Downy Mildew Severity Research Articles

Diagrammatic scale to assess downy mildew severity in melon

The downy mildew, caused by Pseudoperonospora cubensis, is an important melon disease in Northeast Brazil. Considering the lack of standard methods for its assessment, a diagrammatic scale was developed with 2, 4, 8, 16, 32, 64, 82, and 96% of affected leaf area. The scale was then checked for its accuracy, precision, and reproducibility in estimating downy mildew severity. The diagrammatic scale was validated by eight disease raters; using 50 leaves with different severity levels, previously measured using the software Assess®. Two evaluations were performed on the same set of leaves, but in a different sequence order, by the same raters, within a 15-day interval. The accuracy and precision of each rater was determined by simple linear regression between the actual and the estimated severity. The scale provided good levels of accuracy (means of 87.5%) and excellent levels of precision (means of 94%), with absolute errors concentrated around 10%. Raters showed great repeatability (means of 94%) and reproducibility (>90% in 90.3% of cases) of estimates. Therefore, we could conclude that the diagrammatic scale presented here was suitable for evaluating downy mildew severity in melon.

Climatic risk of grape downy mildew (Plasmopara viticola) for the State of São Paulo, Brazil

Viticulture in Brazil has been growing in importance in recent years. In the State of São Paulo, a significant percentage of the production is basically destined to in natura consumption and, more recently, much effort has been made by institutions to revitalize the viticulture in the State. Among fungal diseases, the downy mildew (Plasmopara viticola) is one of the main diseases affecting this crop in Brazil, with extreme damage effects on its production. The objective of this study was to estimate the incidence of the downy mildew on grape under the climatic conditions of the State of São Paulo, based on a mathematical model and using Geographical Information System - GIS tools. The study considered the months from September to April, a period in which the downy mildew can affect grapevines under development. Mean temperature and relative humidity were the basic weather data entered in the GIS database. Leaf wetness duration was estimated from relative humidity measurements. Climatic data entered in the GIS were used to calculate and produce maps depicting the severity of the grape downy mildew, through the application of a disease model. Three cities were evaluated (Jales, Jundiaí, and São Miguel Arcanjo), since they represent the main vineyard centers in the State. The adopted methodology permitted quantifying the severity of the grape downy mildew not only in spatial terms, identifying the variability among the different regions of the State, but also in temporal terms, along the months, making an adequate distinction of the studied cities.

Solar radiation affects grapevine susceptibility to Plasmopara Viticola

Solar radiation plays an important role in the development of some fungal diseases due to its direct action on the microorganisms and also its indirect effect on the production of specific plant compounds. This experiment examined the effect of two light environments (100% and 35% of full strength) on the polyphenolic content of grapevine leaves and quantified their relation to resistance to downy mildew (Plasmopara viticola). Leaf epidermal polyphenolic contents were non-destructively measured during the growing season 2006 using the Dualex chlorophyll fluorescence-based portable leaf-clip. The experimental design consisted of six parcels of 30 vines and measurements were performed on the 12 central vines. The leaves were inoculated with a sporangia suspension containing 50,000 sporangia of P. viticola per mL and the disease severity was assessed after the appearance of symptoms. Leaves maintained at 100% sun had high polyphenolic content and significantly lower disease severity compared to leaves under shading nets. These results indicate an inverse relationship between produced polyphenolics and downy mildew severity.

Induction of systemic resistance against Plasmopara viticola in grapevine by Trichoderma harzianum T39 and benzothiadiazole

