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Sweet basil (Ocimum basilicum, 2n = 4x = 48) is susceptible to downy mildew caused by Peronospora belbahrii. The Pb1 gene exhibits complete resistance to the disease. However, Pb1 became prone to disease because of occurrence of new virulent races. Here, we show that Zambian accession PI 500950 (Ocimum americanum var. pilosum) is highly resistant to the new races. From an interspecies backcross between PI 500950 and the susceptible ‘Sweet basil’ we obtained, by embryo rescue, a population of 131 BC1F1 plants. This population segregated 73 resistant (58) and susceptible (1:1; P = 0.22) plants, suggesting that resistance is controlled by one incompletely dominant gene called Pb2. To determine whether allelic relationship exists between Pb1 and Pb2, we used two differential races: race 0, which is avirulent to both PI 500945 (Pb1) and PI 500950 (Pb2), and race 1, which is virulent to PI 500945 but avirulent to PI 500950. F1 plants obtained from ‘12-4-6’ (BC6F3 derived from PI 500945) and ‘56’ (BC3F3 derived from PI 500950) showed resistant superiority to both races through dominant complementary interaction. F2 plants segregated to race 0 as follows: 12:3:1 (immune/incomplete resistant/susceptible) as opposed to 9:3:4 to race 1, indicating that Pb1 and Pb2 are not alleles. Because joint action is contributed in F1 plants and in advanced [BC3F3(56) × BC6F3(12-4-6) F4] populations that carry both genes, it can be assumed that both accessions carry two unlinked genes but share a common signal transduction pathway, which leads to dominant complementation superiority of the resistance against different races of basil downy mildew.

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.

Quinoa is a potential new crop for New England; however, its susceptibility to downy mildew, caused by Peronospora variabilis, is a key obstacle for cultivation. The objectives of this study were to evaluate differential resistance within the Chenopodium genus, identify novel sources of resistance for use in future genetic studies or breeding programs, and investigate phylogenetic relationships of P. variabilis isolates from different Chenopodium hosts. The long-term goal of this research is to develop a resistant variety of quinoa to be grown in New England. Field trials conducted at the University of New Hampshire evaluated downy mildew disease severity on 10 Chenopodium accessions representing four species. Disease severity for each treatment was compared and significant differences in disease severity were observed between accessions. C. berlandieri var. macrocalycium ecotypes collected from Rye Beach, New Hampshire and Appledore Island, Maine exhibited the lowest disease severity over the growing season. P. variabilis was isolated from each accession, and COX2 sequences were compared. Phylogenetic analyses suggest no effect of host species on P. variabilis sequence similarity; however, isolates are shown to cluster by geographic location. This research provides the first step in identifying potential New England native sources of resistance to downy mildew within the genus Chenopodium and provides preliminary information needed to further investigate resistance at the genomic level in Chenopodium spp.

Protein ubiquitination is one of the most common modifications that can degrade or modify proteins in eukaryotic cells. The E2 ubiquitin-conjugating enzymes (UBCs) are involved in multiple biological processes of eukaryotes and their response to adverse stresses. Genome-wide survey of the UBC gene family has been performed in many plant species but not in cucumber (Cucumis sativus). In this study, a total of 38 UBC family genes (designated as CsUBC1–CsUBC38) were identified in cucumber. The phylogenetic analysis of UBC proteins from cucumber, Arabidopsis and maize indicated that these proteins could be divided into 15 groups. Most of the phylogenetically related CsUBC members had similar conserved motif patterns and gene structures. The CsUBC genes were unevenly distributed on seven chromosomes, and gene duplication analysis indicated that segmental duplication has played a significant role in the expansion of the cucumber UBC gene family. Promoter analysis of these genes resulted in the identification of many hormone-, stress- and development-related cis-elements. The CsUBC genes exhibited differential expression patterns in different tissues and developmental stages of fruit ripening. In addition, a total of 14 CsUBC genes were differentially expressed upon downy mildew (DM) infection compared with the control. Our results lay the foundation for further clarification of the roles of the CsUBC genes in the future.

Pearl millet [Pennisetum glaucum (L.) R. Br.] is the essential food crop for over ninety million people living in drier parts of India and South Africa. Pearl millet crop production is harshly hindered by numerous biotic stresses. Sclerospora graminicola causes downy mildew disease in pearl millet. Effectors are the proteins secreted by several fungi and bacteria that manipulate the host cell structure and function. This current study aims to identify genes encoding effector proteins from the S. graminicola genome and validate them through molecular techniques. In silico analyses were employed for candidate effector prediction. A total of 845 secretory transmembrane proteins were predicted, out of which 35 proteins carrying LxLFLAK (Leucine–any amino acid–Phenylalanine–Leucine–Alanine–Lysine) motif were crinkler, 52 RxLR (Arginine, any amino acid, Leucine, Arginine), and 17 RxLR-dEER putative effector proteins. Gene validation analysis of 17 RxLR-dEER effector protein-producing genes was carried out, of which 5genes were amplified on the gel. These novel gene sequences were submitted to NCBI. This study is the first report on the identification and characterization of effector genes in Sclerospora graminicola. This dataset will aid in the integration of effector classes that act independently, paving the way to investigate how pearl millet responds to effector protein interactions. These results will assist in identifying functional effector proteins involving the omic approach using newer bioinformatics tools to protect pearl millet plants against downy mildew stress. Considered together, the identified effector protein-encoding functional genes can be utilized in screening oomycetes downy mildew diseases in other crops across the globe.

