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Pseudoperonospora humuli is an obligate biotrophic oomycete that causes downy mildew (DM), one of the most destructive diseases of cultivated hop that can lead to 100% crop loss in susceptible cultivars. We used the published genome of P. humuli to predict the secretome and effectorome and analyze the transcriptome variation among diverse isolates and during infection of hop leaves. Mining the predicted coding genes of the sequenced isolate OR502AA of P. humuli revealed a secretome of 1,250 genes. We identified 296 RXLR and RXLR-like effector-encoding genes in the secretome. Among the predicted RXLRs, there were several WY-motif-containing effectors that lacked canonical RXLR domains. Transcriptome analysis of sporangia from 12 different isolates collected from various hop cultivars revealed 754 secreted proteins and 201 RXLR effectors that showed transcript evidence across all isolates with reads per kilobase million (RPKM) values > 0. RNA-seq analysis of OR502AA-infected hop leaf samples at different time points after infection revealed highly expressed effectors that may play a relevant role in pathogenicity. Quantitative RT-PCR analysis confirmed the differential expression of selected effectors. We identified a set of P. humuli core effectors that showed transcript evidence in all tested isolates and elevated expression during infection. These effectors are ideal candidates for functional analysis and effector-assisted breeding to develop DM resistant hop cultivars.

Pseudoperonospora humuli is an obligate biotrophic oomycete that causes downy mildew (DM), one of the most destructive diseases of cultivated hop that can lead to 100% crop loss in susceptible cultivars. We used the published genome of P. humuli to predict the secretome and effectorome and analyze the transcriptome variation among diverse isolates and during infection of hop leaves. Mining the predicted coding genes of the sequenced isolate OR502AA of P. humuli revealed a secretome of 1,250 genes. We identified 296 RXLR and RXLR-like effector-encoding genes in the secretome. Among the predicted RXLRs, there were several WY-motif-containing effectors that lacked canonical RXLR domains. Transcriptome analysis of sporangia from 12 different isolates collected from various hop cultivars revealed 754 secreted proteins and 201 RXLR effectors that showed transcript evidence across all isolates with reads per kilobase million (RPKM) values > 0. RNA-seq analysis of OR502AA-infected hop leaf samples at different time points after infection revealed highly expressed effectors that may play a relevant role in pathogenicity. Quantitative RT-PCR analysis confirmed the differential expression of selected effectors. We identified a set of P. humuli core effectors that showed transcript evidence in all tested isolates and elevated expression during infection. These effectors are ideal candidates for functional analysis and effector-assisted breeding to develop DM resistant hop cultivars. [ABSTRACT FROM AUTHOR]

Highlights • CsWRKY50 encodes a WRKY subgroup IIc transcription factor. • The overexpression of CsWRKY50 enhances cucumber resistance to P. cubensis. • The application of this gene in anti-cucumber downy-mildew was first discovered. Abstract The cucumber (Cucumis sativus L.), an economically important vegetable crop, is often infected by Pseudoperonospora cubensis (P. cubensis), which results in inhibited growth and reduced yield. WRKY transcription factors (TFs) play critical roles in plant disease resistance. However, little is known about the function of WRKY TFs in cucumber downy mildew resistance. In this study, we reported that CsWRKY50, a cucumber WRKY subgroup Ⅱc TF localized in the nucleus, plays an important role in cucumber defense responses to downy mildew. In addition, several putative cis -acting elements involved in abiotic stress responsiveness were also identified in the CsWRKY50 promoter. Expression analysis revealed that CsWRKY50 can be induced by P. cubensis infection, abiotic stress and diverse signaling molecules. The overexpression of CsWRKY50 in cucumber enhanced the resistance of the plant to the fungal pathogen P. cubensis. In addition, less ROS accumulated in 35S:CsWRKY50 transgenic plants infected by the pathogen due to the higher expression levels of antioxidant enzymes. Importantly, after P. cubensis infection, the transcript levels of several hormone-related defense genes were also upregulated in transgenic plants, including SA- and JA-responsive genes and SA-synthesis genes. Collectively, our results indicate that CsWRKY50 positively regulates cucumber disease resistance to P. cubensis via multiple signaling pathways. [ABSTRACT FROM AUTHOR]

Abstract The application of foliar fungicides to horticultural crops has raised public concerns worldwide. In fact, it has been demonstrated that such fungicides have an impact on non-target microorganisms in the rhizosphere. Fluopyram, triadimenol and penthiopyrad are three broad-spectrum fungicides recommended to control foliar diseases. In our experiment, these fungicides were applied to a cucumber crop to mainly control downy mildew caused by Pseudoperonospora cubensis and grey mold caused by Botrytis cinerea. At the same time, we found that these treatments also controlled other fungal pathogens affecting cucumber crops, particularly penthiopyrad, which was more effective. Once the fungicide application period was over, the effect decreased, although fungicide traces remained in the soil. Furthermore, microbial soil community analysis indicated that both fungicide treatments affect fungal communities to a greater extent than bacterial communities. Highlights • Fluopyram-triadimenol and penthiopyrad affect soil borne fungal pathogens. • Fungicides effect decreased when the fungicide applications finished. • Fungicides affected fungal communities in a greater extent than bacterial community. [ABSTRACT FROM AUTHOR]

Severe epidemics of cucurbit downy mildew caused by Pseudoperonospora cubensis have been observed in recent years in major cucumber‐producing regions in Kazakhstan. However, information on the pathogenic variability of P. cubensis that could facilitate screening of candidate cucumber varieties and hybrids prior to deployment for commercial production is currently not available. A total of 112 isolates were collected from 2014 to 2016 during surveys in three major cucumber‐producing regions of Kazakhstan. Single lesion isolates were tested for their compatibility with a differential host set composed of 12 genotypes. Most isolates were compatible with Cucumis sativus and C. melo subsp. melo, while none were compatible with Citrullus lanatus. Isolates differed primarily by their virulence to C. pepo subsp. pepo and Benincasa hispida. Based on the patterns of compatibility with the differential host set, 29 pathotypes were identified. Of these, 18 were associated with a specific production region, six occurred in two regions and five were present in all three regions. The number of virulence factors (VF) ranged from 2 to 10, with 65% of the pathotypes having eight or more VFs. Indices of diversity showed that virulence was very diverse within the pathogen populations. Clustering of pathotypes based on similarity in virulence showed the existence of two main clusters. The high pathogenic variability observed in this study suggests that newly developed cucumber varieties and hybrids with resistance to cucurbit downy mildew need to be evaluated against a wide range of pathotypes prior to their deployment for commercial production in Kazakhstan. [ABSTRACT FROM AUTHOR]

