Downy Mildew Epidemics Research Articles

Cover crops lower the dispersal of grapevine foliar pathogens from the ground and contribute to early-season disease management

Currently, fungicides are widely used to control grapevine foliar diseases. This study explored the possibility of decreasing the use of fungicides to control these diseases using cover crops in the inter-row of vineyards. In small-scale experiments, we found that cover crops (namely horseradish Armoracia rusticana) were able to (i) reduce the numbers of airborne conidia of Botrytis cinerea (originating from an inoculum source above the soil) escaping the cover canopy by >85% with respect to the base soil and (ii) reduce the number of raindrops impacting the soil by 46%–74%, depending on the cover crop height and rain-originated splash droplets that escaped from the ground by 75%–95%, which reduced splash-borne inoculum. In two organic vineyards, for 2 years, fall- (mixture of Lolium perenne, Onobrychis viciifolia, and Trifolium repens) or spring-sown (a mixture of Vicia sativa and Sinapis sp.) cover crops could significantly delay (by 14–30 days) and reduce (till >90%) the development of downy and powdery mildew epidemics. This effect was more evident in plots untreated with fungicides than in treated plots. Cover crops also delayed the onset of epidemics depending on the type of cover crop and disease. Cover crops did not negatively affect grape yield and quality. Overall, the results showed that the introduction of cover crops in vineyard management can significantly contribute to disease control by lowering the load from ground to grapevine canopies of pathogen inocula, delaying disease onset, and reducing diseases severity during the season.

Exploring the association between latent Plasmopara viticola infection and downy mildew epidemic in commercial vineyards: Application of qPCR assay

AbstractIt is critical to quantify latent Plasmopara viticola infections in commercial vineyards rapidly and accurately to estimate the potential for grapevine downy mildew (GDM) epidemics. In this study, a well‐established quantitative PCR (qPCR) assay was employed to quantify the amount of P. viticola in asymptomatic grapevine leaves from various commercial vineyards in the Ningxia Hui Autonomous Region (Ningxia), China. The qPCR revealed efficient detection of P. viticola, with 56.6% and 95.8% of the asymptomatic leaves testing positive to the pathogen in 2018 and 2019, respectively. The molecular disease index (MDI), which offers insights into the progression of latent infection preceding the appearance of visible GDM symptoms, showed relatively modest values in the initial phases. However, it increased variably as grapevines grew. The disease index progress curve and the relative area under the disease progress curve revealed that all assessed vineyards experienced different degrees of epidemics after the initial GDM observation, with most vineyards experiencing more severe GDM epidemics in 2019 than in 2018. Bayesian and non‐linear regression analyses demonstrated that MDI data matched well with occurrence of GDM at early, mid and late stages of disease, and the overall accuracy of predictions exceeded 50%, 64% and 70%, respectively. Furthermore, latent infection levels were positively correlated with subsequent assessments of disease progression (p < 0.001). This study provides evidence for the interrelation between latent P. viticola infections and development of GDM epidemics under field conditions. This insight has substantive implications for the formulation of effective disease management strategies.

Plant Resistance Inducers Affect Multiple Epidemiological Components of Plasmopara viticola on Grapevine Leaves.

Plant resistance inducers (PRIs) harbor promising potential for use in downy mildew (DM) control in viticulture. Here, the effects of six commercial PRIs on some epidemiological components of Plasmopara viticola (Pv) on grapevine leaves were studied over 3 years. Disease severity, mycelial colonization of leaf tissue, sporulation severity, production of sporangia on affected leaves, and per unit of DM lesion were evaluated by inoculating the leaves of PRI-treated plants at 1, 3, 6, 12, and 19 days after treatment (DAT). Laminarin, potassium phosphonate (PHO), and fosetyl-aluminium (FOS) were the most effective in reducing disease severity as well as the Pv DNA concentration of DM lesions on leaves treated and inoculated at 1 and 3 DAT; PHO and FOS also showed long-lasting effects on leaves established after treatment (inoculations at 6 to 19 DAT). PRIs also prevented the sporulation of Pv on lesions; all the PRI-treated leaves produced fewer sporangia than the nontreated control, especially in PHO-, FOS-, and cerevisane-treated leaves (>75% reduction). These results illustrate the broader and longer effect of PRIs on DM epidemics. The findings open up new perspectives for using PRIs in a defense program based on single, timely, and preventative field interventions.