Plasmopara viticola, the causal agent of grapevine downy mildew, severely affects grape yield and wine quality worldwide. Biocontrol agents and resistance inducers could be valid alternatives to chemical pesticides. We demonstrated the capability of Trichoderma harzianum T39 to protect susceptible grapevine cultivars ( Vitis vinifera cv. Pinot Gris and cv. Pinot Noir) against downy mildew under greenhouse conditions. T. harzianum T39 reduced downy mildew severity on grapevine without a direct inhibition of P. viticola sporangia germination. Plant-mediated resistance was activated after preventive T. harzianum treatments, in a manner similar to that observed for benzothiadiazole (BTH) elicitation. Optimal disease control results were seen when BTH was applied 24 h before pathogen inoculation (83% disease reduction) and T. harzianum was applied more than once, 48–72 h before inoculation (63% disease reduction). Whereas leaves treated with the inducers were resistant to the disease, a systemic activation of grapevine defenses was also observed in untreated leaves. In particular, T. harzianum and BTH leaf treatments on one side of the plant activated disease protection, 60% and 56% respectively, in untreated leaves on the opposite side. Additionally, treatments of basal leaves induced acropetal resistance in untreated leaves (more than 40% disease reduction), but root treatments did not induce significant resistance in leaves. The systemic resistance was homogeneously activated, independently of leaf position on the shoots. Differences between T. harzianum T39- and BTH-induced resistance in time of activation, persistence of the effect and systemic protection suggested that in grapevine different pathways than those of salicylic acid-dependent BTH elicitation are activated by T. harzianum.

Effects of garlic (Allium sativum) juice containing allicin on Phytophthora infestans and downy mildew of cucumber caused by Pseudoperonospora cubensis

The volatile antimicrobial substance allicin (diallylthiosulphinate) is produced in garlic when the tissues are damaged and the substrate allicin (S-allyl-l-cysteine sulphoxide) mixes with the enzyme alliin-lyase (E.C.4.4.1.4). Allicin undergoes thiol-disulphide exchange reactions with free thiol groups in proteins and it is thought that this is the basis of its antimicrobial action. At 50 μg ml-1, allicin in garlic juice inhibited the germination of sporangia and cysts and subsequent germ tube growth by Phytophthora infestans both in vitro and in vivo on the leaf surface. Disease severity in P. infestans-infected tomato seedlings was also reduced by spraying leaves with garlic juice containing allicin over the range tested (55–110 μg ml−1) with an effectiveness ranging from approximately 45–100%. Similarly, in growth room experiments at concentrations from 50–1,000 μg ml−1, allicin in garlic juice reduced the severity of cucumber downy mildew caused by Pseudoperonospora cubensis by approximately 50–100%. These results suggest a potential for developing preparations from garlic for use in specialised aspects of organic farming, e.g. for reducing pathogen inoculum potential and perhaps for use under glass in horticulture.

Optically-assessed preformed flavonoids and susceptibility of grapevine to Plasmopara viticola under different light regimes.

The role of flavonoids in the response of plants to Plasmopara viticola, the phytopathogen agent of downy mildew, was studied in the Vitis vinifera L. cultivar Sangiovese. Grapevines in the vineyard were exposed to two light regimes, 100% and 35% of full sunlight in order to induce differences in total leaf polyphenolic content. Epidermal leaf phenolic compounds were assessed optically, using the Dualex chlorophyll fluorescence-based portable leaf-clip. Dualex data were calibrated by means of HPLC analysis of extracts from the same measured leaves. Good correlations were obtained with total flavonoid contents, which consist mainly of quercetin 3-O-glucuronide. From the Dualex non-destructive measurements, we showed that full-sun exposed leaves contained 75% more flavonoids than shaded leaves. Inoculation of leaves with P. viticola sporangia resulted in a significantly lower infected leaf area in sun-lit leaves compared with shaded ones, as seen from subsequent analysis of the downy mildew severity. These results indicated an inverse relationship between preformed flavonoids and the susceptibility of grapevines to downy mildew. The rapid optical method for the non-destructive assessment of flavonoids presented here could be useful for large scale screening and predicting V. vinifera susceptibility to P. viticola.