The efficacy of four oxathiapiprolin (OXPT)-based, novel fungicidal mixtures against downy mildew in cucumber caused by Pseudoperonospora cubensis was examined in growth chambers. OXPT+chlorothalonil (CHT) mixed at weight ratio of 1 + 66.7; OXPT+ azoxystrobin (AZ) 1 + 10.3; OXPT+mandipropamid (MPD) 1 + 8.3; and OXPT+mefenoxam (MFX) 1 + 3, were compared with each other and with individual components. Mixtures performed better than all fungicides alone except for oxathiapiprolin. Of the four mixtures, OXPT+MFX outperformed the other treatments with the highest preventive, curative, translaminar, root treatment and seed treatment efficacies. Deployment in the field of such mixtures with reduced doses of oxathiapiprolin may lower the selection pressure imposed on P. cubensis and delay the buildup of subpopulations resistant to oxathiapiprolin. [ABSTRACT FROM AUTHOR]

Chemical control is currently the most effective method for controlling cu-curbit downy mildew (CDM) caused by Pseudoperonospora cubensis. Most commercial cucurbit cultivars, with the exception of a few new cucumber cultivars, lack adequate disease resistance. Fluopicolide and propamocarb were among the most effective fungicides against CDM in the United States between 2006 and 2009. Since then, reduced efficacy of these two fungicides under field conditions was reported starting around 2013 but occurrence of resistance to fluopicolide and propamocarb in field isolates of P. cubensis had not been established. Thirty-one isolates collected from cucurbits in the eastern United States were tested for their sensitivity to fluopicolide and propamocarb using a leaf disc assay. This same set of isolates and four additional isolates (i.e., 35 isolates) were also used to establish the baseline sensitivity of P. cubensis to ethaboxam, an ethylamino-thiazole-carboxamide fungicide, which was recently granted registration to control CDM in the United States. About 65% of the isolates tested were resistant to fluopicolide with at least one resistant isolate being present in samples collected from 12 of the 13 states in the eastern United States. About 74% of the isolates tested were sensitive to propamocarb with at least one resistant isolate being among samples collected from 8 of the 12 states in the study. The frequency of resistance to fluopicolide and propamocarb was high among isolates collected from cucumber, while the frequency was low among isolates collected from other cucurbit host types. All isolates tested were found to be sensitive to ethaboxam and EC50 values ranged from 0.18 to 3.08 mg a.i./liter with a median of 1.55 mg a.i./liter. The ratio of EC50 values for the least sensitive and the most sensitive isolate was 17.1, indicating that P. cubensis isolates were highly sensitive to ethaboxam. The most sensitive isolates to ethaboxam were collected from New York, North Carolina, and Ohio, while the least sensitive isolates were collected from Georgia, Michigan, and New Jersey. These results show that ethaboxam could be a viable addition to fungicide programs used to control CDM in the United States. [ABSTRACT FROM AUTHOR]

The novel fungicide oxathiapiprolin has potential for the control of downy mildew of cucumber, which is caused by Pseudoperonospora cubensis . In this study, an in vitro bioassay with detached leaves was used to determine the baseline sensitivity to oxathiapiprolin for 77 Ps . cubensis isolates from 11 provinces in China. The baseline sensitivity curve was continuous, and the average EC 50 value was 2.23 × 10 −4  μg ml −1 . In field trials, the control of downy mildew of cucumber was greater with oxathiapiprolin at 20 or 30 g a.i. ha −1 than with dimethomorph at 262.5 g a.i. ha −1 . Oxathiapiprolin was taken up by cucumber roots and transported upwards to stems and leaves. The full length of PscORP1 , the gene that encodes the target protein of oxathiapiprolin in Ps . cubensis , was sequenced for the first time. Our results suggested that oxathiapiprolin will be an excellent alternative fungicide for control of cucumber downy mildew. However, as Ps . cubensis is a high-risk pathogen, resistance development to oxathiapiprolin should be monitored and managed. [ABSTRACT FROM AUTHOR]

Infection risk models of downy mildew of cucumber caused by Pseudoperonospora cubensis were evaluated for their performance in predicting the infection risk of squash and cantaloupe plants under field conditions. Experiments were conducted from 2012 to 2014 in Clayton, NC and Charleston, SC, where disease-free potted plants were exposed to weather conditions during a 24- and 48-h period (hereafter 24- and 48-h models, respectively) within a plot with naturally occurring inoculum. Exposed plants were subsequently placed in a growth chamber where they were monitored for disease symptoms, which was indicative of a successful infection. Disease severity was assessed after 7 days as the proportion of leaf area with disease symptoms. Two predictor variables, day temperature and hours of relative humidity >80% during each exposure were used as inputs to generate model predictions that were compared with observed data. The threshold probability on the receiver operating characteristic (ROC) curve that minimized the overall error rate for the 24-h model was 0.85 for both squash and cantaloupe. The 24-h model was consistently more accurate than the 48-h model in predicting the infection risk for the two hosts. The accuracy of the 24-h model as estimated using area under ROC curve ranged from 0.75 to 0.81, with a correct classification rate ranging from 0.69 to 0.74 across the two hosts. Specificity rates for the model ranged from 0.81 to 0.84, while the sensitivity rates ranged from 0.58 to 0.67. Optimal decisions thresholds (POT) developed based on estimates of economic damage and costs of management showed that POT was dependent on the probability of disease occurrence, with the benefit of using the 24-h model for making management decisions being greatest at low levels of probability of disease occurrence. This 24-h model, previously developed using cucumber as the host, resulted in accurate estimates of the daily infection risk of squash and cantaloupe and could potentially be useful when incorporated into a decision support tool to guide fungicide applications to manage downy mildew in these other cucurbit host types. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