Leaf Microbiome Data for European Cultivated Grapevine (Vitis vinifera) During Downy Mildew (Plasmopara viticola) Epidemics in Three Wine-Producing Regions in France

Grapevine downy mildew ( Plasmopara viticola) is a major disease of European cultivated grapevine ( Vitis vinifera L.) against which a large amount of synthetic pesticides are used. Developing microbial biocontrol of P. viticola could reduce the use of pesticides in viticulture and preserve human and environmental health. To achieve this goal, the ecological interactions that develop during infection between P. viticola and the vine foliar microbiome need to be explored. Here, we present metabarcoding datasets describing the bacterial and fungal communities of pairs of symptomatic and asymptomatic leaf samples collected during downy mildew epidemics in three major wine-producing regions of France. Fungal and bacterial communities were sequenced on a MiSeq Illumina platform, and the abundance of the oomycete P. viticola was quantified using qPCR. We provide the raw metabarcoding datasets, the amplicon sequence variant tables obtained after bioinformatic processing, the metadata describing sampling sites and tissue health conditions, and the code used for bioinformatic analysis. These datasets will enable microbiome comparison within pairs of symptomatic and asymptomatic samples collected at the same time on the same leaf. Such a comparison could help describe the ecological interactions between P. viticola and the grapevine foliar microbiome. [Formula: see text] Copyright © 2023 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license .

Competition Between Plasmopara viticola Clade riparia and Clade aestivalis: A Race to Lead Grape Downy Mildew Epidemics.

There is evidence of five clades of Plasmopara viticola in the world. Only two clades, riparia and aestivalis, have been identified as responsible for downy mildew epidemics in Quebec, Canada. It was reported in 2021 that epidemics caused by clade riparia start 2 or 3 weeks before those caused by clade aestivalis and that clade aestivalis was more aggressive than clade riparia. The objective of this work was to study the competition between P. viticola clade riparia (A) and clade aestivalis (B) and to compare the aggressiveness of both clades in mono- and coinfection situations. Suspensions of sporangia from both clades with six percentage combinations (AB 100-0; AB 89-11; AB 74-26; AB 46-54; AB 23-77; and AB 0-100) were inoculated on leaf discs (cultivar Vidal), and three other combinations (AB 88-12; AB 68-32; and AB 47-53) were inoculated on living leaves of grape plants (cultivar Vidal). Then, sporangium production, expressed as the percentage of sporangia produced by each clade, was estimated on leaf discs after eight cycles of infection-sporulation and then validated on living grape leaves after five cycles. The aggressiveness of clades in monoinfection situations on leaf discs was compared with that in coinfection situations. The results show that the percentage of sporangia produced by clade aestivalis increases with the infection-sporulation cycle while that produced by clade riparia decreases. The area under the sporangium production progress curve (AUSPPC) of clade aestivalis was significantly higher than that of clade riparia. The aggressiveness of P. viticola clades riparia and aestivalis in coinfection situations was different from that in monoinfection situations and was strongly influenced by the percentage of each clade in competition. These results suggest that, on the grapevine cultivar Vidal, P. viticola clade aestivalis is more competitive than clade riparia and that the percentage of each clade present in the vineyard should be considered for management of downy mildew.[Formula: see text] Copyright © 2022 The Author(s). This is an open access article distributed under the CC BY 4.0 International license.

