Onion Downy Mildew Research Articles

Is DOWNCAST an effective model for predicting onion downy mildew in Ontario, Canada?

Is DOWNCAST an effective model for predicting onion downy mildew in Ontario, Canada?

Utility of the adjuvant and infection predicting model to reduce the frequency of chemical spray on onion downy mildew

Utility of the adjuvant and infection predicting model to reduce the frequency of chemical spray on onion downy mildew

Evaluation of Fungicides against Downy Mildew (Peronospora destructor Berk) of Onion (Allium cepa) in Bale Zone, South Eastern Ethiopia

Aims: To evaluate the efficacy of tested fungicides against downy mildew of onion and determine the appropriate interval of fungicides application.
 Study Design: The experiment was laid out down in randomized complete block design with three replications for each treatments.
 Place and Duration of Study: The experiment was carried out in farmers field of Aloshe village; Goba district of Bale zone in 2018/2019 during “Bona Season”.
 Methodology: Locally available onion variety and two tested fungicides (Mancozeb (Mancozeb 80 Wp) and Ridomil (Ridomil Gold MZ 68WG) were used. Locally available variety of onion was planted in a plot size of 3m x 2.4m with three replications. Each two fungicides were applied at interval of seven day, fourteen and twenty one day interval including unapplied fungicide plot as control.
 Results: The result indicated that, both fungicides (Mancozeb and Ridomil) were effectively reduces the severity level of the disease. The higher plant height(42.5cm), bulb weigh (289g), total weight of the bulbs (114Qt/ha) and lower severity (8.35%) were recorded from weekly application of Ridomil, fourteen day application of Ridomil and weekly application interval of Mancozeb. The higher marginal rate of return of 2833.45% and 1999.07% were recorded from weekly and fourteen applications of Mancozeb and Ridomil respectively.
 Conclusion: The minimum application frequency of three times of Ridomil in fourteen days and weekly interval application of Mancozeb were effective against downy mildew and profitable.

Onion downy mildew severity in no-tillage irrigation under nutrient budgeting and population densities

We analyzed the relationship between nutrient (N and K) parceling and population density on the severity of onion downy mildew under no-tillage fertigation cultivation in the conditions of Alto Vale do Itajaí (Barzil). For this purpose, field trials were conducted in the years 2017, 2018 and 2019, in Ituporanga (Barzil). The treatments corresponded to four population densities (300, 400, 500 and 600 mil plants ha-1) subjected to applications of nitrogen (150 kg N ha-1) and potassium (127.5 kg K2O ha-1) distributed throughout the vegetative cycle of the crop via fertigation on a weekly, biweekly and monthly basis, based on the absorption curve of these nutrients for the cultivar Empasc 352-Bola Precoce. In fertigated no-tillage systems, nutrient (N and K) tranches do not influence the severity of downy mildew. The severity of downy mildew increases linearly with increasing population density, especially from 500 mil plants per ha-1.

Primary infection of onion downy mildew on onion seedlings caused by belowground Peronospora destructor inoculum containing germinable oospores

AbstractThis study investigated the possible role of oospores in soil as a primary source of onion downy mildew caused by Peronospora destructor. Oospores were extracted from the debris of diseased leaves collected in spring and stored outdoors during the summer. The resulting oospore suspensions were used for germination assays and inoculation experiments in the laboratory. Germination of oospores buried in the soil in cups occurred at a rate of approximately 0.3%–1.3%. Oospore suspensions were dripped or injected into artificial culture soil in cups at sowing onion seeds or at the seedling stage. After raising these plants in a growth chamber for more than 40 days, approximately 20%–30% of the plants exhibited leaf chlorosis and distortion, reproducing the primary infection symptoms of onion downy mildew observed in commercial fields. The presence of P. destructor in symptomatic plants was confirmed by histological observations and species‐specific PCR. Thus, laboratory experiments have demonstrated for the first time that (i) P. destructor oospores that have oversummered outdoors germinate in soil and (ii) belowground inoculum containing germinable oospores causes primary infection. The results showed that P. destructor survives as oospores during host‐free periods and can be a primary inoculum source for downy mildew during the following growing season in autumn. This research on soil‐borne primary infection by P. destructor may help develop knowledge‐based disease management strategies for onion downy mildew, such as proper disposal of crop residues after harvest, effective crop rotation and chemical soil treatments.