Polyphenol oxidase partial gene PG-PPO was cloned and characterized from Pennisetum glaucum (pearl millet) which showed 42% identity to a PPO sequence isolated from wheat at the region of Copper B with a score of 40 and e-value of 2.8. Multiple sequence alignment results revealed similarity to polyphenol oxidase (PPO) sequences from wheat, trifolium, lettuce, apricot, tobacco, tomato, pokeweed, apple, grape and poplar especially at the Copper B region of PPO. The 395 bp pearl millet PPO sequence was AT rich (53.3%) and contained the highly conserved amino acids of histidine-rich copper binding sites similar to PPO sequences from other crops. Results also indicated that PPO in pearl millet exists in multi copy. The role of the isolated PPO gene during pearl millet-downy mildew interaction was analyzed and the results showed significantly higher and rapid accumulation of PPO mRNAs in resistant pearl millet seedlings inoculated with Sclerospora graminicola in comparison to the susceptible control, demonstrating that the PPO plays a prominent role in pearl millet defense against pathogens, particularly downy mildew pathogen.

Downy and powdery mildews are major grapevine diseases. In organic viticulture, a few fungicides with protectant activities (copper and sulphur in particular) can be used, and their preventative application frequently leads to unneeded spraying. The adoption of an epidemiological disease forecasting model could optimise the timing of treatments and achieve a good level of disease protection. In this study, the effectiveness of the EPI (Etat Potentiel d’Infection) model in predicting infection risk for downy and powdery mildews was evaluated in nine organic vineyards located in Panzano in Chianti (FI), over a 2-year period (2020–2021). The reliability of the EPI model was investigated by comparing the disease intensities, the number of fungicide sprayings, the quantities of the fungicides (kg/ha), and the costs of the treatment achieved, with or without the use of the model, in a vineyard. The results obtained over two seasons indicated that, in most cases, the use of the EPI model accurately signalled the infection risk and allowed for a reduction in the frequency and cost of spraying, particularly for powdery mildew control (−40% sprayings, −20% costs compared to the farmer’s schedule), without compromising crop protection. The use of the EPI model can, therefore, contribute to more-sustainable disease management in organic viticulture.

Pseudoperonospora cubensis (downy mildew) is highly virulent to various Cucurbitaceae crops, including cucumber (Cucumis sativus). We tested chlorine dioxide application in a plastic greenhouse for environmentfriendly control of downy mildew disease. Spraying diluted chlorine dioxide suppressed downy mildew disease with 41.2% control efficacy. Thermal fogging with chlorine dioxide had a high control efficacy of 80.9%, confirming that this approach is useful for environment-friendly downy mildew control. Using thermal fogging to control diseases that are greatly affected by humidity, such as downy mildew, may be more effective compared with conventional dilution spray control methods.

Background. Downy mildew (DM) caused by the fungus Plasmopara halstedii (Farl) Berl. & De Toni) is one of the most harmful diseases of sunflower (Helianthus annuus L.). Due to the pathogen’s attacks, annual seed harvest losses range between 30% and 70%. Lines resistant to new races of the pathogen should be obtained for the development of commercial sunflower hybrids.Materials and methods. Downy mildew resistance of 323 lines and 10 cultivars from the sunflower collection was assessed in the field at the Kuban Experiment Station of VIR in 2017–2018. Line VIR 845, susceptible during all years of observations, was used as a control. Resistance genes were identified by means of the molecular analysis using diagnostic markers of the Plarg, Pl6 and Pl8 genes that confer resistance to many known P. halstedii races.Results. The founder varieties of VIR’s lines were susceptible to a varying extent. Thirty-nine lines were resistant in 2016 and 2018; among those, 36 lines were susceptible in 2017. Presumably, a more virulent P. halstedii race became widespread in 2017, compared to the races that prevailed in 2016 and 2018, so the genes that determined resistance in 2016 and 2018 turned out to be ineffective. Lines ТА 716-18, VIR 768, and VIR 800, having originated from interspecific hybrids, exhibited absence of pathogenic damage during 3 years of the trials. Molecular markers of the Plarg, Pl6 and Pl8 genes were detected in most lines that demonstrated resistance in 2016 and 2018. There were no markers in lines VIR 768 and VIR 800, whereas in ТА 716-18 the markers of Plarg and Pl8 were present.Conclusion. As a result of the long-term studies, a trait-specific genetic collection was established for sunflower. It comprises genotyped lines with various effective DM resistance genes. Lines ТА 716-18, VIR 768 and VIR 800 appeared highly resistant to the pathogen and probably possess new resistance genes/alleles introgressed from wild species.

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

Bananas are among the world’s most important cash and staple crops but are threatened by various devastating pathogens. The phytohormone salicylic acid (SA) plays a key role in the regulation of plant immune response. Tracking the expression of SA-responsive marker genes under pathogen infection is important in pathogenesis elucidation. However, the common SA-responsive marker genes are not consistently induced in different banana cultivars or different organs. Here, we conducted transcriptome analysis for SA response of a banana cultivar, ‘Pei-Chiao’ (Cavendish, AAA genome), and identified three genes, MaWRKY40, MaWRKY70, and Downy Mildew Resistant 6 (DMR6)-Like Oxygenase 1 (MaDLO1) that are robustly induced upon SA treatment in both the leaves and roots. Consistent induction of these three genes by SA treatment was also detected in both the leaves and roots of bananas belonging to different genome types such as ‘Tai-Chiao No. 7’ (Cavendish, AAA genome), ‘Pisang Awak’ (ABB genome), and ‘Lady Finger’ (AA genome). Furthermore, the biotrophic pathogen cucumber mosaic virus elicited the expression of MaWRKY40 and MaDLO1 in infected leaves of susceptible cultivars. The hemibiotrophic fungal pathogen Fusarium oxysporum f. sp. cubense tropical race 4 (TR4) also consistently induced the expression of MaWRKY40 and MaDLO1 in the infected roots of the F. oxysporum f. sp. cubense TR4-resistant cultivar. These results indicate that MaWRKY40 and MaDLO1 can be used as reliable SA-responsive marker genes for the study of plant immunity in banana. Revealing SA-responsive marker genes provides a stepping stone for further studies in banana resistance to pathogens.