Development, Validation, and Utility of Species-Specific Diagnostic Markers for Detection of Peronospora belbahrii

Peronospora belbahrii is an oomycete and the cause of basil downy mildew, one of the most destructive diseases affecting basil production worldwide. Disease management is challenging due to wind-dispersed sporangia and contaminated seed; therefore, identifying P. belbahrii in seed lots before sale or planting or in the field before symptoms develop could allow for timely deployment of disease management strategies. In this study, a draft genome assembly and next-generation sequencing reads for P. belbahrii, as well as publicly available DNA-seq and RNA-seq reads of several other downy mildew pathogens, were incorporated into a bioinformatics pipeline to predict P. belbahrii-specific diagnostic markers. The specificity of each candidate marker was validated against a diverse DNA collection of P. belbahrii, host tissue, and related oomycetes using PCR. Two species-specific markers were identified and used as templates to develop a highly sensitive probe-based real-time quantitative PCR (qPCR) assay that could detect P. belbahrii in leaf tissue and seed samples. Both markers were capable of reliably detecting as low as 500 fg/µl of P. belbahrii genomic DNA and as few as 10 sporangia. The qPCR assay was then validated with seed samples collected from a basil cultivar experiment. In total, 48 seed samples were collected and tested; P. belbahrii was detected in samples of all cultivars at estimated concentrations of 600 fg/µl up to 250 pg/µl and at as few as 10 sporangia up to >1,000 sporangia. The markers and assays are valuable for diagnostics and identifying P. belbahrii-contaminated seed lots to mitigate the effects of future basil downy mildew epidemics.

Temporal Dynamics and Severity of Cucurbit Downy Mildew Epidemics as Affected by Chemical Control and Cucurbit Host Type.

Cucurbit downy mildew caused by the oomycete Pseudoperonospora cubensis is an important disease that affects members of Cucurbitaceae family globally. However, temporal dynamics of the disease have not been characterized at the field scale to understand how control strategies influence disease epidemics. Disease severity was assessed visually on cucumber and summer squash treated with weekly alternation of chlorothalonil with cymoxanil, fluopicolide, or propamocarb during the 2018 spring season and 2019 and 2020 fall seasons in North Carolina and the 2018 and 2020 fall seasons in South Carolina. Disease onset was observed around mid-June during the spring season and early September during the fall season, followed by a rapid increase in severity until mid-July in the spring season and late September or mid-October in the fall season, typical of polycyclic epidemics. The Gompertz, logistic, and monomolecular growth models were fitted to disease severity using linear regression and parameter estimates to compare the effects of fungicide treatment and cucurbit host type on disease progress. The Gompertz and logistic models were more appropriate than the monomolecular model in describing temporal dynamics of cucurbit downy mildew, with the Gompertz model providing the best description for 34 of the 44 epidemics examined. Fungicide treatment and host type significantly (P < 0.0001) affected the standardized area under disease progress curve (sAUDPC), final disease severity (Final DS), and weighted mean absolute rates of disease progress (ρ), with these variables, in most cases, being significantly (P < 0.05) lower in fungicide-treated plots than in untreated control plots. Except in a few cases, sAUDPC, Final DS, and ρ were lower in cases where chlorothalonil was alternated with fluopicolide or propamocarb than in cases where chlorothalonil was alternated with cymoxanil or when chlorothalonil was applied alone. These results characterized the temporal progress of cucurbit downy mildew and provided an improved understanding of the dynamics of the disease at the field level. Parameters of disease progress obtained from this study could serve as inputs in simulation studies to assess the efficacy of fungicide alternation in managing fungicide resistance in this pathosystem.

Genotyping by sequencing suggests overwintering of Peronospora destructor in southwestern Québec, Canada.

Several Peronospora species are carried by wind over short and long distances, from warmer climates where they survive on living plants to cooler climates. In eastern Canada, this annual flow of sporangia was thought to be the main source of Peronospora destructor responsible for onion downy mildew. However, the results of a recent study showed that the increasing frequency of onion downy mildew epidemics in eastern Canada is associated with warmer autumns, milder winters, and previous year disease severity, suggesting overwintering of the inoculum in an area where the pathogen is not known to be endogenous. In this study, genotyping by sequencing was used to investigate the population structure of P. destructor at the landscape scale. The study focused on a particular region of southwestern Québec—Les Jardins de Napierville—to determine if the populations were clonal and regionally differentiated. The data were characterized by a high level of linkage disequilibrium, characteristic of clonal organisms. Consequently, the null hypothesis of random mating was rejected when tested on predefined or nonpredefined populations, indicating that linkage disequilibrium was not a function of population structure and suggesting a mixed reproduction mode. Discriminant analysis of principal components performed with predefined population assignment allowed grouping P. destructor isolates by geographical regions, while analysis of molecular variance confirmed that this genetic differentiation was significant at the regional level. Without using a priori population assignment, isolates were clustered into four genetic clusters. These results represent a baseline estimate of the genetic diversity and population structure of P. destructor.

