Onion Downy Mildew Research Articles

Weather-based decision support reduces the fungicide spraying to control onion downy mildew

Onion downy mildew is a major production constraint. Weather-based decision support has been an important tool for the rational use of fungicides. The present study established a chemical management strategy of the disease based on temperature and relative humidity. We defined daily severity values (DSV) according to temperature range (08–13.9 °C; 14–21.9 °C; 22–29 °C) and relative humidity ≥90% to carry out the spraying. The treatments mancozeb/copper oxychloride or metalaxyl-M/chlorothalonil were applied when the summations of the DSV reached the 6–11 range, or 12–17 range, or ≥18, besides weekly sprays using these fungicides or water. The area under disease progress curve (AUDPC) and yield of commercial bulbs were calculated. In both years evaluated (2014, 2015), we managed to reduce the number of metalaxyl-M/chlorothalonil sprays without compromising commercial yield. In 2014, the yield values for all treatments using metalaxyl-M/chlorothalonil were significantly higher than those which used mancozeb/copper oxychloride and control. In fact, with one spray less the routine weekly application, yield was 48.203 tons per hectare, while following the routine weekly application it was 42.411 tons per hectare. In 2015, we have reduced the number of metalaxyl-M/chlorothalonil sprays by 20%, while maintaining the same levels of AUDPC and yield. For both years, treatments with mancozeb/copper oxychloride did not differ from the control (water weekly). The rational use of fungicides is a key element for establishing an integrated production system of onion.

육묘상 양파 노균병에 대한 몇 가지 살균제의 방제 효과

During the nursery seedlings stage at Muan in 2013, moisture and temperature conditions were favorable for the occurrence of onion downy mildew. Several fungicides were evaluated for controlling onion downy mildew at the stage of seedling. The disease incidence of the untreated control was 51.1%, which was sufficient to evaluate the fungicide efficacy for the control of onion downy mildew. Incidence at the mancozeb treatment (MAN treatment) was 4.4% and it was the highest control value of 91.4%. Incidence at the various fungicides treatment (MIX treatment) consisted of amectotradin plus dimethomorph, floupicolide plus propamocarb HCl, or dimethomorph plus pyraclostrobin was 10.0%, and the control value of the MIX treatment was 80.4%. Among the tested fungicides, amectotradin plus dimethomorph (AME plus DIM treatment) was selected and treated. Incidence of AME plus DIM treatment was 12.2%. There was statistically significant difference among the three different treatments and the untreated control. These results suggest that the application of fungicides on onion nursery seedlings was effective to control the onion downy mildew.

Modelo climático para previsão do míldio da cebola

RESUMO A partir de dados publicados na literatura, elaborou-se um modelo de previsão do míldio da cebola, causado por Peronospora destructor. O modelo desenvolvido baseia-se nas condições climáticas favoráveis para o processo infeccioso, baseado na temperatura e na duração do molhamento foliar. Obteve-se a interação que representa os dois fatores sobre a probabilidade de infecção. Com auxílio da planilha eletrônica Excell, calculou-se a duração do período de molhamento foliar para obter diferentes probabilidades de infecção do patógenos em diferentes temperaturas. Com os dados foi elaborada uma tabela que poderá ser usada na programação computacional que registra a duração do período de molhamento foliar e a temperatura média e automaticamente o valor diário de probabilidade de ocorrência de infecção. O modelo será validado em experimentos conduzidos no campo e com infecção natural, em Rio do Sul, SC.

An Epidemic of Downy Mildew caused by Peronospora destructor on Vidalia Sweet Onions in Georgia in 2012

The first reported occurrence of onion downy mildew (ODM) on Vidalia sweet onions in Georgia was in 1999. Although the disease is not common every year, two epidemics have occurred since 1999. The authors report here on the impact of a recent outbreak of ODM on the $100-million Vidalia onion crop in 2012. Accepted for publication 29 January 2013. Published 28 March 2013.