Efeito de indutores no controle de míldio em Vitis labrusca

A cultura da uva, Vitis labrusca, é de grande importância econômica para a região do vale do Siriji - PE. No entanto, nos últimos anos, o míldio da videira vem pondo em risco essa atividade, fazendo com que os produtores invistam bastante recurso com o uso de fungicida visando o seu controle. Visando substituir o uso de agrotóxico foi avaliado nos municípios de São Vicente Férrer - PE (área 1), Natuba - PB (área 2) e Macaparana - PE (área 3) o efeito da indução de resistência promovida por ácido DL- â - aminobutírico (BABA), Agro-Mos (AM) + Crop-set (Cs), Codavit (Cd) e Ecolife (Ec) em videiras da variedade 'Isabel'. Os resultados obtidos em relação à severidade do míldio demonstraram que o indutor AM + Cs destacou-se dos demais tratamentos, não diferindo estatisticamente dos indutores AM + Cs e Cd, na área 2, e de AM + Cs e Cd, e Ec na área 3. Quanto a eficiência de controle o AM + Cs foi capaz de reduzir a severidade da doença em 37,46%, 35,97% e 18, 57% nas áreas 1, 2 e 3, respectivamente. Nas áreas 1, 2 e 3 a severidade da doença desenvolveu-se obedecendo as seguintes equações, gompertz Y= e-3,36 . e -0,019 t-0,0199(R= 0,66**), logístico Y= 1/1 + 14,63 .e 0,037t (R= 0,84**) e gompertz Y= e-4,13 . e-0,034 t-0,034t,(R= 0,62**), respectivamente. A severidade do míldio da videira em função do tempo na área 1, na maioria dos tratamentos, obedeceu ao modelo de gompertz, exceto o tratamento com BABA cujo modelo que se ajustou foi o logístico. Na área 2, o aumento da severidade da doença nos tratamentos estudados não apresentou predominância de um modelo. O modelo monomolecular apareceu com uma freqüência maior do que os demais, nos tratamentos da área 3.

Incidência de doenças e necessidade de controle em cultivo protegido de videira

O cultivo protegido de videira no Brasil tem-se expandido, em área, visando principalmente à diminuição de danos por adversidades climáticas sobre a produção e a maturação das uvas. Entretanto, não se dispõe de informações sobre o microclima e as necessidades de controle fitossanitário que são impostas por essa tecnologia, as quais constituem os objetivos deste trabalho. O experimento foi instalado no ciclo 2005-2006, em Flores da Cunha-RS, em um vinhedo de 'Moscato Giallo', conduzido em "Y", com cobertura plástica impermeável (160µm), em 12 fileiras com 35m, deixando-se 5 fileiras sem cobertura (controle). Em ambas áreas, avaliou-se o microclima quanto à presença de água livre (registro visual), temperatura (T), umidade relativa (UR) do ar, radiação fotossinteticamente ativa (RFA) e velocidade do vento (VV) próximos ao dossel vegetativo e aos cachos. Na área coberta, foram aplicados fungicidas quando necessário, enquanto na área descoberta foram realizadas aplicações por calendário. Durante a floração e a maturação, avaliaram-se a incidência e a severidade de míldio (Plasmopara viticola), oídio (Uncinula necator), podridão-cinzenta-da-uva (Botrytis cinerea), podridão-da-uva-madura (Glomerella cingulata) e podridão ácida (leveduras imperfeitas e leveduras esporógenas). A cobertura plástica aumentou a temperatura diurna próxima ao dossel vegetativo, não influenciou na umidade relativa do ar, diminuiu a radiação fotossinteticamente ativa e a velocidade do vento e restringiu drasticamente a água livre sobre as folhas e cachos. Nessas condições, na área coberta, realizaram-se apenas duas aplicações para o controle do oídio, enquanto na área descoberta foram realizadas 17 aplicações para o controle de doenças fúngicas. Não houve incidência de doenças na avaliação realizada na floração, nos dois sistemas de cultivo; contudo, no período de maturação, houve decréscimos significativos de incidência de podridão ácida (-77,10%) e a severidade de podridão-da-uva-madura (-89,47%), podridão-cinzenta-da-uva (-57,56%) e podridão ácida (-84,54%) em função da cobertura plástica. De modo geral, as condições microclimáticas do cultivo protegido não permitiram o estabelecimento de míldio e diminuíram a incidência e severidade de podridões de cacho reduzindo as exigências e os custos com controle fitossanitário. Portanto, essa tecnologia pode apresentar-se como uma possibilidade de cultivo com menores impactos de contaminação para o ambiente, produtor e consumidor, desde que sejam consideradas as reduções de tratamentos fitossanitários. Isso fica evidente com os dados de acúmulo residual de fungicidas, que foi maior no cultivo protegido comparado ao convencional, de forma que o manejo fitossanitário deve ser diferenciado em relação ao cultivo convencional.