Modelling the effect of partial resistance on epidemics of downy mildew of grapevine

The cultivation of grape varieties with partial resistance to disease may become an important component for disease management in the future. The impact of partial resistance on downy mildew epidemics according to its components have not been explored so far. This work aims to model, understand, and quantitatively analyse the effect of partial resistance against dual disease epidemics (foliage and clusters) caused by Plasmopara viticola, and rank the efficiency of different resistance components in disease suppression. We use an epidemiological simulation model to integrate the effect of four components of partial resistance, expressed as relative resistance parameters, i.e. infection efficiency (RRIE), latency period (RRLP), sporulation (RRSP), and infectious period (RRIP). Both the individual and combined effect of these components of resistance on downy mildew epidemics are evaluated through a sensitivity analysis. A comparison of simulation runs in different scenarios of disease conduciveness using experimental measurements of components of partial resistance for 16 different grapevine varieties is also performed. Increasing values of RR parameters led to a suppression of disease progress on foliage. The strongest reduction of epidemics on foliage is generated by increases in RRIE, followed by RRSP, RRIP, and last by RRLP. The effect of partial resistance on epidemics is more conspicuous in a scenario of limited disease conduciveness. The strongest suppressive effect of simulated epidemics on clusters is associated with RRIE, and the lowest effect with RRLP, with similar effects of increasing values of RRIP and RRSP. The use of experimentally measured relative resistance parameters to run simulated epidemics shows a reduction of the area under the disease progress curve from 4 × 105 (on a susceptible reference grapevine variety) to 4 × 102 (on cv. Bronner), i.e. a reduction of disease by 1000. The simulation of the varietal effect in intermediate and less favourable scenarios of disease conduciveness strongly suppresses the epidemic on foliage and limits disease on clusters to very low levels. Deploying partial host plant resistance in environments that are not strongly conducive to downy mildew epidemics could represent an effective use of partial resistance.

Evidence for Differences in the Temporal Progress of Plasmopara viticola Clades riparia and aestivalis Airborne Inoculum Monitored in Vineyards in Eastern Canada Using a Specific Multiplex Quantitative PCR Assay.

Four clades of Plasmopara viticola isolated from wild and cultivated Vitis species were described in 2013. Only P. viticola clades riparia and aestivalis have been detected in eastern Canada. To increase our understanding of the epidemiology of these clades of P. viticola, airborne sporangia were monitored with spore samplers at two experimental vineyards from 2015 to 2018 and at 11, 14, and 15 commercial vineyards in 2016, 2017, and 2018, respectively. At each vineyard and in each year, airborne sporangia were assessed three times weekly from grapevine budbreak to harvest. To accurately monitor airborne inoculum, a specific and sensitive quantitative PCR assay for simultaneous quantification of P. viticola clades riparia and aestivalis was developed. At the experimental site, in the vineyard planted with the hybrid grape variety Chancellor, mostly P. viticola clade riparia was detected. In vineyards planted with multiple grape varieties, airborne sporangia of P. viticola clade riparia were prevalent at the beginning of the season, whereas P. viticola clade aestivalis was mostly detected from midseason to harvest. At the commercial sites, airborne sporangia of P. viticola clade riparia were more prevalent in 2016, whereas P. viticola clade aestivalis was more prevalent in 2017 and 2018. The only significant difference between the inoculum progress curves was the time at which 50% of the seasonal inoculum was reached, with an average for the 3 years of 100.8 and 117.9 days since 1 May for P. viticola clade riparia and clade aestivalis, respectively. When airborne sporangium concentrations were expressed as the proportion of the two clades, in general, the proportion of clade aestivalis to that of clade riparia was low at the beginning of the season and increased to reach approximately 0.9 to 1.0 at the end of the season. These results suggest that both clades of P. viticola coexist, but that clade aestivalis is predominant and that downy mildew epidemics caused by P. viticola clade riparia occur 2 to 3 weeks before those caused by clade aestivalis.

