Distribution and association of biophysical factors with onion (Allium cepa L.) downy mildew (Peronospora destructor) disease epidemics in northwestern Ethiopia

Distribution and association of biophysical factors with onion (Allium cepa L.) downy mildew (Peronospora destructor) disease epidemics in northwestern Ethiopia | bioRxiv /* */ /* */ <!-- <!-- /*! * yepnope1.5.4 * (c) WTFPL, GPLv2 */ (function(a,b,c){function d(a){return"[object Function]"==o.call(a)}function e(a){return"string"==typeof a}function f(){}function g(a){return!a||"loaded"==a||"complete"==a||"uninitialized"==a}function h(){var a=p.shift();q=1,a?a.t?m(function(){("c"==a.t?B.injectCss:B.injectJs)(a.s,0,a.a,a.x,a.e,1)},0):(a(),h()):q=0}function i(a,c,d,e,f,i,j){function k(b){if(!o&&g(l.readyState)&&(u.r=o=1,!q&&h(),l.onload=l.onreadystatechange=null,b)){"img"!=a&&m(function(){t.removeChild(l)},50);for(var d in y[c])y[c].hasOwnProperty(d)&&y[c][d].onload()}}var j=j||B.errorTimeout,l=b.createElement(a),o=0,r=0,u={t:d,s:c,e:f,a:i,x:j};1===y[c]&&(r=1,y[c]=[]),"object"==a?l.data=c:(l.src=c,l.type=a),l.width=l.height="0",l.onerror=l.onload=l.onreadystatechange=function(){k.call(this,r)},p.splice(e,0,u),"img"!=a&&(r||2===y[c]?(t.insertBefore(l,s?null:n),m(k,j)):y[c].push(l))}function j(a,b,c,d,f){return q=0,b=b||"j",e(a)?i("c"==b?v:u,a,b,this.i++,c,d,f):(p.splice(this.i++,0,a),1==p.length&&h()),this}function k(){var a=B;return a.loader={load:j,i:0},a}var l=b.documentElement,m=a.setTimeout,n=b.getElementsByTagName("script")[0],o={}.toString,p=[],q=0,r="MozAppearance"in l.style,s=r&&!!b.createRange().compareNode,t=s?l:n.parentNode,l=a.opera&&"[object Opera]"==o.call(a.opera),l=!!b.attachEvent&&!l,u=r?"object":l?"script":"img",v=l?"script":u,w=Array.isArray||function(a){return"[object Array]"==o.call(a)},x=[],y={},z={timeout:function(a,b){return b.length&&(a.timeout=b[0]),a}},A,B;B=function(a){function b(a){var a=a.split("!"),b=x.length,c=a.pop(),d=a.length,c={url:c,origUrl:c,prefixes:a},e,f,g;for(f=0;f<d;f++)g=a[f].split("="),(e=z[g.shift()])&&(c=e(c,g));for(f=0;f<b;f++)c=x[f](c);return c}function g(a,e,f,g,h){var i=b(a),j=i.autoCallback;i.url.split(".").pop().split("?").shift(),i.bypass||(e&&(e=d(e)?e:e[a]||e[g]||e[a.split("/").pop().split("?")[0]]),i.instead?i.instead(a,e,f,g,h):(y[i.url]?i.noexec=!0:y[i.url]=1,f.load(i.url,i.forceCSS||!i.forceJS&&"css"==i.url.split(".").pop().split("?").shift()?"c":c,i.noexec,i.attrs,i.timeout),(d(e)||d(j))&&f.load(function(){k(),e&&e(i.origUrl,h,g),j&&j(i.origUrl,h,g),y[i.url]=2})))}function h(a,b){function c(a,c){if(a){if(e(a))c||(j=function(){var a=[].slice.call(arguments);k.apply(this,a),l()}),g(a,j,b,0,h);else if(Object(a)===a)for(n in m=function(){var b=0,c;for(c in a)a.hasOwnProperty(c)&&b++;return b}(),a)a.hasOwnProperty(n)&&(!c&&!--m&&(d(j)?j=function(){var a=[].slice.call(arguments);k.apply(this,a),l()}:j[n]=function(a){return function(){var b=[].slice.call(arguments);a&&a.apply(this,b),l()}}(k[n])),g(a[n],j,b,n,h))}else!c&&l()}var h=!!a.test,i=a.load||a.both,j=a.callback||f,k=j,l=a.complete||f,m,n;c(h?a.yep:a.nope,!!i),i&&c(i)}var i,j,l=this.yepnope.loader;if(e(a))g(a,0,l,0);else if(w(a))for(i=0;i (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0];var j=d.createElement(s);var dl=l!='dataLayer'?'&l='+l:'';j.src='//www.googletagmanager.com/gtm.js?id='+i+dl;j.type='text/javascript';j.async=true;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-M677548'); Skip to main content Home About Submit ALERTS / RSS Search for this keyword Advanced Search New Results Distribution and association of biophysical factors with onion ( Allium cepa L.) downy mildew ( Peronospora destructor ) disease epidemics in northwestern Ethiopia Mitku Bitew , Addisu Mandefro , View ORCID Profile Yehizbalem Azmeraw doi: https://doi.org/10.1101/2025.08.26.672276 Mitku Bitew a Department of Plant Sciences, Debre Tabor University , P.O. Box 272, Debre Tabor, Ethiopia Find this author on Google Scholar Find this author on PubMed Search for this author on this site Addisu Mandefro a Department of Plant Sciences, Debre Tabor University , P.O. Box 272, Debre Tabor, Ethiopia Find this author on Google Scholar Find this author on PubMed Search for this author on this site Yehizbalem Azmeraw b Department of Plant Sciences, Injibara University , P.O. Box 40, Injibara, Ethiopia Find this author on Google Scholar Find this author on PubMed Search for this author on