Grapevine downy mildew (GDM) is one of the most serious diseases of grapevines. Limitations to the use of copper-based products in organic agriculture according to the European Union (EU) regulation EU/2002/473 and the later EU Commission implementing regulation 2018/1981 have promoted a search for alternatives. This 5-year field trial evaluated the effectiveness of several strategies against GDM using different chitosan-based formulations and application rates in comparison with other natural compounds applied individually or with copper hydroxide. Trials were performed in commercial vineyards with different environmental conditions and grapevine cultivars. For the natural compounds applied as individual treatments, a 0.5%/0.8% chitosan formulation provided the best protection against GDM; the other compounds and formulations were less effective. When copper hydroxide use was halved by combining it with the natural compounds according to three different strategies, the GDM incidence, severity, and McKinney index were reduced, particularly for copper hydroxide applied in combination with the 0.5%/0.8% chitosan formulation. The 0.5%/0.8% chitosan formulation alone and with copper hydroxide provided good protection against GDM during both high-pressure and low-pressure disease seasons. Therefore, chitosan represents a good alternative to copper formulations for the control of GDM and both organic and integrated disease management.

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.

Downy mildew is one of the most serious diseases of Papaver somniferum. Endophytes isolated from different parts of P. somniferum were screened for their ability to enhance resistance against downy mildew caused by the obligate biotrophic oomycete Peronospora meconopsidis. Two endophytes (SMR1 and SMR2) reduced the downy mildew on three P. somniferum genotypes (Sampada, J-16, and I-14). SMR1 (Microbacterium sp.) also enhanced the resistance of P. somniferum against downy mildew under field conditions. The biochemical markers of plant susceptibility under biotic stresses (proline and malondialdehyde) were found to be reduced in P. somniferum upon SMR1 treatment. To understand the mechanisms underlying the enhanced resistance to downy mildew in SMR1 endophyte-treated P. somniferum genotype J-16, we compared the expression profiles using the next-generation RNA sequencing approach between P. somniferum pretreated with SMR1 and untreated endophyte-free control plants following exposure to downy mildew pathogen. Comparative transcriptome analysis revealed differential expression of transcripts belonging to broad classes of signal transduction, protein modification, disease/defense proteins, transcription factors, and phytohormones in SMR1-primed P. somniferum after infection with downy mildew pathogen. Furthermore, enhanced salicylic acid content was observed in SMR1-primed P. somniferum after exposure to downy mildew pathogen. This study sheds light on molecular mechanisms underlying enhanced resistance to downy mildew in SMR1-primed P. somniferum. Finally, we propose that the SA-dependent defense pathway, the hallmark of systemic acquired resistance, is activated in SMR1-primed P. somniferum, triggering the endophyte-induced resistance.

Commercial essential oils (EO) obtained from different parts of black cumin (N. sativa), mustard (S. nigra), St. John’s wort (H. perforatum), garlic (A. sativum), grape (V. vinifera), and ginger (Z. officinale) plants were evaluated for in vivo anti-mildew activity against pathotypes ‘771’ and ‘773’ of P.halstedii. The EOs were examined at concentrations of 0.2%, 0.4%, and 0.6% in susceptible sunflower variety ‘08-TR-003’ as seed, foliar, and seed + foliar treatments.It was concluded that seed treatments could be applicable and appropriate, due to the phytotoxic effects of foliar treatments at increasing concentrations. Moreover, it was observed that seed treatments with St. John’s wort, mustard, grape, ginger, or garlic EOs at a concentration of 0.6% exhibited anti-mildew activity with a decrease in sporangium quantity above 80% for pathotypes ‘771’ and ‘773’. Seed treatment with grape EO had the highest anti-mildew activity at a concentration of 0.6% and was found to be the most effective with a decrease in sporangium quantity above 90% for both pathotypes of the pathogen.

Downy mildew is a serious disease in cucumber production worldwide, which is caused by Pseudoperonospora cubensis (Berk. & Curt.) Rostov. Underlying the mechanism of cucumber response to downy mildew infection is important for breeding improvement and production; however, the research remains largely elusive. A comparative proteomic approach was used to reveal the differential accumulation of the proteomes from leaves of cucumber (susceptible line and resistant line) that were inoculated with P. cubensis or not by two-dimensional electrophoresis and MALDI-TOF/TOF MS. A total of 76 protein spots were successfully identified with significant changes in abundance (> 2-fold, P < 0.05) in downy mildew infected or not leaves for the susceptible line and resistant line. By the functional annotation, these proteins were classified into 8 groups including photosynthesis (29%), energy and metabolism (29%), cell rescue and defense (17%), and protein biosynthesis, folding and degradation (13%), unclassified (7%), nucleotide metabolism (3%), signal transduction (1%) and cellular process (1%). Among the 17 differentially expressed proteins between the resistant and susceptible cucumber line, most of the protein spots were concentrated in cell rescue and defense (4) and energy and metabolism (4). Moreover, a schematic diagram containing majority of the metabolic pathways of cucumber leaves in response to downy mildew was proposed. This network revealed the positive effect of several functional components in cucumber seedlings' resistant to downy mildew such as accumulation of energy supply and resistance-related proteins, hastened protein metabolism and photorespiratory, inhibited photosynthesis, and triggered photosystem repair and programmed cell death. Taken together, these results have advanced a further understanding of the key metabolic pathways of cucumber resistance to downy mildew and pathogen control in the proteomic level.