Temporal disease dynamics and relative importance of sexual and asexual reproduction of grape downy mildew (Plasmopara viticola) in an isolated vineyard in the North Georgia Mountains, USA

AbstractThe North Georgia Mountains are the southernmost region along the United States East Coast where European wine grapes (Vitis vinifera) are grown commercially. Epidemics of downy mildew, caused by Plasmopara viticola, are frequent and severe, but little is known about the epidemiology and population biology of the pathogen in this region. Disease monitoring in an experimental vineyard from 2015 to 2017 indicated that times of disease onset and progress rates were highly variable across years and cultivars. Oospores were observed microscopically, and simulation with a process‐based model indicated presence of conditions favourable for oospore germination in the spring and early summer each year. A total of 409 P. viticola isolates collected over three years were genotyped with seven microsatellite markers, revealing very high genotypic diversity, which when combined with the observation of oospores is indicative of a sexually reproducing population. Among the 409 isolates, 225 multilocus genotypes (MLGs) were identified, of which 164 were detected only once and 61 were repeated (clonal). Eight MLGs (represented by 28 isolates) were detected across years, suggesting the possibility of asexual overwintering of P. viticola in this region. Across sampling dates, the percentage of isolates belonging to nonrepeated (unique) MLGs ranged from 27.3% to 63.2%. Even towards the end of the annual epidemic, the percentage of isolates in nonrepeated MLGs was still relatively high, around 30%. These MLGs may have originated from oospores germinating late during the growing season, although incomplete sampling at earlier dates and contribution by immigration cannot be fully excluded.

Simulation of potential epidemics of downy mildew of grapevine in different scenarios of disease conduciveness

The parameterisation process of a previously developed modelling structure of the grapevine-downy mildew pathosystem is described. The model incorporates primary and secondary infections, host crop growth and development, along with a linkage between disease on foliage and disease on clusters. This process-based model was developed with a main objective of understanding the behaviour of the pathosystem under different, variable, environmental conditions, or under climate change. Six scenarios of disease conduciveness were developed in order to capture the range of environmental conditions under which potential downy mildew of grapevine epidemics can develop. These climate scenarios were based on moisture and temperature factors. The scenarios were translated into vectors of parameters for primary and secondary infections in the model. Model testing was performed in three steps: (i) an analysis of potential epidemics was conducted from the literature on grapevine downy mildew in order to delineate the behaviour of the pathosystem under different scenarios; (ii) a simulation experiment was conducted to investigate the response of the model to different patterns of environmental conditions, corresponding to six scenarios of disease conduciveness; and (iii) expected and simulated epidemics under these scenarios were compared. In scenarios, the model mobilised existing quantitative information on downy mildew of grapevine and generated outputs that are congruent with expected patterns of potential epidemic. This study indicates that the model is a reliable tool for simulating accurate and robust potential epidemics of downy mildew of grapevine in a scenario analysis. This can have many applications, such as the understanding of the behaviour of the pathosystem under climate change or when partial host resistance is involved.