To identify new bacterial antagonists for cucurbit downy mildew (CDM) caused by Pseudoperonospora cubensis, 163 bacterial isolates were recovered from different microenvironments of field-grown cucumber plants. In the greenhouse, 19 representative isolates were applied to cucumber plants as a foliar spray (FS); 7 isolates achieved the efficacy over 60% against CDM, with 5 (DS22, HS10, DP 14, HP4, and DS57) identified as Bacillus pumilus, B. licheniformis, Enterobacter sp., Bacillus sp. and Stenotrophomonas maltophilia, respectively. Strains DP 14, DS22, and HS10 were assessed for their biocontrol effect on naturally occurring CDM in 2-year field trials (2010 and 2011), in which their overall efficacy relative to that of propamocarb was 106.25 to 117.17% with foliar spray plus root drench (FS+RD) but only 70.98 to 84.03% with FS. Coincidently, DP14 and HS10 applied as root drench (RD) alone also significantly reduced CDM. Under field conditions, DP14. DS22, and HS10 all successfully colonized cucumber leaves and the rhizosphere, and also significantly increased fruit yield by 37.60 to 51.03%, as well as nutrient levels. Taken together, Enterobacter sp. DP 14, B. licheniformis HS10, and B. pumilus DS22 are plant-growth-promoting rhizobacteria effective in controlling CDM in the field, whose efficacy increased with FS+RD compared with FS alone. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

In the present study, the impact of potassium phosphite (KPhi) was investigated at molecular and biochemical levels in response to Pseudoperonospora cubensis infection in cucumber plants. Real-time PCR was employed to explore the differential expression of defense genes against P. cubensis . The highest expression for thaumatin-like protein ( TLP ), Ribosome-inactivating protein ( RIP ) and Defensin genes was observed in pre-inoculation plants at 96, 72 and 48 h, respectively. These findings revealed the involvement of these genes in the defense response of cucumber leaves after KPhi treatment and pathogen inoculation. At the biochemical level, more induction in the contents of some end-products of phenylpropanoid pathway, such as phytoalexin, phenolic component, flavonoid and anthocyanin as well as phenylalanine ammonia-lyase (PAL) enzymatic activity was detected in pre-inoculation plants compared to post-inoculation plants at all-time points. Data suggest that KPhi primes rapid and robust response in plants against infection via activation of defense responses. The negative effects of P. cubensis on cucumber plants could be considerably mitigated by KPhi application before infection. [ABSTRACT FROM AUTHOR]

The effect of potassium phosphite (KPhi) on the photosynthetic parameters of cucumber plants inoculated withPseudoperonospora cubensiswas investigated in the present study. Cucumber plants were treated with KPhi before or after inoculation withP. cubensisand leaf samples were collected at different time courses for assessments. Results showed that in pathogen-inoculated plants Fv/Fm was decreased up to 3%. The rate of quantum photosynthetic was also decreased significantly in inoculated plants. Downy mildew led to a decrease in chlorophyll amount which in turn reduced the efficiency of photosystem II. In the KPhi-treated leaves, chlorophyll a and b decreased by 72% and 68%, respectively. Remarkable reduction in the efficiency of photosystem II as well as increased lipid membrane disruption, led to increased lipid peroxidation rate of the membranes up to 52%. The results of this study indicate the mitigating role of potassium phosphite in reducing the adverse effects of pathogen and maintaining the photosynthetic apparatus efficiency in cucumber plants. [ABSTRACT FROM AUTHOR]

Pseudoperonospora cubensis, the causal agent of cucurbit downy mildew ( CDM), is known to exhibit host specialization. The virulence of different isolates of the pathogen can be classified into pathotypes based on their compatibility with a differential set composed of specific cucurbit host types. However, the genetic basis of host specialization within P. cubensis is not yet known. Total genomic DNA extracted from nine isolates of P. cubensis collected from 2008 to 2013 from diverse cucurbit host types ( Cucumis sativus, C. melo var. reticulatus, Cucurbita maxima, C. moschata, C. pepo, and Citrullus lanatus) in the United States were subjected to whole-genome sequencing. Comparative analysis of these nine genomes confirmed the presence of two distinct evolutionary lineages (lineages I and II) of P. cubensis. Many fixed polymorphisms separated lineage I comprising isolates from Cucurbita pepo, C. moschata, and Citrullus lanatus from lineage II comprising isolates from Cucumis spp. and Cucurbita maxima. Phenotypic characterization showed that lineage II isolates were of the A1 mating type and belonged to pathotypes 1 and 3 that were not known to be present in the United States prior to the resurgence of CDM in 2004. The association of lineage II isolates with the new pathotypes and a lack of genetic diversity among these isolates suggest that lineage II of P. cubensis is associated with the resurgence of CDM on cucumber in the United States. [ABSTRACT FROM AUTHOR]

Background: Cucumber downy mildew, caused by P. cubensis, is an important leaf disease that can severely affect cucumber production. In recent years, cucumber target spot, caused by C. cassiicola, has been reported in both Asia and Europe and is now considered as a major disease disrupting cucumber production. Single-disease-resistant cucumber varieties have been unable to satisfy production needs. To explore the molecular mechanisms of cucumber resistance to these two diseases, cucumber cultivars D9320 (resistant to downy mildew and target spot) and D0401 (susceptible to downy mildew and target spot) were used as experimental materials in this study. We used transcriptome sequencing technology to identify genes related to disease resistance and verified using transgenic technology. Results: We screened out the cucumber resistance-related gene CsERF004 using transcriptome sequencing technology. Induction by pathogens, salicylic acid (SA), and ethylene (ET) resulted in the up-regulation of CsERF004. Three treatments, namely, inoculation with C. cassiicola alone, inoculation with P. cubensis alone, and simultaneous inoculation with both pathogens, all resulted in the significant and sustained up-regulation of CsERF004 in the resistant cultivar D9320, during the early stage of infection. In the susceptible cultivar D0401, CsERF004 expression was also significantly up-regulated at the later stage of infection but to a lesser extent and for a shorter duration than in the resistant cultivar D9320. The CsERF004 gene encodes a protein localizes to the nucleus. The over-expression of CsERF004 in the susceptible cultivar D0401 resulted in the significant up-regulation of the CsPR1 and CsPR4 genes and increased the levels of SA and ET, which enhanced the resistance of cucumber to downy mildew and target spot. Conclusions: Analyses of the CsERF004 expression pattern in disease-resistant and susceptible cucumber cultivars and transgenic validation indicate that CsERF004 confers resistance to P. cubensis and C. cassiicola. The findings of this study can help to better understanding of mechanisms of response to pathogens and in establishment the genetic basis for the development of cucumber broad-spectrum resistant cultivars. [ABSTRACT FROM AUTHOR]