Fungicidal management of downy mildew of isabgol (Plantago ovata) simulating farmers's field conditions

This work aimed to assess the feasibility of economical management of downy mildew of isabgol (Plantago ovata) under simulated conditions of farmers’ fields at Gujarat, India. Seed treatment and three foliar sprays of the combination fungicide Ridomil MZ were found to be the best treatment for suppressing downy mildew (Peronospora plantaginis) severity and increasing seed yield. However, seed treatment and a single dose of the Ridomil MZ followed by two applications of mancozeb produced maximum net return. Mancozeb residue in seed was 15 times lower than the acceptable daily intake level recommended by the World Health Organisation.

Effect of intra row spacing on the incidence and severity of pearl millet downy mildew and grain yield

The effect of seven intra row spacing (10, 20, 30, 40, 50, 60, 70 cm) on the incidence and severity of downy mildew of pearl millet and yield components of a local pearl millet variety (Zango) were studied under field conditions in 2002 and 2003 at Samaru, Nigeria. Intra row spacing did not have significant effect on the incidence and severity of the disease but significantly increased plant height, panicle length and weight, 1000-grain weight and grain yield.

Thermal imaging of cucumber leaves affected by downy mildew and environmental conditions

Pathogenesis of Pseudoperonospora cubensis causing downy mildew of cucumber resulted in changes in the metabolic processes within cucumber leaves including the transpiration rate. Due to the negative correlation between transpiration rate and leaf temperature, digital infrared thermography permitted a non-invasive monitoring and an indirect visualization of downy mildew development. Depending on the stage of pathogenesis and the topology of chloroses and necroses, infection resulted in a typical temperature pattern. Spatial heterogeneity of the leaf temperature could be quantified by the maximum temperature difference (MTD) within a leaf. The MTD increased during pathogenesis with the formation of necrotic tissue and was related to disease severity as described by linear and quadratic regression curves. Under controlled conditions, changes in temperature of infected leaves allowed the discrimination between healthy and infected areas in thermograms, even before visible symptoms of downy mildew appeared. Environmental conditions during thermographic measurement, in particular air temperature and humidity, as well as water content and age of the leaf influenced the temperature of its surface. Conditions enhancing the transpiration rate facilitated the detection of changes in leaf temperature of infected leaves at early stages of infection. As modified by environmental conditions, MTD alone is not suitable for the quantification of downy mildew severity in the field.

Effect of fertilizer levels on grain yield, incidence and severity of downy mildew in pearl millet

The effect of various levels of nitrogen (0.0, 30.0, 60.0, 120.0) and phosphorus (0.0, 6.5, 13.0, 36.0) on the incidence and severity of downy mildew of pearl millet and yield of two pearl millet varieties (Zango and GB8375) were studied under field conditions in 2000 and 2001 respectively. Both nitrogen and phosphorus significantly increased incidence and severity of the disease in the two varieties. Grain yield and 1000 grain weight of the varieties also increased with nitrogen and phosphorus levels.

Effect of Metalaxyl on the incidence and severity of Pearl millet downy mildew disease in Northern Nigeria

Pearl millet downy mildew (DM) incidence, severity and yield losses of two pearl millet varieties (local and improved) due to the disease were determined in the field. Significant differences in the disease incidence and severity were recorded in the plots sown with metalaxyl-treated seeds and those sown with non-treated seeds, indicating the efficacy of the fungicide on the fungus. Yield losses due to non-treatment of seeds with metalaxyl was 40.88 and 45.39% in a local variety and 43.00 and 18.60% in an improved variety in the 2000 and 2001 cropping seasons respectively. Significant differences between plots sown with metalaxyl-treated and those sown with non-treated seeds were obtained for other yield components such as 1000-grains weight, panicle length and weight.