Factors Influencing the Occurrence of Onion Downy Mildew (Peronospora destructor) Epidemics: Trends from 31 Years of Observational Data

Onion downy mildew (ODM) caused by Peronospora destructor has been increasing annually in south-western Québec since the early 2000s, reaching 33% of affected onion fields in 2014. Using observational data collected over a period of 31 consecutive years, this study aimed to investigate the variations in ODM incidence and epidemic onset and identify the meteorological variables that influence its polyetic development. A logistic model was fitted to each ODM epidemic to estimate and compare the onset of epidemics on a regional basis. Results of this analysis showed that the first observation date, 10% epidemic onset (b10) and mid-time (b) were, on average, 30.4, 15.1 and 11.3 days earlier in 2007–2017 than in 1987–1996. Results of a principal component analysis suggested that regional disease incidence was mostly influenced by the precipitation regime, the final regional disease incidence the previous year, and warmer temperature during the harvest period the previous fall. Subsequently, the data were divided in three periods of 10, 10 and 11 years, and a discriminant analysis was performed to classify each year in the correct period. Using a sufficient subset of five discriminating variables (temperature and rainfall at harvest the previous fall, winter coldness, solar radiation, and disease incidence the previous year), it was possible to classify 93.5% of the ODM epidemics in the period where they belong. These results suggest that P. destructor may overwinter under northern latitudes and help to highlight the need for more research on overwintering and for the development of molecular-based tools enabling the monitoring of initial and secondary inoculum.

Genome Sequencing and Transcriptome Analysis of the Hop Downy Mildew Pathogen Pseudoperonospora humuli Reveal Species-Specific Genes for Molecular Detection.

Pseudoperonospora humuli is an obligate oomycete pathogen of hop (Humulus lupulus) that causes downy mildew, an important disease in most production regions in the Northern Hemisphere. The pathogen can cause a systemic infection in hop, overwinter in the root system, and infect propagation material. Substantial yield loss may occur owing to P. humuli infection of strobiles (seed cones), shoots, and cone-bearing branches. Fungicide application and cultural practices are the primary methods to manage hop downy mildew. However, effective, sustainable, and cost-effective management of downy mildew can be improved by developing early detection systems to inform on disease risk and timely fungicide application. However, no species-specific diagnostic assays or genomic resources are available for P. humuli. The genome of the P. humuli OR502AA isolate was partially sequenced using Illumina technology and assembled with ABySS. The assembly had a minimum scaffold length of 500 bp and an N50 (median scaffold length of the assembled genome) of 19.2 kbp. A total number of 18,656 genes were identified using MAKER standard gene predictions. Additionally, transcriptome assemblies were generated using RNA-seq and Trinity for seven additional P. humuli isolates. Bioinformatics analyses of next generation sequencing reads of P. humuli and P. cubensis (a closely related sister species) identified 242 candidate species-specific P. humuli genes that could be used as diagnostic molecular markers. These candidate genes were validated using polymerase chain reaction against a diverse collection of isolates from P. humuli, P. cubensis, and other oomycetes. Overall, four diagnostic markers were found to be uniquely present in P. humuli. These candidate markers identified through comparative genomics can be used for pathogen diagnostics in propagation material, such as rhizomes and vegetative cuttings, or adapted for biosurveillance of airborne sporangia, an important source of inoculum in hop downy mildew epidemics.

Cryptic species and population genetic structure of Plasmopara viticola in São Paulo State, Brazil

Downy mildew (Plasmopara viticola) is one of the most important diseases in grape‐growing areas worldwide, including Brazil. To examine pathogen population biology and structure, P. viticola was sampled during the 2015/16 growing season from 516 lesions on nine grape cultivars in 11 locations in subtropical areas of São Paulo State, Brazil. For identification of cryptic species, a subsample of 130 isolates was subjected to cleaved amplified polymorphic sequence (CAPS) analysis, and for 91 of these isolates the ITS1 region was sequenced. These analyses suggest that the population of P. viticola in São Paulo State consists of a single cryptic species, P. viticola clade aestivalis. Seven microsatellite markers were used to determine the genetic structure of all 516 P. viticola isolates, identifying 23 alleles and 55 multilocus genotypes (MLGs). Among these MLGs, 34.5% were clonal and represented 93% of the isolates sampled. Four dominant genotypes were present in at least five different locations, corresponding to 65.7% of the isolates sampled. Genotypic diversity (Ĝ = 0.21–0.89) and clonal fraction (0.58–0.96) varied among locations (populations). Most populations showed significant deviation from Hardy–Weinberg expectations; in addition, excess of heterozygosity was verified for many loci. However, principal coordinate analysis revealed no clusters among locations and no significant isolation by distance was found, suggesting high levels of migration. The results indicate that downy mildew epidemics result from multiple clonal infections caused by a few genotypes of P. viticola, and reproduction of P. viticola in São Paulo State is predominantly asexual.