Cucumber downy mildew, caused by Pseudoperonospora cubensis, is a worldwide disease that causes severe damage to cucumber production. The effects of temperature and moisture on sporulation and infection by P. cubensis were investigated by inoculating cucumber ('85F12') cotyledons with sporangia and examining the sporangia produced on the inoculated cotyledons under artificially controlled environments. The result showed that the temperature required for sporangium infection by P. cubensis and sporulation of the downy mildew lesions occurred at 5 to 30°C. The optimal temperature estimated by the fitted model was 18.8°C for sporangium infection and 16.2°C for downy mildew lesion sporulation. The pathogen formed plenty of sporangia when disease cotyledons were wetted or in the environment with relative humidity = 100%. The downy mildew lesions produced only a few sporangia when placed in the environment with relative humidity = 90%. The inoculated cotyledons, which incubated for 5 days at about 20°C in a dry greenhouse, began to form sporangia 4 h after being wetted when incubated in darkness. The quantity of sporangia produced on the downy mildew lesions increased with extension of incubating period (within 12 h), and the relationship between produced sporangia and the incubation period at 15, 20, and 25°C can be described by three exponential models. The observed minimum wetness durations (MWD) required for sporangia to complete the infection process and cause downy mildew were 12, 4, 2.5, 1, 1, and 6 h for 5, 10, 15, 20, 25, and 30°C, respectively. The effect of temperature and wetness duration on infection by sporangia of P. cubensis can be described by the modified Weibull model. The shortest MWD was 0.45 h, about 27 min, estimated by model. The experimental data and models will be helpful in the development of forecasting models and effective control systems for cucumber downy mildew. [ABSTRACT FROM AUTHOR]

During the past two decades, a resurgence of cucurbit downy mildew has occurred around the world, resulting in severe disease epidemics. In the United States, resurgence of the disease occurred in 2004 and several hypotheses, including introduction of a new genetic recombinant or pathotype of the pathogen, have been suggested as potential causes for this resurgence. Occurrence and distribution of mating types of Pseudoperonospora cubensis in the United States were investigated using 40 isolates collected from cucurbits across 11 states from 2005 to 2013. Pairing of unknown isolates with known mating-type tester strains on detached leaves of cantaloupe or cucumber resulted in oospore formation 8 to 10 days after inoculation. Isolates differed in their ability to form oospores across all coinoculation pairings, with oospore numbers ranging from 280 to 1,000 oospores/cm2 of leaf tissue. Oospores were hyaline to golden-yellow, spherical, and approximately 36 μm in diameter. Of the 40 isolates tested, 24 were found to be of the A1 mating type, while 16 were of the A2 mating type. Mating type was significantly (P < 0.0001) associated with host type, whereby all isolates collected from cucumber were of the A1 mating type, while isolates from squash and watermelon were of the A2 mating type. Similarly, mating type was significantly (P = 0.0287) associated with geographical region, where isolates from northern-tier states of Michigan, New Jersey, New York, and Ohio were all A1, while isolates belonging to either A1 or A2 mating type were present in equal proportions in southern-tier states of Alabama, Florida, Georgia, North Carolina, South Carolina, and Texas. Viability assays showed that oospores were viable and, on average, approximately 40% of the oospores produced were viable as determined by the plasmolysis method. This study showed that A1 and A2 mating types of P. cubensis are present and the pathogen could potentially reproduce sexually in cucurbits within the United States. In addition, the production of viable oospores reported in this study suggests that oospores could have an important role in the biology of P. cubensis and could potentially influence the epidemiology of cucurbit downy mildew in the United States. [ABSTRACT FROM AUTHOR]

The F-box protein FBXW7 is the substrate-recruiting subunit of an SCF ubiquitin ligase and a major tumor-suppressor protein that is altered in several human malignancies. Loss of function of FBXW7 results in the stabilization of numerous proteins that orchestrate cell proliferation and survival. Little is known about proteins that directly regulate the function of this protein. In the current work, we have mapped the interactome of the enigmatic pseudophosphatase STYX. We reasoned that a catalytically inactive phosphatase might have adopted novel mechanisms of action. The STYX interactome contained several F-box proteins, including FBXW7. We show that STYX binds to the F-box domain of FBXW7 and disables its recruitment into the SCF complex. Therefore, STYX acts as a direct inhibitor of FBXW7, affecting the cellular levels of its substrates. Furthermore, we find that levels of STYX and FBXW7 are anti-correlated in breast cancer patients, which affects disease prognosis. We propose the STYX- FBXW7 interaction as a promising drug target for future investigations. [ABSTRACT FROM AUTHOR]

The article presents abstracts on the annual meeting of the American Phytopathological Society (APS) which include the importance of dispersal in botanical epidemics, the dispersal of Pseudoperonospora cubensis and the impact of climate change to the migration of crop pest.