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

Downy mildew, caused by the biotrophic oomycete Plasmopara viticola, is one of the most economically significant grapevine diseases worldwide. Current strategies to cope with this threat rely on the massive use of chemical compounds during each cultivation season. The economic costs and negative environmental impact associated with these applications increased the urge to search for sustainable strategies of disease control. Improved knowledge of plant mechanisms to counteract pathogen infection may allow the development of alternative strategies for plant protection. Epigenetic regulation, in particular DNA methylation, is emerging as a key factor in the context of plant-pathogen interactions associated with the expression modulation of defence genes. To improve our understanding of the genetic and epigenetic mechanisms underpinning grapevine response to P. viticola, we studied the modulation of both 5-mC methylation and gene expression at 6 and 24 h post-infection (hpi). Leaves of two table grape genotypes (Vitis vinifera), selected by breeding activities for their contrasting level of susceptibility to the pathogen, were analysed. Following pathogen infection, we found variations in the 5-mC methylation level and the gene expression profile. The results indicate a genotype-specific response to pathogen infection. The tolerant genotype (N23/018) at 6 hpi exhibits a lower methylation level compared to the susceptible one (N20/020), and it shows an early modulation (at 6 hpi) of defence and epigenetic-related genes during P. viticola infection. These data suggest that the timing of response is an important mechanism to efficiently counteract the pathogen attack.

Downy mildew is, globally, one of the most significant diseases in viticulture. Control of this pathogen is achieved through fungicide application. However, due to restrictions (from upcoming regulations) and growing environmental conscientiousness, it is critical to continuously enhance forecasting models to reduce fungicide application. Infection potential has traditionally been based on a 50 h–degree calculation (temperature multiplied by leaf wetness duration) measured by weather stations; the main climatic parameters for forecast modelling are temperature, relative humidity, and leaf wetness. This study took these parameters measured by a weather station and compared them with the same parameters measured inside a grape canopy. The study showed that the temperature readings by the weather station compared to inside the canopy recorded differences during the day but not at night; the relative humidity showed significant differences during both daytime and night; leaf wetness showed the highest differences and was statistically significant during both daytime and night. In conclusion, the measurement differences between inside of the canopy and at the weather station have significant impacts on the precision of forecasting models. Thus, using data from inside of a canopy for the prediction should lead to even less fungicide applications.

Downy mildew caused by Peronospora variabilis is a major fungal disease limiting yield of quinoa (Chenopodium quinoa) around the world. Existing quinoa cultivars do not have adequate levels of resistance to the disease. In the present study, 165 quinoa accessions were screened for resistance to the disease in two consecutive years. Disease ratings were performed by using a three-leaf scoring technique. In 2018, of 70 quinoa accessions, 47.14% were moderately resistant and 18.57% were highly resistant. However, in 2019, of 95 accessions, 44.21% showed moderately resistant reaction and 26.31% highly resistant reaction to the disease. The average of AUDPC (area under disease progress curve) of all the accessions was 1293.42 in 2018, and 931.68 in 2019. The disease caused significant (P < 0.01) cumulative defoliations in susceptible accessions. AUDPC was positively correlated (r = 0.796 in 2018, r = 0.896 in 2019) with defoliation rate and negatively correlated (r = –0.834 in 2018, r = –0.599 in 2019) with plant height. There were also negative correlations (r = –0.755 in 2018, r = –0.580 in 2019) between defoliation rate and plant height. The study not only revealed resistant germplasm but also showed the importance of screening downy mildew resistance by comparing disease-related agronomic characteristics.

Grapevine (Vitis vinifera L.) is prone to fungal and oomycete diseases. Downy and powdery mildews and grey mold, are caused by Plasmopara viticola, Erisiphe necator and Botrytis cinerea, respectively. P. viticola and E. necator are obligatory biotrophs whereas B. cinerea is a necrotroph. In tolerant grapevine cultivars, plant-pathogen interaction induces defence responses, including metabolite and protein accumulation and hypersensitive reaction. Lipid and lipid-derived molecules may have a key role in the activation of defence mechanisms. Previous results suggest that V. vinifera cv Regent tolerance to P. viticola may be mediated in the first hours post inoculation by fatty acid (FA) associated signalling. In the present study we characterized FA modulation in V. vinifera cv Regent leaves upon inoculation with P. viticola, E. necator and B. cinerea and correlated FA modulation with the expression profiles of genes encoding the FA desaturases FAD6 and FAD8. In all the interactions, a progressive desaturation of stearic acid to α-linolenic acid, precursor of jasmonic acid, occurred, which was observed for a longer period against B. cinerea. Our results provide evidence of a distinct FA meditated signalling pattern in grapevine interaction with biotrophs and necrotrophs. While the interaction with the biotrophs may trigger a higher synthesis of polyunsaturated FA (PUFA) at early time-points with a tendency to return to basal levels, the interaction with B. cinerea may trigger a later and more durable induction of PUFA synthesis. In all interactions, membrane fluidity modulation occurred, which may be crucial to maintain cellular function during infection.