this site ORCID record for Yehizbalem Azmeraw For correspondence: yehizbalem2002{at}gmail.com Abstract Full Text Info/History Metrics Preview PDF ABSTRACT Onion ( Allium cepa L.) is a significant food bulb crop that can be used for medicine, diverse recipes, and income. However, downy mildew disease limits productivity and production of onion in onion growing-regions. Thus, this survey was conducted to assess the distribution and intensity of onion downy mildew and determine the associations of disease parameters with biophysical factors. During the 2024/2025 onion cropping season, 130 onion fields were surveyed in order to measure biophysical and disease data. The associations of disease parameters and biophysical factors were analyzed using the binary logistic regression model by employing the SAS GENEMODE procedure. The survey’s findings verified that downy mildew was 100% prevalent. The highest disease intensity was assessed from Fogera (50.68% and 23.48%) and Libokemkem (45.16% and 21.62%) districts, respectively. High disease incidence (>40%) and severity (>20%) were strongly associated with maturity crop growth stages, field size (>0.25 ha), blub previous crop, late October transplanting, less than four times fungicide application and less than three times land preparation. Lower disease incidence (≤ 40%) and severity (≤ 20%) had a strong association with early December transplanting, more than three times land preparation, fungicide spray (>4 times), field size (≤ 0.25 ha), fields previously planted with cereals, could be considered as relative management options to minimize the disease’s effects in onion growing areas of northwest Ethiopia and other similar onion cultivating areas. In addition, In order to develop effective management strategies, future research may examine pathogen variability. 1. INTRODUCTION The onion ( Allium cepa L.), a bulb crop in the Amaryllidaceae family and essential spices of vegetable crops grown in many countries of the world. It originated in southwest Asia and is the most important commercial bulb crop in the vegetable bulb crop group. In terms of yearly global production of vegetable crops, onion ranks second after tomato [ 1 ]. Most smallholder farmers and a few commercial producers cultivate it year-round for both domestic and export markets, mostly under irrigation and to a lesser degree under rain-fed conditions [ 2 , 3 ]. According to [ 1 ] report, onion production exceeds 93.22 megatons (Mt) in world annually. China is the world’s largest producer of onions with production of 23.91 Mt, followed by India (19.42 Mt). In Ethiopia, about 3.46 megatons (Mt) were produced in 38,952.58 ha land in 2020/21 cropping season of with an average yield of 8.8t ha -1 [ 4 ], which is lower than the other onion-producing countries. The onion is a crop that thrives in semi-arid regions, high elevations, and tropical climates. Climate characteristics that are favorable for onion production in Ethiopia include temperatures between 17 and 27 °C, 500 to 700 mm of annual rainfall, and elevations up to 2200 m. a. s. l. Onion production can also be achieved in fertile sandy and silt loam soils with a pH range of 6.0 to 6.8, while excessive rainfall and extremely hot or cold temperatures can reduce yield [ 5 ]. Onion is one of the most significant vegetable crops grown globally for food, as well as a relative and a valuable addition to many recipes [ 6 , 7 , 8]. In world including Ethiopia, The crop is valued for its distinct pungency and form essential ingredients for flavoring varieties of dishes, sauces, soup, sandwiches and snacks as onion rings and also exhibits a number of therapeutic properties, such as antibacterial, antifungal, anti-helminthic, anti-inflammatory, antiseptic, antispasmodic, etc. and it contributes to the national economy through export products like, bulbs and cut flowers to different countries of the world [ 9 , 10 , 11 , 12 ]. Because of these significant advantages, onion production is growing in various agro-ecologies across the nation in small-scale production systems as a part of commercialization. Despite its importance for household consumption, medicinal uses, income sources and contribution to the national economy through exports, the onion production and productivity are constrained by several abiotic and biotic stresses [ 13 ]. The national average production of onion under farmers’ situations is considerably deficit in Ethiopia (8.9 t per ha -1 ) [ 4 ], which are significantly lower than global average (19.1t ha −1 ). The Amhara region is a major contributor, accounting for about 50% of the national production with a yield of 12.3 tons per hectare [ 14 ]. This low productivity is mainly associated with different diseases and insect pests. In this regard, diseases, such as purple blotch ( Alternaria porri ) and downy mildew ( Peronospora destructor are the major economically important diseases that lowering onion yields [ 15 , 16 , 17 , 18 , 19 ]. Among diseases, downy mildew is one of the most widespread and devastating disease and cause highest yield losses in onion producing areas in the world [ 15 , 20 , 21 , 22 ]. It’s characterized by yellowing and dieback of leaves, with a white and then purple fungal growth on the leaf surface. It can also cause premature sprouting or shriveling of bulbs. It is a water mold or oomycete disease of alliums caused by Peronospora destructor . It causes irregular foliar lesions that begin as pale-green, and then progress to yellow or brown necrotic tissue. Eventually, lesions coalesce and lead to the collapse of the leaf. During periods of high moisture, fuzzy, gray-to-violet sporangia appear on leaf surfaces. These symptoms can also be seen on seed stalks and flowers [ 23 , 24 ]. When conditions are favorable for the disease to occur, downy mildew outbreaks can cause yield decreases of onion bulbs of 30% to 75% [ 25 , 26 ]. In northwestern Ethiopia, yield loss due to downy mildew has recorded up to 80% of the total production with the relative occurrence of the disease [ 27 ]. Various management strategies are employed to lessen the disease’s transmission and infection sources. The management of onion downy mildew involves a number of strategies, including the Planting timing, resistant cultivars [ 28 , 29 ]. Soil drainage [ 30 ], seed or bulb health stock [ 31 ], wind block and the elimination of infected plants [ 32 , 33 ]. However, the most effective means of controlling the downy mildew is the use of fungicides [ 34 ]. Despite the fact that the disease is a significant barrier to onion production, intensive field survey on the disease epidemic is not done in the study areas rather than preliminary survey and fungicide evaluation studies. Thus, compressive field survey on downy mildew epidemics in onion producing-areas in northwestern Ethiopia could aid to provide information on the association of disease epidemic with cultural and environmental factors and to identify the existing management options at farmers’ level. Therefore, the study was conducted to assess the distribution and intensity of onion downy mildew and determine the association of the disease parameters with biophysical factors in northwestern Ethiopia. 2. MATERIALS AND METHODS 2.1. Description of the study areas A field survey was conducted in the South and Central Gondar Zones of the Amhara Region Ethiopia, which are major onion-producing regions. During the 2024–2025 onion growing season, field assessments were conducted in three South Gondar districts (Fogera, Libokemkem, and Farta) and one Central Gondar zone district (Gondar Zuria), as shown in Figure 1 . The districts are situated between latitudes 8°45’ and 13°45’ N and longitudes 36° 20’ and 40° 20’ E in the northwest of the Amhara Region. The districts were found between elevations of 1790 and 2186 meters above sea level, and they receive 1000 to 1600 millimeters of rainfall annually [ 35 ]. The study areas’ have an average temperature of ranged from 17.8 to 27.05 °C. The two main soil types found in the survey areas are nitisol and vertisol. Rainfall and minimum and maximum temperature of the districts were obtained from meteorological stations and are presented in Figure 2 . Download figure Open in new tab Figure 1. Map showing the districts for onion downy mildew diseases study in northwestern Ethiopia in 2024/25 cropping season. Download figure Open in new tab Figure 2. Average Temperatures (°C) and Relative humidity (%) of surveyed districts in northwestern of Ethiopia from sowing to surveyed periods of 2024/25 onion growing season. 