A study to determine the effect of Bacillus chitinosporus on Pseudoperonospora cubensis, the causal agent of cucumber downy mildew under plastic house conditions was carried out. The optimum incubation period for bacterial bio-agent growth and metabolite production was estimated. Disease control with B. chitinosporus was effective at the optimum condition for an incubation period of 5 days. Area under diseased plants progress curve (AUDPC) was reduced by using B. chitinosporus metabolites within 35 days. A scanning electromicroscopic examination was carried out to show the effect of B. chitinosporus metabolites on sporangia and sporangiophores of P. cubensis. Twisting, loss turgor, collapsing of sporangiophores and sporangium explosion were observed after application of B. chitinosporus metabolites. Bacterial metabolites of B. chitinosporus were analyzed by gas chromatography-mass spectrophotometry (GC-MS). The efficacy of it in relation to Ridomil Gold MZ 68 WG fungicide on growth parameters was investigated. A significant increase in chlorophyll content, growth and yield parameters was observed with application of B. chitinosporus metabolites compared to control treatment. Metabolites of B. chitinosporus could be a powerful and an alternative way to chemical fungicides for controlling the downy mildew disease of cucumber. [ABSTRACT FROM AUTHOR]

The host range of Pseudoperonospora cubensis, the causal agent of cucurbit downy mildew, includes many taxa of the Cucurbitaceae. Little is known of the interactions of this pathogen with wild and weedy Cucurbita species, the genus encompassing two important crops, pumpkin and squash. Interactions of 97 Cucurbita spp. accessions, mostly wild and weedy forms representing 10 species, with 11 P. cubensis isolates originating from Cucumis sativus were studied by using a leaf-disc method under controlled conditions. Among the 97 accessions, 57 distinct reaction patterns were observed. Fifteen accessions were resistant and 12 susceptible to all 11 pathogen isolates. Within-accession, heterogeneous reactions were observed in only nine interactions. Of the 15 accessions resistant to all pathotypes, 13 were wild and weedy C. argyrosperma. Reactions of Cucurbita spp. to P. cubensis are race-specific. The development of methodology for P. cubensis race determination and denomination should include a differential set of genotypes for each species separately. C. argyrosperma, which is partially cross-compatible with the economically important C. pepo, offers a potential source for breeding downy mildew-resistant squash and pumpkins. [ABSTRACT FROM AUTHOR]

Downy mildew (Pseudoperonospora cubensis) and powdery mildew (Podosphaera xanthii) are two of the most economically important and widespread cucurbit diseases. Disease management relies primarily on fungicide use, but frequent fungicide applications can lead to the development of resistant pathogen populations. In addition, more vegetables are being produced with organic practices, which prohibit the use of many fungicides. Incorporating biorational products into a disease management program may help mitigate the risk of fungicide resistance development while being compatible with organic production. Field trials were conducted for two years on organically managed land in Maryland with cucumber, muskmelon, pumpkin, and butternut squash to evaluate the efficacy of four biorational products (i.e., Actinovate AG, OxiDate, Regalia, and Serenade Soil) when applied in a rotational program with copper against foliar cucurbit diseases. Generally, all biorational treatments resulted in significantly lower downy and powdery mildew severity compared with the nontreated plants, but the level of disease management was not significantly different than that provided by copper alone. However, Actinovate AG, OxiDate, and Serenade Soil each improved disease management on at least one crop, as compared with copper alone. Rotational programs with biopesticides are a viable disease management option for organic production of field-grown cucurbits in Maryland. [ABSTRACT FROM AUTHOR]

Advances in next-generation sequencing (NGS) allow for rapid development of genomics resources needed to generate molecular diagnostics assays for infectious agents. NGS approaches are particularly helpful for organisms that cannot be cultured, such as the downy mildew pathogens, a group of biotrophic obligate oomycetes that infect crops of economic importance. Unlike most downy mildew pathogens that are highly host-specific, Pseudoperonospora cubensis causes disease on a broad range of crops belonging to the family Cucurbitaceae. In this study, we identified candidate diagnostic markers for P. cubensis by comparing NGS data from a diverse panel of P. cubensis and P. humuli isolates, two very closely related oomycete species. P. cubensis isolates fo ra diverse hosts and geographical regions in the United States were selected for sequencing to ensure that candidates were conserved in P cubensis isolates infecting different cucurbit hosts. Genomic regions unique to and conserved in P. cubensis isolates were identified through bioinformatics. These candidate regions were then validated using PCR against a larger collection of isolates from P cubensis, P. humuli, and other oomycetes. Overall seven diagnostic markers were found to be specific to P. cubensis. These markers could be used for pathogen diagnostics on infected tissue, or adapted for monitoring airborne inoculum with real-time PCR and spore traps. [ABSTRACT FROM AUTHOR]

Downy mildew, caused by the oomycete pathogen Pseudoperonospora cubensis (Berkeley & Curtis) Rostov, is a major foliar disease of cucumber. Ten years after the reemergence of P. cubensis, downy mildew continues to be a major threat to cucumber production in the United States. Cucumber accessions with high levels of resistance have been identified. Development of cultivars with high levels of resistance remains an important objective of cucumber breeding programs. We tested a set of cucumber cultigens, including highly resistant PI accessions and susceptible control lines, to observe the effect of plant age on resistance. Cultigens responded differently to disease across plant developmental stages. In general, older plants had more disease symptoms, even those classified as resistant, such as PI 197088. However, PI 330628 and PI 605996 held their resistance even at late developmental stages. It is possible that these lines were resistant at late stages due to other factors, such as their rapid, indeterminate growth, that allows them to outgrow the disease. However, although PI 197088 appears to have a rapid, indeterminate growth habit, it did not have more resistance at later stages of plant maturity. Regardless of the mechanism involved, plant breeders should use the genetic resistance in PI 330628 and PI 605996 over PI 197088. [ABSTRACT FROM AUTHOR]