A Chitosan formulation Elexa™ induces downy mildew disease resistance and growth promotion in pearl millet

A commercially developed aqueous Chitosan formulation Elexa was used in different concentrations viz., 1:5, 1:10, 1:15, 1:19 and 1:25 as seed soaking treatment to pearl millet for 3, 6 and 9 h duration to test for its effect on germination and vigor index. Among the treatments used 1:19 for 6 h soaking recorded maximum germination and seedling vigor. Seed treatment, foliar spray and combination of seed treatment and foliar spray were tested for control of downy mildew disease caused by Sclerospora graminicola in pearl millet under greenhouse and field conditions. Metalaxyl at the rate of 2.1% a.i. in the form of Apron 35 SD seed treatment was used as check. Under greenhouse conditions seed treatment offered 48% protection. Foliar spray was carried out to two, seven and 14-day-old seedlings and there was marked reduction in downy mildew disease incidence. Maximum protection of 67% was recorded with foliar spray to 2-day-old seedlings. With the combination of seed treatment and foliar spray to 2-day-old seedlings offered 71% protection. Under field conditions Elexa treatments were evaluated for their effect on incidence and severity of downy mildew disease. Seed treatment reduced downy mildew severity to 42.5% and recorded 38% protection, whereas foliar spray to 7-day-old seedlings gave 67% protection and reduced severity to 25%. Combination of seed treatment and foliar spray to 7-day-old seedlings recorded 69% protection and reduced severity by 23%. The nature of disease control mechanisms has been investigated and the results indicated that it is due to induction of systemic resistance. The induction of resistance was observed as early as 24 h time gap between the inducer treatment and pathogen inoculation and the maximum resistance developed at 24–48 h time gap and maintained thereafter. Elexa treated to pearl millet seeds offered growth promoting effect under greenhouse conditions and recorded increase in plant height, earhead length and seed weight. Hence, it is inferred that Elexa is a good downy mildew disease management commercial formulation and also exhibits growth-promoting effects in pearl millet.

Evaluation of disease assessment methods in quinoa for their ability to predict yield loss caused by downy mildew

Seven disease assessment methods to measure severity of downy mildew ( Peronospora farinosa f.sp. chenopodii) were tested in eight cultivars of quinoa ( Chenopodium quinoa) in two field experiments in Peru, one at low (Lima) and one at high (Huancayo) disease pressure. Two levels of downy mildew were established by means of fungicide treatments. Area under disease progress curve (AUDPC) values were calculated for each method and correlated to the grain yield. In addition to the severity measurements, spectral reflectance was measured throughout the growth season in Huancayo and area under reflectance curve (AURC) values calculated and correlated to yield. The disease level was ten-fold higher in Huancayo compared to Lima. AUDPC based on the Three-leaf method (mean disease severity of three leaves per plant selected randomly from the lower, middle and upper part of the plant), showed the highest negative correlation to yield ( r=−0.736) and is regarded as the best method to predict yield loss caused by downy mildew. In addition, this method proved to be the quickest and easiest compared to the other six methods included in the study, which were based on assessments of disease severity of the entire plant or parts of the plant. Yield losses recorded in the tested cultivars ranged from 33% to 99% under the high-disease conditions. The reflectance method was superior to all other methods by showing a higher correlation between AURC and yield ( r=0.853), which is explained by its ability to measure pathogen-induced defoliation. Remote sensing is an excellent tool to compare the effect of treatments on disease severity within a cultivar. However, the method does not allow for comparison of disease severity among cultivars as the spectral reflectance is determined by size, shape and colour of the plant. The Three-leaf method provides an easy, cheap and readily applicable tool for researchers and plant breeders to study downy mildew of quinoa, characterise germplasm for resistance and compare results across regions and years.

Pea yield decline syndrome in South Australia: the role of diseases and the impact of agronomic practices

Sixty commercial pea crops were surveyed in 1995 to determine the causes of declining yields. Blackspot (Mycosphaerella pinodes and Phoma medicaginis var. pinodella) and downy mildew (Peronospora viciae) were prevalent in most crops and were identified as probable major contributors to the syndrome. Short rotation intervals (<5 years) between pea crops in paddocks were correlated with increased levels of blackspot and lower grain yields. Early sowing dates were correlated with increased levels of blackspot. A detailed survey of blackspot development was conducted in 5 commercial paddocks in 1996. The relative importance of sowing time and rotation varied between regions and seasons. The impact of a range of herbicides and the micronutrients manganese and zinc on blackspot, caused by the M. pinodes component of the blackspot complex, was investigated in a field trial during 1996. The herbicides diuron, metribuzin, and fluazifop significantly increased blackspot crown lesions compared with the nil treatment. There was a significant interaction between blackspot severity, herbicides, and the micronutrients manganese and zinc. Manganese concentration in pea plants was negatively correlated with the severity of blackspot crown lesions and positively correlated with severity of downy mildew.