The productivity of rapeseed-mustard in India is quite low as compared to the world scenario. It is mainly due to important diseases, Alternaria blight, white rust, downy mildew, powdery mildew, and white or Sclerotinia rot. Knowledge of epidemiologyand forecasting provide the basic information to developefficient and workable plant disease control models. The various weather variableslike temperature (T), relative humidity (RH), rainfall, wind velocity, and direction, leaf wetness duration, and solar radiation influence differentparameters of infection process, and disease development. Interaction between these weather variables and disease development pave the way for the development of the prediction models. Prediction models developed for the management of important diseases of rapeseed-mustard revealed that Alternaria blight is favoured by Tmax of 20–25 °C, Tmin of 15 °C, RHmor > 90% and RHeve > 50% where as white rust influencedby > 15 °C and RH > 65% with intermittent rains. Similarly, for downy mildew, Temprange of 15–20 °C with high RH was considered optimal for its progress. Leaf wetness duration of 4–6 h at 20 °C and 6–8 h at 15 °C is essential to initiate the downy mildew infection. Stag-head due to mixed infection of downy mildew and white rust is favoured by a Temp 20 °C with high RH and reduced period of sunshine (2–6 h/day) with rainfall up to 161 mm. Powdery mildew development is favoured by Temprange of 16–28 °C, mean RH 80%), Tmax up to 25 °C and Tmin of 5–12 °C. Often prediction models developed at one location may not fit atother locations. It indicates that data needs to be generated for a longer period and the model be tested atMultilocation. The disease-forecasting models must be developed by taking into account the crop variety, the prevalence of a particular pathotype and the microclimatic factors.

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

Cucurbit downy mildew (CDM), caused by the oomycete pathogen Pseudoperonospora cubensis, is a devastating foliar disease on cucumber resulting in reduced yields. In 2004, the pathogen re-emerged in the United States, infecting historically resistant cucumber cultivars and requiring the adoption of an intensive fungicide program. The pathogen cannot overwinter in Michigan fields but because of an influx of airborne sporangia CDM occurs annually. In Michigan, spore traps are used to monitor the presence of airborne P. cubensis sporangia in cucumber growing regions to guide the initiation of a fungicide program. However, Pseudoperonospora humuli sporangia, the causal agent of downy mildew on hop, are morphologically indistinguishable from P. cubensis sporangia. This morphological similarity reduces the ability to accurately detect P. cubensis from spore trap samples when examined with the aid of light microscopy. To improve P. cubensis detection, we adapted a qPCR-based assay to allow the differentiation between P. cubensis and P. humuli on Burkard spore trap samples collected in the field. Specifically, we evaluated the specificity and sensitivity of P. cubensis detection on Burkard spore trap tapes using a morphological-based and quantitative-PCR (qPCR)-based identification assay and determined whether sporangia of P. cubensis and P. humuli on Burkard samples could be distinguished using qPCR. We found that the qPCR assay was able to detect a single sporangium of each species on spore trap samples collected in the field with Cq values q values of P. cubensis and P. humuli, respectively, suggesting a limited capacity of the qPCR assay for the absolute quantification of sporangia in field samples. After 2 years of monitoring using Burkard spore traps coupled with the qPCR in cucumber fields, P. humuli sporangia were detected more frequently than P. cubensis early in the growing season (May and June). P. cubensis sporangia were detected ∼5 to 10 days before CDM symptoms were first observed in cucumber fields during both years. This research describes an improved sporangial detection system that is key for the monitoring and management of P. cubensis in Michigan.

Downy mildew (Peronospora belbahrii) is a severe disease in sweet basil. The aim of the present research was to study the effect of total nitrogen (N) concentration and NH4+ partial concentration in sweet basil fertigation on sweet basil downy mildew (SBDM). The study was conducted under controlled conditions, semi-commercial conditions, and commercial-like (field) conditions. Increasing the total N concentration in the irrigation water from 1.1 to 10.7 mM resulted in a gradual increase in N concentration in the shoots, and SBDM severity in potted sweet basil plants. Similarly, under commercial-like greenhouse conditions, higher N in the irrigation water resulted in higher SBDM severity. A high rate of 30–40% of the total N of NH4+ in the irrigation water, was associated with lower SBDM severity. Mineral concentration in the shoots of sweet basil was measured in all greenhouse plots. Shoot concentrations of N, P, K, and Ca were positively correlated with SBDM severity, while shoot concentrations of Mg and Cl negatively correlated with disease severity. In conclusion, N fertigation affected SBDM severity. We suggest that the effect of N fertigation on disease may be due to the change in N concentration in the tissue, or the change in other nutritional element (s).

Seedlings of 431 accessions representing six and five Armenian natural populations of the wild Lactuca georgica and L. altaica, respectively, and 32, 23, and 20 populations of L. saligna, L. serriola, and L. aculeata, respectively (mostly Israeli natural populations), were screened at seedling stage for resistance to six highly virulent races of Bremia lactucae - the causal of lettuce downy mildew - that were found on several widely grown cultivars and represent the virulence spectrum in the main lettuce production areas in Europe and California. This study is likely the first detailed screening of resistance to B. lactucae races in natural populations of L. georgica and L. altaica. The highest average resistance probability and frequency of highly resistant accessions across races were detected in L. georgica. These were even higher than those shown by the germplasm of known non-host resistance (NHR) species L. saligna, which classified to the secondary lettuce gene pool (LGP-2). Thus, we suggest that L. georgica is a new LGP-2 source of resistance to B. lactucae. Our findings also support previous observations that L. aculeata, a species within the LGP-1, should be prioritized as a source of downy mildew resistance in domesticated lettuce (L. sativa) breeding programs, since some L. aculeata accessions expressed highly resistance across races. For L. serriola, the closest relative of cultivated lettuce, and for L. altaica, also a closer or even a primitive form of L. sativa, probabilities of resistance to each of the investigated B. lactucae races were very low.