Quantitative studies on downy mildew (Peronospora destructor Berk. Casp.) affecting onion seed production in southern Uruguay

Onion downy mildew (Dm) symptoms and damage on seed production fields in southern Uruguay were quantified during two seasons as the progress of incidence, severity, and as the effect of the level of seed-stalks infections on seed yield and quality. In addition, the effects of two plantation dates and two plant densities on Dm were studied in a factorial experiment. Maximum incidence along the season ranged from 15 to 65% in four commercial fields in 2005 and two fields in 2006. Maximum severity ranged from 4.5 to 9.3% of leaf area affected in 2005, and 0.35 to 1.17% in 2006. Whereas Dm incidence varied among studied fields, disease progress as Dm severity varied mainly between studied years. Crop rotation, crop vigour and plant density were identified as major factors affecting Dm variation in the field. Intensive fungicide schedules did not control Dm when other conditions favoured the disease. A high level of Dm severity defined as large necklace spots on seed-stalks significantly reduced seed yield in comparison with healthy seed-stalks in 2005, as well as seed yield and germination in 2006. Weight of 1,000 seeds was not significantly reduced by Dm infections on seed-stalks. Six genera of fungi were recovered from seeds harvested on highly infected seed-stalks (Stemphylium, Fusarium, Alternaria, Penicillium, Aspergillus and Botrytis), but not Peronospora. Late planting date and low plant density had significantly lower Dm incidence and severity. This finding questioned the early planting dates and high densities previously recommended in order to achieve high yielding crops. The combination of several practices to reduce initial inoculum, susceptibility status of the host, and environmental conditions promoting the disease is discussed as forming a basis for effective disease control.

Development of detection systems for the sporangia of Peronospora destructor

A monoclonal antibody that recognises components of the wall of sporangia of Peronospora destructor was raised. Tests using spores of higher fungi and other species of mildew demonstrated the specificity of the monoclonal. The antibody was used to develop lateral flow devices for sporangia of P. destructor. A competitive lateral flow format was developed which could detect onion downy mildew sporangia. Five-microliter gold anti-mouse IgM solution pre-mixed with 10 μl of P. destructor monoclonal antibody (EMA 242) proved the optimal concentration for detection of sporangia of P. destructor when applied to sample pads of lateral flow devices. Limits of approximately 500 sporangia of P. destructor could be detected by the absence of a test line on the lateral flow device within test samples. Using a scanning densitometer improved the sensitivity of detection. Further development and validation of the test is required if it is to be used for risk assessments of onion downy mildew in the field.

PROTECTION COMPLEXES’ INFLUENCE UPON ONION YIELD UNDER WESTERN FIELD CONDITIONS

The information concerning yield losses represent the best synthesis of the effects exerted by pathogen agents and pests upon onion crop. These information originate in the observations performed during 2003-2005 in the untreated control variant, compared to the variants in which we have applied protection complexes and also in the comparisons between the variants with the experience mean ( M ) and with the protection complexes mean (M cp). Yield differences compared to the control variant (Mt) are between 8.93-17.83 t/ha. The yield achieved in all seven variants is statistically assured, and the significances of yield differences are very significantly positive. Analyzing the significance if yield differences, by reporting it to the experience mean (M ), we may notice an evident differentiation per variants of the significance degree, namely a very significantly positive significance is available in only three variants (V5, V6 and V3). In V1 the significance is just positive, while in the variants V2 and V4 there is no significance, and in variants V7 and V8 it is very significantly negative. When the reference of yield differences was made only to the yield average achieved under the influence exerted by the protection complexes, excluding the yield achieved in V8 – untreated control variant, the situation changed dramatically, namely the yield has statistical cover only in two variants, difference significance being very significantly positive in V5 (Ridomil Gold MZ 68 0,25 % + Victenon 50 WP 0,075%) and distinctly significantly positive in V6 (Ridomil Gold Plus 42,5 WP 0,3% + Mospilan 20 SP 0,025%). There is no significance in the variants V3, V2 and V1, and in the variants V7 and V4 yield difference significance is very significantly negative, respectively distinctly significantly negative. As a general conclusion, we may say that the protection complexes from V5 (Ridomil Gold MZ 68 0,25 % + Victenon 50 WP 0,075%) and V6 (Ridomil Gold Plus 42,5 WP 0,3% + Mospilan 20 SP 0,025%) had the best effects in fighting against the two diseases (onion downy mildew - Peronospora destructor (Berk.) Casp and bulb rot - Botrytis allii Munn.) and the pest (onion fly – Delia antiqua Meig.), fact also proved by the level of yields achieved, by their statistical cover and the highest assessment degree of yield difference significance. The weakest effect in fighting against the two diseases and the pest belonged to the protection complex from V7 (Dithane M45 0,2% + Fastac 10EC 0,02%), because the significance of yield differences compared to M and M cp is very significantly negative.

