and the results were largely consistent between all the methods applied. Strong support is achieved for monophyly of a clade comprising both the genus Phytophthora and the obligate biotrophic species. The facultatively parasitic genus Phytophthora is shown to be at least partly paraphyletic. Monophyly of a cluster nested within Phytophthora containing all obligate parasites is strongly supported. Within the obligate biotrophic downy mildews, four morphologically or ecologically well-defined subgroups receive statistical support: (1) A cluster containing all species with brownish-violet conidiosporangia, i.e., the genera Peronospora and Pseudoperonospora; (2) a clade comprising the genera with vesicular to pyriform haustoria (Basidiophora, Benua, Bremia, Paraperonospora, Plasmopara, Plasmoverna, Protobremia); (3) a group containing species included in Hyaloperonospora and Perofascia which almost exclusively infect Brassicaceae; (4) a clade including the grass parasites Viennotia oplismeni and Graminivora graminicola. Phylogenetic relationships between these four clades are not clearly resolved, and neither is the position of Sclerospora graminicola within the downy mildews. Character analysis indicates an evolutionary scenario of gradually increasing adaptation to plant parasitism in Peronosporales and that at least the most important of these adaptive steps occurred only once, including major host shifts within downy mildews.