Occurrence of Peronosclerospora sorghi in Uganda.

A study was conducted to determine the occurrence and severity of sorghum downy mildew on three major hosts (maize, sorghum, and Johnson-grass) in Uganda. Five surveys were conducted in four growing seasons, between 1994 and 1995. The disease was encountered in 11 of the 22 districts surveyed. In the majority of the areas, incidence was less than 10%, but it was significantly higher (P < 0.05) in the second season of 1994 than in the first season of 1995. Soil type significantly (P = 0.001) influenced the incidence of sorghum downy mildew, with high incidence being associated with sandy soils, followed by loamy soils, and lowest incidence in clay soils. Incidence, shredding, and oospore production were comparatively higher on sorghum and Johnson-grass than on maize.

Comparison of Lettuce Diseases and Yield Under Subsurface Drip and Furrow Irrigation

ABSTRACT Subsurface drip and furrow irrigation were compared on lettuce (Lactuca sativa) cvs. Salinas and Misty Day for yield and incidence and severity of three important diseases of lettuce in the Salinas Valley, CA. Experiments were conducted between 1993 and 1995 during the spring and fall seasons. The diseases examined included lettuce drop (Sclerotinia minor), downy mildew (Bremia lactucae), and corky root (Rhizomonas suberifaciens). Replicated plots of subsurface drip and furrow irrigation were arranged in a randomized complete-block design. All plants were inoculated with S. minor at the initiation of the experiment during the 1993 spring season. Plots were not inoculated for downy mildew and corky root during any season nor were the plots reinoculated with S. minor. During each season, all plots were sprinkler irrigated until thinning, and subsequently, the irrigation treatments were begun. The furrow plots were irrigated once per week, and the drip plots received water twice per week. The distribution of soil moisture at two soil depths (0 to 5 and 6 to 15 cm) at 5, 10, and 15 cm distance on either side of the bed center in two diagonal directions was significantly lower in drip-irrigated compared with furrow-irrigated plots. Plots were evaluated for lettuce drop incidence and downy mildew incidence and severity at weekly intervals until harvest. Corky root severity and yield components were determined at maturity. Lettuce drop incidence and corky root severity were significantly lower and yields were higher in plots under subsurface drip irrigation compared with furrow irrigation, regardless of the cultivar, except during the 1994 fall season. Incidence and severity of downy mildew were not significantly different between the two irrigation methods throughout the study. The differential microclimates created by the two irrigation treatments did not affect downy mildew infection, presumably because the mesoclimate is usually favorable in the Salinas Valley. Subsurface drip irrigation is a viable, long-term strategy for soilborne disease management in lettuce in the Salinas Valley.

Fusarium proliferatumas a Biocontrol Agent Against Grape Downy Mildew

Postinfection applications of microconidial suspensions of Fusarium proliferatum G6 reduced sporangial production of Plasmopara viticola on grape leaf disks by 97% and prevented resporulation. Microscopic examination of the hyphal interaction in vitro showed hyphae of F. proliferatum G6 coiled around and inside sporangiophores of P. viticola. In the field from 1992 to 1995, where substantial natural grape downy mildew developed, weekly applications of F. proliferatum G6 microconidia reduced disease development on leaves and fruit clusters of Vitis interspecific hybrid cultivars Chancellor and Lakemont. Severity of downy mildew on Chancellor clusters was reduced by 77% in 1992, 80% in 1993, and 53% in 1994 and was reduced on leaves by 71% in 1992. On Lakemont, disease severity on clusters was reduced by 99% in 1993, 94% in 1994, and 81% in 1995 and was reduced on leaves by 79% in 1992, 67% in 1994, and 60% in 1995. Fumonisin mycotoxins, which are produced by F. proliferatum, including F. proliferatum G6, were not found in grape berries or juice made from berries sprayed with microconidia in the field. Agar tests with fungicides showed sulfur, copper, and metalaxyl to be minimally detrimental to F. proliferatum G6. Biological control with F. proliferatum might be practical in conjunction with a management program in which sulfur and copper are used or in an antiresistance management program with metalaxyl, on moderately resistant cultivars, or in areas where downy mildew is not severe.

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%.