The sorghum culture TNS 648 (APK 1 x M35-1) recorded an average grain yield of 2445 kg/ha and fodder yield of 6490 kg/ha with an increase of 10.30 per cent and 10 per cent, respectively over CO 30 under rainfed condition. It also excelled K 12 by recording a 9.17 per cent and 13 per cent increase for grain and fodder yield, respectively. In irrigated situation, TNS 648 recorded an average grain yield of 2911 Kg/ha and fodder yield of 11710 kg/ha with a yield increase of 12 per cent and 10, respectively overruling sorghum variety CO 30. TNS 648 also performed better over K12 by recording an 11.32 per cent and 14 per cent increase for grain and fodder yield, respectively. Under AICRP on Sorghum national level testing, TNS 648 recorded a mean yield of 3100 Kg/ha and fodder yield of 11453 Kg/ha and excelled CSV 17 by recording 6.11 and 29.99 per cent increase of grain and fodder yield, respectively. It matures in 105 - 110 days, Grains are highly acceptable, yellowish white in colour, borne on medium semi-compact ear heads. The sorghum culture TNS 648 is moderately resistant to important shoot pests viz., shoot fly and stem borer. It is moderately resistant to downy mildew and grain mold. It has high protein (11.31-14.66 %) and fiber content (5.8 %) along with better cooking quality traits. Hence, the promising culture TNS 648 has been proposed for new release during 2020 and released as sorghum CO 32 for general cultivation in Tamil Nadu for both under rainfed and irrigated condition.

Lettuce downy mildew, caused by Bremia lactucae Regel, is the most economically important foliar disease of lettuce (Lactuca sativa L.). The deployment of resistant cultivars carrying dominant resistance genes (Dm genes) plays a crucial role in integrated downy mildew disease management; however, high variability in pathogen populations leads to the defeat of plant resistance conferred by Dm genes. Some lettuce cultivars exhibit field resistance that is only manifested in adult plants. Two populations of recombinant inbred lines (RILs), originating from crosses between the field resistant cultivars Grand Rapids and Iceberg and susceptible cultivars Salinas and PI491224, were evaluated for downy mildew resistance under field conditions. In all, 160 RILs from the Iceberg × PI491224 and 88 RILs from the Grand Rapids × Salinas populations were genotyped using genotyping by sequencing, which generated 906 and 746 high-quality markers, respectively, that were used for quantitative trait locus (QTL) analysis. We found a QTL in chromosome 4 that is present in both Grand Rapids × Salinas and Iceberg × PI491224 populations that has a major effect on field resistance. We also found two additional significant QTLs in chromosomes 2 and 5 in the Iceberg × PI491224 RIL population. Marker-assisted gene pyramiding of multiple Dm genes in combination with QTLs for field resistance provide the opportunity to develop cultivars with more durable resistance to B. lactucae.

An outbreak of downy mildew disease of onion, caused by Peronospora destructor, in Japan in 2016 necessitated a reevaluation of the primary inoculum sources to optimize disease management. Detection of the P. destructor pathogen in plants with asymptomatic infection and in soil would guide the application of fungicides according to the extent of infection before disease development. Here, we detected P. destructor in both plants and soil using newly developed primer sets (Pd ITS and Pd ITS 614) by both conventional and real-time PCR. Validation by real-time PCR with Pd ITS 614 showed that P. destructor DNA was amplified from symptomless seedlings at 3.7 × 102 to 1.0 × 100 conidium cells/50 mg leaf tissue, suggesting the detection of asymptomatic infection. Real-time PCR with Pd ITS amplified pathogen DNA from field soils at 1.6 × 103 to 8.3 × 101 oospore cells/g of soil. This real-time PCR assay provides a useful tool for identifying and quantifying inoculum sources, which may be the foundation of the design of integrated disease management strategies.

In the growing season 2016–2017, downy mildew disease was initially occurred on pea pods followed by plantlets leaves that observed in different locations where pea plants were grown at Kafr-El-Dawar district, Beheria governorate, Egypt. This irregular observation leads to thought that the cause of disease is new to this particular region. Molecular identification of the isolated fungus proved that the causal agent is Peronospora viciae. The sequences of the fungal isolate sent to GenBank and registered under accession number MN620384. Under field trials, it was observed that throughout the two cultivated seasons the fungicides application had greater effect on downy mildew incidence comparing with applied chemical inducers. Along the two cultivation seasons, pea plants sprayed with salicylic acid, potassium sorbate and ascorbic acid highly suppressed the disease incidence between 12.4 and 17.1%. Moderate effect was recorded as 19.2–22.7% disease reduction with potassium bicarbonate, lemongrass oil, lemongrass oil and humic acid application. The lowest effect was recorded as 32.5% at sodium benzoate treatment; meanwhile, 54.3% disease incidence was recorded at untreated control. Under field conditions, significant reduction in pea downy mildew disease was achieved by the use of fungicide alternative as foliar application. Therefore, these approaches could be utilized successfully for controlling this foliar disease especially they distinguished with safely use and avoidance of environmental pollution.

Pseudoperonospora cubensis is an obligate oomycete and cause of cucurbit downy mildew (CDM), the most destructive foliar disease affecting cucurbit hosts. Annual epidemics develop throughout the United States as windborne sporangia travel great distances and survive prolonged exposure to solar radiation. Recent genomic evidence suggests that P. cubensis isolates display host adaptation based on their respective clade. Early detection is key for fungicide application timing, and identification of the host-adapted clade provides information on the risk of infection for specific cucurbit crops. In this study, a multiplex quantitative PCR assay was developed based on species- and clade-specific nuclear genomic markers. The assay detected as few as 10 sporangia or DNA at 100 fg/ml for both clades and was validated in the field by deploying rotorod spore samplers in cucurbit sentinel plots located at two research stations in North Carolina. Using this assay, sporangia DNA was detected in spore trap sampling rods before signs of P. cubensis or CDM symptoms were observed in the sentinel plots. Both clade 1 and clade 2 DNA were detected in late-season cucumber and watermelon plots but only clade 2 DNA was detected in the early-season cucumber plots. These results will significantly improve disease management of CDM by monitoring inoculum levels to determine the cucurbit crops at risk of infection throughout each growing season.