Key message: Host resistance in WI7120 cucumber to prevailing downy mildew pathogen field populations is conferred by two major-effect, one moderate-effect and two minor-effect QTL. Abstract: Downy mildew (DM) caused by the obligate oomycete Pseudoperonospora cubensis is the most devastating fungal disease of cucumber worldwide. The molecular mechanism of DM resistance in cucumber is poorly understood, and use of marker-assisted breeding for DM resistance is not widely available. Here, we reported QTL mapping results for DM resistance with 243 F families from the cross between DM-resistant inbred line WI7120 (PI 330628) and susceptible '9930'. A linkage map was developed with 348 SSR and SNP markers. Phenotyping of DM inoculation responses were conducted in four field trails in 2 years at three locations. Four QTL, dm2.1, dm4.1, dm5.1, and dm6.1 were consistently and reliably detected across at least three of the four environments which together could explain 62-76 % phenotypic variations ( R). Among them, dm4.1 and dm5.1 were major-effect QTL ( R = 15-30 %) with only additive effects; dm2.1 ( R = 5-15 %) and dm6.1 ( R = 4-8 %) had moderate and minor effects, respectively. Epistatic effects were detected for dm2.1 and dm6.1 with both dm4.1 and dm5.1. One additional minor-effect QTL, dm6.2 ( R = 3-5 %) was only detectable with the chlorosis rating criterion. All alleles contributing to DM resistance were from WI7120. This study revealed two novel QTL for DM resistance and the unique genetic architecture of DM resistance in WI7120 conferring high level resistance to prevailing DM populations in multiple countries. The effects of disease rating scales, rating time and criteria, population size in phenotyping DM resistance on the power of QTL detection, and the use of DM resistance in WI7120 in cucumber breeding were discussed. [ABSTRACT FROM AUTHOR]

Cucurbit downy mildew (CDM), caused by the oomycete pathogen Pseudoperonospora cubensis, is a devastating disease that affects cucurbit species worldwide. This obligate, wind-dispersed pathogen does not overwinter in Michigan or other northern regions and new isolates can enter the state throughout the growing season. To evaluate the regional and temporal population structure of P. cubensis, sporangia from CDM lesions were collected from cucurbit foliage grown in Michigan and Ontario field locations in 2011. Population structure and genetic diversity were assessed in 257 isolates using nine simple sequence repeat markers. Genetic diversity was high for isolates from Michigan and Canada (0.6627 and 0.6131, respectively). Five genetic clusters were detected and changes in population structure varied by site and sampling date within a growing season. The Michigan and Canada populations were significantly differentiated, and a unique genetic cluster was detected in Michigan. [ABSTRACT FROM AUTHOR]

Melon ( Cucumis melo ) is seriously affected by Pseudoperonospora cubensis , the causal agent of cucurbit downy mildew. Within C. melo , several genotypes resistant to P. cubensis have been described; however, we lack detailed experimental studies focused on deciphering variation in this host-pathogen interaction. The response of 115 accessions of C. melo to 8 isolates of P. cubensis was studied under controlled conditions. Pathogen isolates with low, medium and high virulence represented 8 distinct pathotypes. The virulence of isolates expressed by their published pathotypes conformed poorly to their virulence as observed on the C. melo accessions in this experiment. Collectively, 45 different reaction patterns were recorded. Most C. melo accessions (67) were highly susceptible to all isolates. Two accessions of C. melo subsp. agrestis (PI 614174 and PI 614442) were incompletely resistant to all isolates. The reaction of the remaining 46 accessions was race-specific. None of screened C. melo accessions expressed complete resistance to all isolates. Accession PI 315410 from India was resistant to 5 isolates and incompletely resistant to 2 others. The reduced development of at least one P. cubensis isolate was recorded on leaf discs of 17 accessions. Accessions with incomplete resistance to at least one isolate originated most frequently from India, Zimbabwe and Zambia. The study: (a) confirmed the race-specificity of interactions between C. melo and P. cubensis , (b) revealed C. melo subsp. agrestis as a taxon with potential field resistance to a number of races P. cubensis , and (c) showed that within C. melo populations from India and southern Africa there are accessions completely or incompletely resistant to European races of P. cubensis . [ABSTRACT FROM AUTHOR]

To develop a molecular method to identify Pseudoperonospora cubensis in cucumber leaves with signs of downy mildew, we compared the nucleotide sequences of ribosomal DNA-internal transcribed spacer, cytochrome oxidase II, and β- tubulin genes of P. cubensis and P. humuli isolates. Seven single nucleotide polymorphisms (SNPs) distinguished P. cubensis and P. humuli based on variations in β- tubulin sequences, and one specific primer set was designed for further analysis. Real-time PCR and high resolution melting analysis showed that the primer set can be used to specifically identify P. cubensis in cucumber leaves with downy mildew. [ABSTRACT FROM AUTHOR]

Over a dozen fungicides are registered in the United States to manage cucurbit downy mildew caused by Pseudoperonospora cubensis. Efficacy varies greatly among them, due, in part, to reduced sensitivity to some fungicides in some pathogen populations. The objective of this study was to determine whether fungicide efficacy could be assessed using fungicide-treated cucumber (Cucumis sativus) exposed to natural inoculum for a brief period. Potted cucumber plants were treated with water or 1 of 13 fungicides registered to control cucurbit downy mildew. One day later, they were placed in a field among cucumber plants that had symptoms and signs of downy mildew. After a 48-h exposure to P. cubensis, potted plants were moved to a growth chamber held at day and night temperatures of 21 and 18°C, respectively, and 50% relative humidity. Severity (leaf area with symptoms) of downy mildew was rated 5 and 7 days later. The assay was done eight times, twice each in July and October 2013 and 2014. Year, season, trial, and interactions among these factors affected downy mildew development. Severity at 7 days on plants treated with mandipropamid, azoxystrobin, dimethomorph, cymoxanil, fluopicolide, and propamocarb was not significantly different from the water control treatment in eight, five, three, three, two, and two of eight bioassays, respectively. Severity on plants treated with cyazofamid, fluazinam, mancozeb + zoxamide, mancozeb, chlorothalonil, and ametoctradin + dimethomorph was less than on plants treated with water in all bioassays. These six fungicides should be effective when applied early in the season to prevent initial infections. Cyazofamid and mancozeb + zoxamide prevented an increase in severity between rating times. In conclusion, the assay consistently detected resistance to mandipropamid and azoxystrobin and demonstrated the efficacy of six other fungicides. [ABSTRACT FROM AUTHOR]