Development of ZWIPERO, A Model Forecasting Sporulation and Infection Periods of Onion Downy Mildew based on Meteorological Data

The weather-based forecasting model ZWIPERO was developed by the German Weather Service and determines the risk of sporulation and infection of Peronospora destructor quantitatively based on actual as well as predicted weather data (temperature, relative humidity, leaf wetness, precipitation). The model allows precise planning of disease monitoring and infection-related application of fungicides. ZWIPERO is a more complex mathematical model than the previously published models for downy mildew. In order to operate ZWIPERO independently of the actual field location and season, the time of sunrise and sunset of the location are exactly determined by a subroutine. Another subroutine provides simulated microclimatic input variables based on local production data as well as actual and hourly predicted (up to 4 days) standard weather data. Starting at the time of 'sunrise + 7 h', ZWIPERO calculates the number of sporangia produced, the time of onset of sporangia release, as well as the number of infections possible and the number of sporangia which may survive the day for each 24-h time step. Field evaluations of sporulation periods of downy mildew showed that the simulated micrometeorological input variables are reliable. As the actual plant development, the susceptibility and the disease incidence in the field are not taken into account, ZWIPERO has to be considered primarily as a decision support system for extension services and growers.

Development of an Integrated Disease Management Model for Control of Onion Downy Mildew

Development of an Integrated Disease Management Model for Control of Onion Downy Mildew

First Report of Downy Mildew (Caused by Peronospora destructor) of Onion in Georgia.

In February 1999, localized areas in a commercial field of Vidalia sweet onions in Toombs County, GA, exhibited symptoms that included elongated pale to light tan lesions on older leaves as well as some totally collapsed leaves. A dark, sooty to purple-gray fungal growth also was observed on affected leaves. Both symptoms and signs were observed during a period of wet, cool weather. Microscopic observation of affected tissues revealed nonseptate mycelia and dichotomously branched sporangiophores, which tapered to short sterigmata, giving rise to pyriform to fusiform sporangia. Based on these observations, the disease was determined to be downy mildew of onion caused by Peronospora destructor (Berk.). The dimensions of 25 sporangia averaged 25.1 ± 3.8 × 64.2 ± 4.3 μm, falling within the range of those currently reported for P. destructor (1). The disease was isolated to the field in which it was initially reported and did not cause extensive damage. Its failure to progress and spread may have been due to the warm, dry conditions that prevailed subsequent to symptom detection. This is the first report of P. destructor in Georgia. Reference: (1) H. F. Schwartz and S. K. Mohan. 1995. Diseases of aerial parts caused by fungi. Pages 20-24 in: Compendium of Onion and Garlic Diseases. The American Phytopathological Society, St. Paul, MN.