The downy mildew pathogen Pseudoperonospora humuli is a major disease of cultivated and wild hop (Humulus lupulus) in many hop-growing areas of the world. Despite its frequent occurence and economic impact, so far very little is known with respect to its population structure, which would be a prerequisite for understanding population dynamics and for successful resistance breeding efforts. We collected a set of 70 P. humuli samples in five European countries (Austria, the Czech Republic, France, Germany, Italy) around the Alps, a high mountain range and a known major hurdle for dispersal, and examined them with microsatellite markers. For this purpose we developed a set of new markers and used them along with markers published for P. humuli by other authors. While the 11 microsatellites that were reported as polymorphic for North America did not show variation for the European samples, the five markers newly established in this study were variable and could be used to investigate the population structure of hop downy mildew in Europe. The overall genetic variation found in European populations was low, with only few heterozygous loci detected. Overall, 20 multilocus genotypes were observed, which clustered into 2 or 4 groups, depending on clustering criteria. We observed a random distribution of genotypes on the surveyed area in line with predominant selfing and long range dispersal. Thus, our investigations suggest that Pseudoperonospora humuli forms a large, potentially panmictic population with little local adaptation and frequent long-distance exchanges. This suggests that a wide array of Pseudoperonospora humuli strains should be used in any resistance breeding programs. In addition, the low degree of variation in Europe in contrast to North America suggests that, in line with its report in Europe as late as the 1930‘s, hop downy mildew might be an introduced species in Europe. Whether the introduction occurred from North America or from East Asia needs to be clarified in future studies.

In the United States, fungicides are the primary management option for cucumber growers to protect their crops from Pseudoperonospora cubensis, the causal agent of cucurbit downy mildew. Pathogen resistance to some fungicides can quickly develop with the repeated applications needed to protect yield. In order to determine fungicide efficacy and monitor it over time, bioassays were conducted from 2016 to 2019 in Delaware, Maryland, Pennsylvania, and New York. Potted cucumber plants were either sprayed with fungicides or not treated, placed next to field-grown plants with cucurbit downy mildew for up to 2 days, and then kept in a greenhouse until symptoms developed. Severity of symptoms or number of lesions on leaves was recorded 6 to 14 days after exposure started and used to determine fungicide efficacy. Quadris (azoxystrobin) was ineffective in seven of the nine bioassays, and Revus (mandipropamid) was ineffective in six of seven bioassays. Forum (dimethomorph) and Presidio (fluopicolide) were ineffective in three of eight and four of nine bioassays, respectively. The most effective fungicides were Bravo (chlorothalonil), Zing! (zoxamide + chlorothalonil), and Orondis (oxathiapiprolin), all of which consistently suppressed disease severity more than 90% when compared with the untreated control. Previcur Flex (propamocarb hydrochloride) and Ranman (cyazofamid) were also effective in every bioassay.

Considerable progress has been made in managing Peronospora belbahrii, an oomycete pathogen that causes basil downy mildew, since 2007, when it was first detected in the United States. Conventional fungicides have been registered and shown effective against P. belbahrii in replicated experiments in recent years. Unfortunately, because of their specific modes of action and P. belbahrii biology, some are at risk for resistance development, which has been documented outside the United States. Sweet basil varieties have been developed and commercialized, with most exhibiting good to high levels of resistance to basil downy mildew. Knowledge about conditions favorable for infection and disease development has resulted in the identification of cultural practices for managing basil downy mildew in the greenhouse. Practices being implemented include fans to move leaves, thus preventing water deposition and decreasing relative humidity, lighting at night to mitigate sporulation, and temperature modification to suppress disease development. Although downy mildew can be more effectively managed today, growers still experience losses, particularly when conditions are highly favorable for disease development. None of the organic fungicides or programs tested have provided adequate control for susceptible varieties, and limited testing has been done on resistant varieties to date. This review aims to summarize effective basil downy mildew management tools, in particular downy mildew-resistant varieties, environment modifications, and fungicide applications.

Downy mildew (DM), caused by P. cubensis, is harmful to cucurbits including luffa, with increased shortcomings associated with its control through cultural practices, chemical fungicides, and resistant cultivars; there is a prompt need for an effective, eco-friendly, economical, and safe biocontrol approach. Current research is therefore dealt with the biocontrol of luffa DM1 through the endophytic fungi (EF) consortium. Results revealed that T. harzianum (ThM9) and T. virens (TvA1) showed pathogen-dependent inducible metabolic production of squalene and gliotoxins by higher gene expression induction of SQS1/ERG9 (squalene synthase) and GliP (non-ribosomal peptide synthetase). Gene expression of lytic enzymes of EF was also induced with subsequently higher enzyme activities upon confrontation with P. cubensis. EF-inoculated luffa seeds showed efficient germination with enhanced growth potential and vigor of seedlings. EF-inoculated plants showed an increased level of growth-promoting hormone GA with higher gene expression of GA2OX8. EF-pre-inoculated seedlings were resistant to DM and showed an increased GSH content and antioxidant enzyme activities (SOD, CAT, POD). The level of MDA, H2O2, REL, and disease severity was reduced by EF. ACC, JA, ABA, and SA were overproduced along with higher gene expression of LOX, ERF, NCED2, and PAL. Expression of defense-marker genes (PPO, CAT2, SOD, APX, PER5, LOX, NBS-LRR, PSY, CAS, Ubi, MLP43) was also modulated in EF-inoculated infected plants. Current research supported the use of EF inoculation to effectively escalate the systemic immunity against DM corresponding to the significant promotion of induced systemic resistance (ISR) and systemic acquired resistance (SAR) responses through initiating the defense mechanism by SA, ABA, ET, and JA biosynthesis and signaling pathways in luffa.