Examination and characterization of key factors to seasonal epidemics of downy mildew in native grape species Vitis davidii in southern China with the use of disease warning systems

Spine grape (Vitis davidii) is a major native grape species cultivated in south central China, and the industry is threatened by downy mildew outbreaks caused by Plasmopara viticola in this rainy and warm region. To examine and characterize the local epidemics of this disease in this region, experiments were conducted in Jingzhou in 2012–2013 with variety ‘Gaoshan-2’ and Zhongfang in 2013–2016 with variety ‘Xiangzhenzhu’ in Hunan Province. The number of diseased leaves with new infections (DL_Sum) and disease severity were assessed every 5 days. First, we used a mechanistic simulation model originally developed in Europe to simulate the primary inoculum seasons in these years and found that they ended in early May, suggesting little influences from oospores to the disease in summer. Then we used negative binomial regression to model the temporal changes of DL_Sum in summer. The predictive variables included weather variables and two disease risk indices derived based on two other empirical disease warning systems developed in Europe and North America. Relative humidity (RH) was identified the best predictive variable among all variables. Ordinal logistic regression identified mean temperature and days of heavy rainfall (>7 mm) as key weather factors for disease development. In general, RH was more important for infection process, and temperatures and rainfall were more important for symptom development and spore dispersal. Canopy structures could also have significant effects on disease development. These results may help for the development of new disease warning or forecast systems more suitable for this disease in southern China.

Detection of Latent Peronospora effusa Infections in Spinach.

Downy mildew disease of spinach, caused by Peronospora effusa, is managed in conventional fields by a combination of host resistance and scheduled fungicide applications. Fungicides are currently applied to prevent downy mildew epidemics regardless of the infection status of spinach crops. A more streamlined approach would be to develop methods to target either latent infections for fungicide application in conventional production systems or to hasten harvest in organic production. In this study, conventional polymerase chain reaction (PCR) was applied to detect P. effusa DNA in symptomless spinach leaves in three spatially and temporally separated field plots, each containing four 2-m beds, 35 m in length. Spinach leaves were sampled weekly at 3-m intervals at 48 locations throughout each plot. Initial samples were asymptomatic and yet PCR enabled detection of P. effusa DNA extracted from sampled spinach leaves. Detection of latent downy mildew infection in spinach leaves was confirmed by PCR as early as 7 days prior to symptom development. The limit of pathogen DNA detection in spinach leaves was calculated at 10 pg using the conventional PCR approach. Quantitative PCR with TaqMan methodology revealed the presence of inhibitors from spinach leaf DNA extracts and affected amplification efficiencies, but not when diluted, enabling detection of P. effusa DNA at a concentration of <0.1 pg. In conclusion, detection of latent infections may enable management decisions for earlier-than-normal harvest of infected, symptomless organic crops, and for timing fungicide applications on symptomless plants in conventional production.

Pseudoperonospora cubensis virulence and pathotype structure in Kazakhstan

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.

Advances in Diagnostics of Downy Mildews: Lessons Learned from Other Oomycetes and Future Challenges.