The genetic basis for the interaction(s) between plants and pathogens has been classically illustrated as a gene-for-gene relationship, through which a single gene product from the pathogen interacts within a single gene product found within the plant. In the simplest terms, it is the outcome of these interactions that underpin resistance and/or susceptibility signalling processes. While this basic concept shapes our understanding of many of the molecular-genetic mechanisms controlling immune signalling in plants, current research has revealed that the timing and ultimate outcome of these interactions are far more complex, and are typically regulated in a genome-by-genome manner. As a central theme for this review, we will focus on recent discoveries in the field of plant-oomycete interactions, primarily on the downy mildews, an important group of obligate oomycete pathogens of plants. Recent transcriptome-based studies have shown that survival, adaptation and virulence rely on complex, bi-directional interactions between the host and pathogen genomes. Taking advantage of the obligate nature of the downy mildews, insight into the transcriptional plasticity of these genomes has revealed a remarkable ability to adapt to host and environmental stressors. Herein, we will highlight a recent body of research using thePseudoperonospora cubensis – Cucumis sativusinteraction, which has identified a suite of alternative splicing and sRNA-based regulatory signals that are induced in a temporal and host-specific manner. In combination with recent studies in other plant-oomycete pathosystems, a comprehensive transcriptional profile of resistance and susceptibility within the host and pathogen illustrates the remarkable ability of this group of pathogens to adapt to host and environment to infect and cause disease in a diverse array of agriculturally important crops. [ABSTRACT FROM AUTHOR]

Pseudoperonospora cubensis is an oomycete pathogen causing downy mildew disease on a variety of Cucurbitaceae, and has recently re-emerged as a destructive disease on crops in this family, mainly on cucumber and squash. Multilocus sequence analysis ( MLSA) of four mitochondrial and two nuclear DNA regions was used to detect changes in the genetic structure of P. cubensis populations occurring in the Czech Republic that might be associated with recently reported shifts in virulence. The analysed sample set contains 67 P. cubensis isolates collected from 1995 to 2012 in the Czech Republic and some other European countries. Sequence analyses revealed differences and changes in the genetic backgrounds of P. cubensis isolates. While all isolates sampled before 2009 exhibited the genotype of the subspecies of Clade II and were collected from cucumber, all samples collected from other hosts belonged to Clade I ( P. cubensis sensu stricto) or were sampled from 2009 onwards. In addition, 67·16% of all post-2009 isolates from Clade II had two heterozygous positions in their nr ITS sequence, which suggests sexual reproduction and/or a mutational origin. Thus, the results indicate that, apart from the rise in prevalence of Clade I, the change in the genetic structure of P. cubensis populations may be linked with a hybridization or, less likely, a mutation event that rendered strains able to infect a broader spectrum of host species. [ABSTRACT FROM AUTHOR]

RNA sequencing (RNA-seq) and genotyping-by-sequencing (GBS) were used for single nucleotide polymorphism (SNP) identification from two economically important obligate plant pathogens, Pseudoperonospora cubensis and P. humuli. Twenty isolates of P. cubensis and 19 isolates of P. humuli were genotyped using RNA-seq and GBS. Principle components analysis (PCA) of each data set showed genetic separation between the two species. Additionally, results supported previous findings that P. cubensis isolates from squash are genetically distinct from cucumber and cantaloupe isolates. A PCA-based procedure was used to identify SNPs correlated with the separation of the two species, with 994 and 4,231 PCA-correlated SNPs found within the RNA-seq and GBS data, respectively. The corresponding unigenes (n = 800) containing these potential species-specific SNPs were then annotated and 135 putative pathogenicity genes, including 3 effectors, were identified. The characterization of genes containing SNPs differentiating these two closely related downy mildew species may contribute to the development of improved detection and diagnosis strategies and improve our understanding of host specificity pathways. [ABSTRACT FROM AUTHOR]

Downy mildew of cucumber ( Cucumis sativus L.), caused by Pseudoperonospora cubensis (Berk. et Curt.) Rostovzev, is one of the most important foliar diseases of cucurbit crops. Two parental lines resistant PI 197085, susceptible PI 175695 and their F2 generation were used in our study. Inheritance of resistance to Pseudoperonospora cubensis in PI 197085 was quantitative. JoiMap 4.1 and MapQTL 6.0 software was used for a linkage groups construction and QTL mapping. Three QTL were detected: DM1, DM2, DM3. The loci were mapped on chromosome 5 of cucumber genome. Molecular analysis confirmed results of classical quantitative genetics indicating that resistance to Pseudoperonospora cubensis in PI 197085 is polygenic trait. [ABSTRACT FROM AUTHOR]

MicroRNAs (miRNAs) regulate the expression of genes related to several stress responses, including fungal infection, in plants. However, the miRNA-mediated gene regulatory networks in cucumbers that respond to Pseudoperonospora cubensis stress remain unexplored. In this study, the miRNA expression patterns in response to P. cubensis stress in cucumbers were investigated through high-throughput sequencing. A total of 123 known miRNAs and 4 novel miRNAs were identified, and their corresponding expressions were detected in mock- and P. cubensis -inoculated leaves. Three novel and 39 known miRNAs were found to be differentially expressed in P. cubensis -infected leaves. The results of 5′-RLM-RACE confirmed that miR164b, miR156h, miR171e, miR160b, and miR159f targeted No Apical Meristem domain protein, squamosa promoter binding protein-like class transcription factor, GRAS family transcription factor, Auxin response factor ARF16, and a conserved gene of unknown function, respectively. The expression patterns of these miRNAs were also determined through quantitative reverse transcription polymerase chain reaction (qRT-PCR). All of these miRNAs, except for miR156h, can respond to P. cubensis infection in cucumber leaves. In addition, the results of qRT-PCR revealed that the targets negatively correlated with their corresponding miRNAs (miR164b, miR171e, miR160b, and miR159f). [ABSTRACT FROM AUTHOR]