Radish downy mildew (DM) caused by the oomycete Hyaloperonospora brassicae f. sp. raphani is a serious problem in radish crop, an edible root vegetable of the Brassicaceae family. The objective of this research was to assess radish germplasm for DM resistance and to evaluate the response of different radish organs to the disease under controlled conditions. Forty-four radish accessions were inoculated at cotyledons and true-leaves with H. brassicae isolate R10, collected in cotyledons of field plants. The roots were tested with isolates R10 and R6, this last one collected in roots of field radish. DM symptoms varied with the radish genotype and plant organ analysed. Twenty-seven resistant and partially resistant accessions were identified in all plant stages and are promising sources of resistance to DM, namely 16 commercial varieties, 10 breeding lines, and one landrace. A significant correlation was observed between cotyledon and leaf (1st and 2nd leaves) DM resistance, but low and no correlation was found between the resistance of true-leaves or cotyledons and roots, respectively. Cotyledon and leaf evaluation cannot be used to predict root resistance response in radish. However, cotyledon resistance has its own value because non-infected cotyledons will act as a barrier to slow disease progression to true-leaves and roots. Interesting sources of DM resistance were identified that can be used in radish breeding programs.

Downy mildew of cucumber caused by Pseudoperonospora cubensisis is a serious threat to the cucurbits production in high tunnels. In this study, the disease incidence and severity were recorded from nine high tunnel farms, located at Vehari District, Pakistan. The results revealed that downy mildew incidence was ranged from 24 to 80%. The disease development was correlated with plant genotype, sowing time, commonly used fungicides, and plant growth stages. "Kaan" was the most resistant while "Yayla" was the highly susceptible plant genotype against the disease. Cucumber sowing at mid-November showed significantly less disease over early and late sowing. Flowering was the most vulnerable stage of plants for disease development. ‘Champion’ (Copper hydroxide) followed by ‘Score’ (Difenoconazole) fungicides reduced the disease to 55% and 43%, respectively, which increased the yield 8-15 %. We conclude that the use of resistant genotype “Kaan”, protectant fungicide ‘Champion’ and appropriate sowing time Mid-November may reduce the disease incidence appreciably in high tunnels.

Onion downy mildew (ODM), caused by Peronospora destructor, is a serious threat for onion growers worldwide. In southwestern Québec, Canada, a steady increase in occurrence of ODM has been observed since the mid-2000s. On onion, P. destructor can develop local and systemic infections producing numerous sporangia which act as initial inoculum locally and also for neighboring areas. It also produces oospores capable of surviving in soils and tissues for a prolonged period of time. A recent study showed that ODM epidemics are strongly associated with weather conditions related to production and survival of overwintering inoculum, stressing the need to understand the role of primary (initial) and secondary inoculum. However, P. destructor is an obligate biotrophic pathogen, which complicates the study of inoculum sources. This study aimed at developing a molecular assay specific to P. destructor, allowing its quantification in environmental samples. In this study, a reliable and sensitive hydrolysis probe-based assay multiplexed with an internal control was developed on the internal transcribed spacer (ITS) region to quantify soil- and airborne inoculum of P. destructor. The assay specificity was tested against 17 isolates of P. destructor obtained from different locations worldwide, other members of the order Peronosporales, and various onion pathogens. Validation with artificially inoculated soil and air samples suggested a sensitivity of less than 10 sporangia g−1 of dry soil and 1 sporangium m−3 of air. Validation with environmental air samples shows a linear relationship between microscopic and real-time quantitative PCR counts. In naturally infested soils, inoculum ranged from 0 to 162 sporangia equivalent g−1 of dry soil, which supported the hypothesis of overwintering under northern climates. This assay will be useful for primary and secondary inoculum monitoring to help characterize ODM epidemiology and could be used for daily tactical and short-term strategic decision-making.

This study was performed to evaluate vine performance, physiological disorders of the berries, and disease of two grape (Vitis labrusca) cultivars, ‘Campbell Early’ and ‘Jinok’, in response to climate changes in private vineyards of two different latitudes, the Naju region (35°N) and the Okcheon (36°N) region, between the years 2016 and 2018. The climates from April to August or during the period of berry ripening (July and August in 2016–2018 in the Naju region was warmer than that of the 10-years average, causing approximately a 10-day longer duration of leafing to harvesting than during 2011 and 2012 for ‘Campbell Early’ vines. Total shoot length was the highest for both grape cultivars grown in Naju vineyard, when temperatures ranged between 20 and 25 °C from April to June, resulting in increased yield of ‘Campbell Early’ vines. Severe cracking of up to 59.1% of ‘Campbell Early’ berries occurred in the vineyard in Okcheon in 2017 when a high amount of precipitation occurred during the period of ripening. Also, an incidence of berry blight occurred on up to 30.6% of berries in ‘Campbell Early’ vines in the vineyard in Naju in 2018, which was subjected to a mean daily maximum temperature of 33.6 °C. Mean daily maximum temperature and number of heatwave days during the ripening period were positively related to the occurrence of berry cracking over the three years of this study, with a significant correlation observed between precipitation and mean temperature and berry blight. Downy mildew and powdery mildew occurred on both grape cultivars in the vineyard in Naju over the three years, with severe leaf spot observed on both cultivars in the region.