Evaluation of onion downy mildew disease risk in New Zealand using meteorological forecasting criteria

Meteorological prediction criteria for onion downy mildew (Peronospora destructor), adapted from the disease forecasting system “Downcast”, were used to examine patterns of disease risk in two climatically contrasted New Zealand onion growing regions. Potential sporulation and infection events were predicted to occur in both regions in all months between September and April, although South Auckland had more events (27-43% of days per month) than Canterbury (10-18% of days per month). Predicted disease risk was most often limited in Canterbury when relative humidity failed to reach the level required for sporulation. In South Auckland, risk was most often limited by insufficient leaf wetness duration for infection, but in one low rainfall year, relative humidity was the most often limiting factor, as in Canterbury. It appears that in Canterbury, and in some seasons in South Auckland, forecasting criteria could be used to modify fungicide timing

Management of downy mildew ( Peronospora destructor) of onion ( Allium cepa)

Studies were conducted during winter seasons (rabi) of 1990–1991, 1991–1992, 1992–1993, 1993–1994 to evaluate the effect of heat treatment of bulbs and fungicide sprays on the management of downy mildew ( Peronospora destructor [Berk.] Casp.) of onion ( Allium cepa L.). Heat treatment of bulbs at 35, 40 and 45 °C for 4, 8, 12, 16 and 24 h reduced the disease significantly. Heat treatment of bulbs at 40 and 45 °C for more than 12 h although resulting in a significant suppressive effect on disease severity, also retarded plant growth drastically. Amongst the fungicides evaluated through bulb dip, metalaxyl (Apron 35 SD and Ridomil MZ 72 WP) effectively reduced the initial inoculum and delayed the appearance of disease. In vitro evaluation of thirteen fungicides through inhibition of sporangial germination revealed that Peronospora destructor showed more sensitivity to metalaxyl (Ridomil MZ) and least to fosetyl-A1 with ED 50 values of 2.8 μg ml −1 and 58.6 μg ml −1, respectively. Amongst the co-ordinated products of acylalanines tested, Peronospora destructor was more sensitive to metalaxyl + mancozeb followed by oxadixyl + copper oxychloride and least to benalaxyl + mancozeb with ED 50 values of 2.6, 4.5 and 8.3 μg ml −1, respectively. Similarly, out of nine fungicides evaluated in the field as foliar sprays, three 14-day sprays of metalaxyl + mancozeb (0.3%) effectively controlled the disease. Integration of heat treatment (35 °C for 12 h), bulb dip in metalaxyl (3.5 g kg −1 bulbs for 1 h) and three 14-day sprays of metalaxyl + mancozeb (0.2%) were most effective and resulted in more than 85% control of onion downy mildew.

ONIMIL, a forecaster for primary infection of downy mildew of onion 1

Onion downy mildew, caused by Peronospora destructor, can be controlled effectively only when sprays are begun in the infection period following first sporulation. Systems analysis was applied to the onion/P. destructor pathosystem in order to understand better the relationships between the factors influencing the establishment of primary infection. A preliminary model able to forecast primary infection, ONIMIL (ONIon downy MILdew), was developed. This model was able to determine, for each day, the probability of P. destructor establishing an infection on onion and its infectivity level, compared with the maximum. The model uses empirical rules and mathematical functions based on meteorological factors.

Control of onion downy mildew in the presence of phenylamide-resistant strains of Peronospora destructor (Berk.) Caspary

Two isolates of Peronospora destructor differed in their sensitivities to phenylamide fungicides. An isolate (3014) from the Lockyer Valley did not respond to increasing concentrations of metalaxyl, benalaxyl and oxadixyl between 0.1 and 200 mg a.i./L. An isolate (2967) from Bundaberg was sensitive to these fungicides, with EC50 values (concentrations which reduce disease severity by 50%) of 1.6, 6.0 and 12.1 mg a.i./L, respectively. Isolates 3014 and 2967 were both sensitive to dimethomorph, with EC50 values of 4.3 and 1.4 mg a.i./L, respectively. Field trials compared the efficacy of fungicides for the control of onion downy mildew in the Lockyer Valley in 1989 and 1990. In both trials, treatments containing mancozeb gave the best control, resulting in yield increases of 60% in 1990. There was no difference from the check treatment in either disease severity or yield when metalaxyl was applied, suggesting that phenylamide-resistant strains were prevalent. Other fungicide treatments which did not significantly reduce the severity of downy mildew in the 1990 trial included copper hydroxide, chlorothalonil, fosetyl-Al, and propamocarb. The experimental compound dimethomorph, although less effective than mancozeb, significantly reduced disease severity and increased yield by 24%.