The resurgence of cucurbit downy mildew has dramatically influenced production of cucurbits and disease management systems at multiple scales. Long-distance dispersal is a fundamental aspect of epidemic development that influences the timing and extent of outbreaks of cucurbit downy mildew. The dispersal potential of Pseudoperonospora cubensis appears to be limited primarily by sporangia production in source fields and availability of susceptible hosts and less by sporangia survival during transport. Uncertainty remains regarding the role of locally produced inoculum in disease outbreaks, but evidence suggests multiple sources of primary inoculum could be important. Understanding pathogen diversity and population differentiation is a critical aspect of disease management and an active research area. Underpinning advances in our understanding of pathogen biology and disease management has been the research capacity and coordination of stakeholders, scientists, and extension personnel. Concepts and approaches developed in this pathosystem can guide future efforts when responding to incursions of new or reemerging downy mildew pathogens. [ABSTRACT FROM AUTHOR]

The downy mildew pathogen, Pseudoperonospora cubensis, which infects plant species in the family Cucurbitaceae, has undergone major changes during the last decade. Disease severity and epidemics are far more destructive than previously reported, and new genotypes, races, pathotypes, and mating types of the pathogen have been discovered in populations from around the globe as a result of the resurgence of the disease. Consequently, disease control through host plant resistance and fungicide applications has become more complex. This resurgence of P. cubensis offers challenges to scientists in many research areas including pathogen biology, epidemiology and dispersal, population structure and population genetics, host preference, host-pathogen interactions and gene expression, genetic host plant resistance, inheritance of host and fungicide resistance, and chemical disease control. This review serves to summarize the current status of this major pathogen and to guide future management and research efforts within this pathosystem. [ABSTRACT FROM AUTHOR]

Real-time PCR assays using locked nucleic acid (LNA) probes and high resolution melt (HRM) analysis were developed for molecular differentiation ofPseudoperonospora cubensisandP. humuli, causal agents of cucurbit and hop downy mildew, respectively. The assays were based on a previously identified single nucleotide polymorphism (SNP) in the cytochrome oxidase subunit II (cox2) gene that differentiates the two species. Sequencing of the same region from 15P. cubensisisolates collected in New York State for the current study confirmed that all isolates shared the conserved SNP. LNA probes were specific and sensitive, detecting as few as 10 sporangia for both species and as little as 1 fgP. cubensistotal DNA and 10 fgP. humulitotal DNA. The LNA assay detected both pathogens from air sampled using spore traps placed in vegetable fields and a hop yard during the summers of 2013 and 2014 and correctly diagnosed symptomatic leaf tissue. High resolution melt analysis (HRM) correctly identified all tested isolates as well as those isolates from symptomatic plants collected in the field. The LNA and HRM assays correctly identified both organisms when tested independently in a second laboratory. The results confirm that the LNA and HRM assays developed can provide reliable identification of both species despite the high molecular similarity of thecox2gene. [ABSTRACT FROM AUTHOR]

Downy mildew, caused by Pseudoperonospora cubensis, is a severe foliar disease of many cucurbit crops worldwide. Forty-one cucurbit cultigens (commercial cultivars and plant introductions) from five genera (Cucumis, Citrullus, Cucurbita, Lagenaria, and Lujfa) were assessed for susceptibility to P. cubensis in a research field exposed to natural inoculum in Michigan. Eight cultigens from a differential set for pathotype determination were included within the 41 cultigens to detect changes in dominant P. cubensis pathotypes present. No pathotype differences were found between 2010 and 2011. Cucumis melo cultigen MR-1 was less susceptible to Michigan P. cubensis populations than other C. melo cultigens. No symptoms or signs of infection were detected on cultigens of Cucurbita moschata and C. pepo. Disease onset was later in 2011 than 2010; greater than 90% disease severity in pickling cucumber 'Vlaspik' was observed in both years. This study confirmed that Cucumis is the most susceptible cucurbit genus, while Citrullus and Cucurbita cultigens were the least susceptible genera to populations of P. cubensis in Michigan. Area under the disease progress curve values indicated that disease progress was limited on all Citrullus cultigens compared with Cucumis cultigens, and pathogen sponilation was not detected under field conditions. Future studies should evaluate the ability of a reduced fungicide program to control downy mildew on less susceptible Cucumis melo 'Edisto 47', 'Primo', 'Athena', 'Strike', 'Ananas', 'Banana', and 'Tarn-Dew'. Many of the melon cultivars evaluated were selected on the basis of reported resistance to downy mildew, yet they showed significant disease symptoms. It is important to evaluate candidate cultigens for resistance to local P. cubensis populations. [ABSTRACT FROM AUTHOR]

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-generation-sequencing was performed on a mapping population 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 Washington (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 = 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. [ABSTRACT FROM AUTHOR]

The physiological research regarding the effects of the treatment with fungicides on the intensity of the physiological processes has been carried in cucumber plants (Cucumis sativus L.) cultivated in the Oltenia region. After applying the treatment with fungicides, there were carried out physiological analysis, on the 10th of July 2012, in the attacked plants and also in the plants treated with fungicides. The research carried out on the plants attacked by the pathogen, in comparison to the plants treated with fungicides, emphasized that these presents lower values of photosynthesis and transpiration intensity due to the reduction of the assimilation surface and the malfunctioning of the closing and opening mechanisms of stomata, as a result of the formation of the spots, which, at the beginning, are yellow, then brown, and to the necrosis of the tissue corresponding to the spots. In the plants attacked by the pathogen, it was observed a decrease of the chlorophyll content and the decrease of the water content, which caused hydric and metabolic imbalances in the attacked plants, in comparison with the plants analysed after the treatment with fungicides. [ABSTRACT FROM AUTHOR]

Pseudoperonospora cubensis, an obligate biotrophic oomycete causing devastating foliar disease in species of the Cucurbitaceae family, was never reported in seeds or transmitted by seeds. We now show that P. cubensis occurs in fruits and seeds of downy mildew-infected plants but not in fruits or seeds of healthy plants. About 6.7% of the fruits collected during 2012–2014 have developed downy mildew when homogenized and inoculated onto detached leaves and 0.9% of the seeds collected developed downy mildew when grown to the seedling stage. This is the first report showing that P. cubensis has become seed-transmitted in cucurbits. Species-specific PCR assays showed that P. cubensis occurs in ovaries, fruit seed cavity and seed embryos of cucurbits. We propose that international trade of fruits or seeds of cucurbits might be associated with the recent global change in the population structure of P. cubensis. [ABSTRACT FROM AUTHOR]