Monitoring of Peronospora destructor oospores from field samples using real‐time PCR

AbstractOnion downy mildew caused by Peronospora destructor has been widespread in Japan since 2016. Soil disinfection and use of fungicides have been implemented as control measures against oospores in the soil, which are the primary source of infection. Measurements of oospore density are needed to clarify the risk of disease development and inform disease management. In this study, an experimental system capable of detecting P. destructor DNA from field samples was developed. A TaqMan probe‐type primer was used to target the coxII (cytochrome c oxidase subunit II) region of the mitochondrial genome of P. destructor and no false positives were observed. Using this method, the detection limit was equivalent to 5 oospores of P. destructor/g soil in grey lowland soil and andosol. The correlation coefficient between oospore density and the primary infection rate was investigated by testing soil samples from 115 fields. Although no correlations between oospore density and the primary infection were observed in pesticide‐treated fields, significant correlations were observed in untreated fields. Continuous cropping of onions increased the oospore density. The correlation between primary and secondary infection was relatively weak, and the negative predictive value of primary infection was relatively high (89.0). These results suggest that the disease‐risk potential of onion downy mildew is very high, and thus it is necessary to set a low pathogen detection threshold. The system presented here provides a highly sensitive method for supporting decision‐making aimed at disease control.

Characteristics of Meteorological Conditions during a Severe Outbreak of Onion Downy Mildew and Metalaxyl Sensitivity of Peronospora destructor in Saga, Japan, in 2016

In 2016, an onion downy mildew epidemic caused by Peronospora destructor severely damaged the commercial onion fields in Saga Prefecture, western Japan. To identify the factors underlying the outbreak, we investigated the symptoms of downy mildew caused by secondary infections and examined P. destructor’s sensitivity to metalaxyl, the most effective traditional fungicide used against this onion pathogen, in 2016–2018. Disease symptoms developed in late March 2016, which was earlier than symptom development in 2017 and 2018. Furthermore, there were synchronous repeated disease development and favourable meteorological conditions for infection in early and late April resulting in the development of polycyclic epidemics. In field trials from 2016 to 2018, the efficacy of chlorothalonil + metalaxyl-M application ranged 18–45%, as calculated by comparing disease severity at the final stage of each treatment to that in the untreated plots. On the basis of the metalaxyl sensitivity observed in 2016, the effective concentration, which reduced germ-tube elongation in P. destructor by 50%, exceeded 200 µg ai/mL for certain strains. Our observations indicate that these characteristic meteorological conditions were major factors contributing to the severe disease outbreak in 2016. The emergence of less-metalaxyl-sensitive P. destructor strains may be an additional predisposing factor.

Evaluation and Application of Normalized Difference Vegetation Index for Precision of Onion Downy Mildew Prediction

Evaluation and Application of Normalized Difference Vegetation Index for Precision of Onion Downy Mildew Prediction

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.

Method for occurrence of onion downy mildew caused by primary infection with <i>Peronospora destructor</i> by using a soil mixed with diseased foliage carrying oospores as an inoculum.

Method for occurrence of onion downy mildew caused by primary infection with <i>Peronospora destructor</i> by using a soil mixed with diseased foliage carrying oospores as an inoculum.

Severidade do míldio da cebola em plantio direto fertirrigado sob parcelamento de nutrientes e densidades populacionais

The study analyzed the relationship between nutrient splitting (N and K) and population density on the severity of onion downy mildew under no-tillage cultivation within the conditions of Alto Vale do Itajaí (Brazil). For this purpose, field trials were conducted in the years 2017, 2018 and 2019, in Ituporanga (Brazil). The treatments corresponded to four plant densities (300, 400, 500 and 600 thousand plants ha-1) submitted to applications of nitrogen (150 kg N ha-1) and potassium (127.5 kg K2O ha-1). The doses of nutrients were distributed along the vegetative cycle of the culture via fertigation on a weekly, biweekly and monthly, based on the absorption curve of nutrients for the cultivar Empasc 352 - Bola Precoce. In fertigated no-tillage systems, the distribution of nutrients (N and K) does not influence the severity of downy mildew. Mildew severity increases linearly with increasing population density especially from 500 thousand plants per ha-1.