タマネギべと病菌分生胞子の形態異常の発生

タマネギべと病菌分生胞子の形態異常の発生

Evaluation of a forecaster for downy mildew of onion ( Allium cepa L.)

A forecaster of onion downy mildew, referred to as DOWNCAST, was evaluated under field conditions to determine its validity and effectiveness for timing fungicide sprays. DOWNCAST utilized criteria for predicting sporulation and infection by the downy mildew pathogen, Peronospora destructor. The criteria were based on quantitative and temporal relationships of temperature, rain, high humidity, rate of dew deposition and dew duration with the infection cycle of the pathogen. DOWNCAST correctly predicted sporulation incidence on 111 of 119 nights during two growing seasons. Refinement of a temperature criterion was needed to predict sporulation correctly on the remaining eight nights. Indirect evidence showed that DOWNCAST correctly predicted infection incidence. Sequences of weather conditions were used to predict ‘sporulation-infection periods’, when the pathogen sporulated, survived and infected the onions, and to identify infection cycles. Fungicide programmes of Ridomil-MZ, Sandofan M8, Bravosan, Vinicur M-SC, Manzate 200 or Bravo 500, begun on the sixth or eighth day of the first infection cycle, did not usually reduce sporulation incidence early in the second infection cycle. However, all of the programmes except Bravosan and Bravo effectively restored disease management a few days later. Programmes of Ridomil-MZ begun in the second infection cycle usually were less effective than those begun in the first cycle. Relative effectiveness of the various fungicide programmes was reflected in increased yield and size of the onions. It was concluded that fungicide programmes should start at about the time of the first sporulation-infection period.

DOWNY MILDEW OF ONION AND ITS CONTROL IN THE BRITISH COLUMBIA INTERIOR

Downy mildew of onion appeared initially in the semi-arid British Columbia Interior about 50 years after onion crops had been first grown commercially. The year of its appearance, 1942, was abnormally wet, and this condition suggested that the weather had previously been unfavourable for its establishment. However, the disease has since persisted and has caused damage in most seasons. The pathogen was probably present on infected living plants introduced shortly before the appearance of the disease.Systemic overwintering of the pathogen in diseased bulbs occurs only occasionally. Heat treatment of seed mother bulbs was found to injure them sufficiently to seriously reduce the number and height of seed stalks.Zineb provided best control in fall-planted, spring-planted, and seed crop onions. Maneb and ferbam were less effective. Captan and NP–1282 were ineffective. Control was found to be effective only if a spray was applied at first indication of mildew in a district.

Preliminary experiments on the use of oil‐soluble copper compounds as fungicides

Tests were made of the solubility of a number of copper compounds in mineral and vegetable oils. Phytocidal tests on onion foliage showed that turpentine, pine oil and white spirit were damaging whereas cottonseed oil and white oil were innocuous. Copper salts in white oil, white spirit, turpentine or pine oil showed a higher fungicidal value than in cottonseed oil. In laboratory tests, a solution of copper 3:5‐di‐isopropyl salicylate was more fungicidal than Bordeaux mixture of the same copper concentration. In a small field trial, copper 3:5‐di‐iospropyl salicylate dissolved in white oil to give a concentration of 0.01% Cu was atomized three times at monthly intervals on to onion foliage without damage. The same material at 0.1% Cu in white oil was phytocidal. No control of onion downy mildew (Peronospora schleideniana) was obtained in this trial. The method of atomization was convenient and practicable for small‐scale operation. It gave perfect wetting of the foliage and was very economical of material.