Downy mildews are plant pathogens that damage crop quality and yield worldwide. Among the most severe and notorious crop epidemics of downy mildew occurred on grapes in the mid-1880s, which almost destroyed the wine industry in France. Since then, there have been multiple outbreaks on sorghum and millet in Africa, tobacco in Europe, and recent widespread epidemics on lettuce, basil, cucurbits, and spinach throughout North America. In the mid-1970s, loss of corn to downy mildew in the Philippines was estimated at US$23 million. Today, crops that are susceptible to downy mildews are worth at least $7.5 billion of the United States' economy. Although downy mildews cause devastating economic losses in the United States and globally, this pathogen group remains understudied because they are difficult to culture and accurately identify. Early detection of downy mildews in the environment is critical to establish pathogen presence and identity, determine fungicide resistance, and understand how pathogen populations disperse. Knowing when and where pathogens emerge is also important for identifying critical control points to restrict movement and to contain populations. Reducing the spread of pathogens also decreases the likelihood of sexual recombination events and discourages the emergence of novel virulent strains. A major challenge in detecting downy mildews is that they are obligate pathogens and thus cannot be cultured in artificial media to identify and maintain specimens. However, advances in molecular detection techniques hold promise for rapid and in some cases, relatively inexpensive diagnosis. In this article, we discuss recent advances in diagnostic tools that can be used to detect downy mildews. First, we briefly describe downy mildew taxonomy and genetic loci used for detection. Next, we review issues encountered when identifying loci and compare various traditional and novel platforms for diagnostics. We discuss diagnosis of downy mildew traits and issues to consider when detecting this group of organisms in different environments. We conclude with challenges and future directions for successful downy mildew detection.

Temperature and plant age drive downy mildew disease epidemics on oilseed Brassica napus and B. juncea

Studies were undertaken on the effects of temperature (14/10 °C and 22/17 °C day/night) and plant age (15, 23, 31 and 40 day-old-plants) on the severity of downy mildew (Hyaloperonospora parasitica) on oilseed Brassica cultivars (temperature: Brassica juncea Montara, B. napus Atomic, ATR-Hyden, Hyola 432, Hyola 450 TT, Thunder TT; plant age: B. juncea Dune, B. napus Surpass 402 and Hyola 450 TT). For temperature studies, there were significant (P < 0.001) effects of temperature, cultivar, and cultivar x temperature interaction. On cotyledons of susceptible cultivars (B. napus Hyola 450 TT and Thunder TT), plants were symptomatic at 22/17 °C by 48 h post inoculation (hpi) and with abundant sporulation evident by 72 hpi, and with all cotyledons of B. napus Thunder TT collapsed by 7 days post inoculation (dpi). However, at 14/10 °C, there were no symptoms on the same cultivars until 5 dpi, and sporulation only observed at 7 dpi. Percent disease index values (DI%) at 22/17 °C of B. juncea Montara and B. napus ATR-Hyden, Hyola 432, Atomic, Hyola 450 TT and Thunder TT were 4.5, 49.0, 51.4, 65.8, 86.3 and 96.0, respectively, with all except B. juncea Montara having significantly lower (P < 0.001) disease at 14/10 °C with DI% values of 2.8, 30.4, 27.9, 31.1, 44.4 and 76.4, respectively. For plant age studies, there were significant (P < 0.001) effects of plant age, cultivar, and cultivar x plant age interaction. DI% was significantly higher at 15 compared to 40 day-old-plants (dop) across all cultivars. B. juncea Dune showed greatest resistance, particularly on 40 dop, with DI% values of 25.8, 24.6, 22.9 and 7.5, for 15, 23, 31 and 40 dop, respectively. B. napus Surpass 402 showed high susceptibility on cotyledons of 15 dop but moderate resistance on leaves of other ages, with DI% values of 59.0, 31.2, 27.1 and 26.2 for 15, 23, 31 and 40 dop, respectively. B. napus Hyola 450 TT showed very high susceptibility at the cotyledon stage on 15 dop, but some resistance on 23 dop and more so on 31 and 40 dop, with DI% values of 84.0, 41.2, 35.4 and 32.9 for 15, 23, 31 and 40 dop, respectively. Together, these findings explain for the first time why development of downy mildew epidemics on susceptible cultivars occurs early in the growing season when warmer seasonal temperatures in autumn coincide with presence of seedlings; in contrast to later in the growing season on less susceptible older plants coinciding with cooler and less favourable winter temperatures. Increasing maximum and minimum temperatures associated with climate change have likely fostered the increased severity of downy mildew over the past 15 years.