Inheritance of resistance against Peronospora destructor in onion cv. ‘Regia’

Onion downy mildew (DM) caused by Peronospora destructor is a very destructive leaf disease. Genetic resistance is an economic and environmentally friendly way of disease control. In this research, the segregation of DM resistance was analysed in six offspring from crosses between national cultivars and the cv. ‘Regia’, a partial resistance source. F1 plants were self-fertilised, and F1S1 derived progenies were evaluated. The segregations in DM severity were skewed towards susceptibility, with transgressive segregation in five of six progenies. The recessive inheritance could be associated with loss of susceptibility mechanisms. In another experiment, the response of advanced F1S2 lines ‘Regia’ x ‘Pantanoso del Sauce’ were evaluated two times (August and November 2017) in DM disease severity, DM histological quantitative differences, and agronomic traits. F1S2 lines, which have had an earlier selection process, presented an intermediate DM severity between the parents. Disease severity was positively correlated with histological differences in the proportion of infected stomata. ‘Regia’ presented the lowest DM severity and the highest percentage of healthy stomata. Most resistant F1S2 lines did not differ from control cultivars in bulb yield, bulb quality traits and postharvest behaviour, and could be the basis to develop a DM resistant cultivar in comparison to currently grown cultivars

Monitoring of Peronospora destructor Primary and Secondary Inoculum by Real-Time qPCR.

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.

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.

Machine vision-based automatic disease symptom detection of onion downy mildew

The effective crop management is major issue in recent agriculture because the cultivation area per farmer is increasing consistently while the aging-related reductions in the labor force. To manage crop cultivation effectively, it needs automatic monitoring in farmland. This paper presents an image-based field monitoring system for automatically crop monitoring and consists of constructing field monitoring system for periodic capturing of onion field images, training the deep neural network model for detecting the disease symptom, and evaluating performance of the developed system. The field monitoring system was composed of a PTZ camera, a motor system, wireless transceiver, and image logging module. The deep learning model was trained based on weakly supervised learning method that can classify and localize objects only with image-level annotation. It is effective to recognize crop disease symptom which has ambiguous boundary. The model was trained using captured onion images using the filed monitoring system, and 6 classes including the disease symptom were classified. The detected disease symptom was localized from background through thresholding of the class activation map. The 60% of maximum value in class activation map was determined as an Optimal threshold for disease symptom localization. Identification performance of disease symptom was evaluated using mAP metric by IoU. The results show that the mAP at IoU criteria 0.5, which should have over 50% overlap, was the highest in all models from 74.1 to 87.2. The results showed that the developed field monitoring system could automatically detect onion disease symptoms in real-time.

Field efficacy of fungicides to control downy mildew of onion

Downy mildew is the main leaf disease of onion in southern Brazil. The management of the disease is carried out through frequent applications of fungicides. The objective of this study was to evaluate the fungicide efficiency in controlling downy mildew, under field conditions, during 2016–18 crop seasons. We evaluated the effect of weekly sprays of different active ingredients on the area under disease progress curve (AUDPC) and on commercial yield. There was variation among the best treatments in the three years of the test. However, we observed that sprays with metalaxyl-M (100 g/ha) + mancozeb (1600 g/ha) or mancozeb (1875 g/ha) were always allocated to the most productive groups: i) metalaxyl-M + mancozeb in the most productive group in 2016 (41.61 ton/ha), in the most productive group in 2017 (21.25 ton/ha), and in the second most productive group in 2018 (16.70 ton/ha); ii) mancozeb in the second most productive group in 2016 (32.62 ton/ha), in the most productive group in 2017 (19.88 ton/ha), and in the second most productive group in 2018 (16.00 ton/ha). The results demonstrated that metalaxyl-M and mancozeb are important molecules in the management of the disease. The use of fungicides composed only of molecules with systemic action is not recommended for the management of onion downy mildew under Brazilian conditions.

Aspectos epidemiológicos do míldio da cebola na região do alto vale do Itajaí em Santa Catarina

RESUMO Com objetivo de estudar a epidemiologia do míldio da cebola causada por Peronospora destructor na região do alto vale do Itajaí/SC, um ensaio foi conduzido com plantas naturalmente infectadas, nas safras 2014 e 2015. A cada sete dias e, durante 19 semanas, foi quantificada a população de esporângios no ar, registrada as condições ambientais e avaliada a severidade nas plantas. Observou-se, que mesmo com ambiente favorável à doença, seu início ocorreu após nove semanas após o transplante das plantas. Constatou-se que a epidemia iniciou uma semana antes da primeira detecção do inóculo no ar. O progresso da doença foi representado pelo modelo de Gompertz y = 0,42636*(exp(-442,06555*exp(-0,50749*x))). A produção foi de 34.930 e 15.773 Kg.ha-1 de bulbos nos respectivos anos de avaliação. Este estudo epidemiológico pode fornecer informações do momento do início da epidemia e ser útil na validação de um sistema de previsão do míldio da cebola.

Influence of Humate Substances and Fungicides on the Control of Onion Downy Mildew

Downy mildew of onion (Allium cepa), caused by Peronospora destructor, is one of the most important onion diseases that mainly controlled by fungicides. This investigation aimed to evaluate the effect of foliar application of two forms of humic substances (HS) individually or in combinations with two fungicides against downy mildew, growth, photosynthetic capacity and metabolism changes of onion. The trials were carried out during the growing seasons 2016 and 2017. Final disease severity (FDS%) andarea under disease progress curve (AUDPC) were used to assess the potential of twoforms of (HS), and two fungicides to control downy mildew of onion. All treatments resulted in minimizing the FDS (%) and AUDPC, compared to control treatment, during the two seasons. The most effective treatment was (HS)-leonardite combined with fungicide Ridomil Gold 68% (WP), which exhibited a significant decrease in FDS (%) and AUDPC in relation to other treatments and control. All treatments improved onion growth parameters i.e., bulb diameter, plant height, dry eight/plant, dry matter, average yield compared to untreated control plants. The chemical changes in treated plants i.e., enzymes activities, total carbohydrates, total nitrogen (%), total soluble solids (TSS %) were also assessed. Principal component analysis gave evidence to the importance of all parameters in this study, especially FDS (%), as a reliable indicator for evaluation of these materials against downy mildew disease.

Compatibility between Chemical Inducers and Amistar Top Fungicide for Controlling Onion Downy Mildew and Purple Blotch Diseases

Compatibility between Chemical Inducers and Amistar Top Fungicide for Controlling Onion Downy Mildew and Purple Blotch Diseases

Influência da temperatura e da duração do molhamento foliar na severidade do míldio da cebola

RESUMO No presente trabalho foram estudadas, em condições controladas para o desenvolvimento de um modelo climático baseado na influência da temperatura (10, 15, 20 e 25°C) e da duração do molhamento foliar (6, 12, 24 e 48 horas) na severidade do míldio da cebola incitado por Peronospora destructor. A densidade relativa de lesões foi influenciada pela temperatura e pela duração do molhamento foliar (P<0,05). A doença foi mais severa na temperatura de 15°C. Os dados foram submetidos à análise de regressão não linear. A função beta generalizada foi usada para ajuste dos dados de severidade e temperatura, enquanto uma função logística foi escolhida para representar o efeito do molhamento foliar na severidade do míldio da cebola. A superfície de resposta obtida pelo produto das duas funções foi expressa por SE = 0,1506 * (((x-8)0,0614) * ((30-x)0,1419)) * (0,71642/(1+0,56954 * exp (-0,04460*y))), onde SE, representa o valor da severidade estimada (0,1); x, a temperatura (ºC) e y, o molhamento foliar (horas). Este modelo climático deverá ser validado em condições de campo para aferir o seu emprego como um sistema de previsão computacional para o míldio da cebola