2.2. Sampling procedure and diseases assessment The districts were specifically selected on the basis of their potential for onion production. Five to seven representative Farmers Associations were selected from each district, and each Farmers’ Association had six to ten fields examined. Random sampling was used to select onion fields at 4-to 5-kilometer intervals. The plant population was assessed using a 1 m 2 quadrat arranged in a “X” layout with 5 m between each field. The mean plant population densities were calculated by averaging the number of populations in the quadrat for analysis. Five spots were evaluated diagonally in each field (using a 1m2 quadrat) to avoid bias in recording the disease’s severity. A total of 130 fields were assessed across four districts throughout the survey. The number of onion plants in each quadrat was counted, and the number of infected plants was noted. Ten randomly selected plants were evaluated for disease severity in each quadrat using a 1–9 rating system [ 34 ]; where, 1 = no disease symptoms, 2 = only a few leaves 1% affected, 3 = less than half of the plants (5%) affected, 4 = most of the plants affected, 10% attack is restricted to one leaf per plant, 5 = all plants attack, (20%) attack restricted to one or two leaves, 6 = three to four leaves of each 50% of plants affected, crops looks fairly green, 7 = all leaves affected 75% of crop gives blight appearance, 8 = all leaves severely affected 90%, greenness restricted to central shoot only, 9 = foliage completely blighted 100%. Then, the severity scale was converted to percent severity index (PSI) [ 36 ]. Disease prevalence [ 37 ], incidence [ 38 ], and percent severity index in each sampled field were calculated as follows: Additional data, including previous field history, seed source, transplanting date, land preparation, and frequency of fungicide application, were gathered during the survey by interviewing farmers using questionnaires. Visual observation was used to record the field size and crop growth stage. The Global Positioning System (GPS) was used to record the altitude, latitude, and longitude. 2.3. Data analysis Descriptive analysis was used to show the distribution and association of the incidence and severity of onion downy mildew disease and biophysical parameters in each district. To develop a binary dependent variable, class boundaries were established for both disease incidence and severity into two separate groups’ binomial qualitative data: ≤ 40 and > 40% were chosen for disease incidence, while ≤ 20 and >20% were selected for severity. To represent the bivariate distribution of fields based on data classifications, a contingency table of disease incidence and severity as well as the independent variables was constructed ( Table 1 ). View this table: View inline View popup Table 1. Categorization of variables used in analysis for the distribution of onion downy mildew epidemics in four districts ( n = 130) of northwestern Ethiopia, during the 2024/25growing season. To investigate the associations between biophysical factors and onion downy mildew disease, a binary logistic regression model was applied [ 39 ], using SAS procedure of GENMOD (SAS, 2016). The importance of the independent variables was evaluated twice in terms of their effect on the incidence and severity. First, using a single-variable model, the relationship between each independent variable and the incidence and severity of downy mildew was analyzed. Second, when all other factors in the model were entered first and last, the relationship between an independent variable and the incidence or severity of the disease was investigated. Finally, a reduced multiple-variable model was constructed by adding those independent variables that had a significant association with the incidence or severity of the disease. The odds ratio, which is understood as the relative risk, was calculated by exponentiating the parameter estimates for comparing the effect based on a reference point. A complete analysis of the deviance reduction was calculated for each variable as it was added to the reduced multiple-variable model [ 39 ]. The GENMOD process uses a maximum likelihood estimation of the parameter to fit a generalized linear model to the data. The comparison between the single and multiple variable models was done using the deviation, which is the logarithm of the ratio of two likelihoods. To assess the significance of the variable, the difference between the likelihood ratio tests (LRTs) was compared to the Chi-square value [ 40 ]. 3. RESULTS 3.1. General descriptions of surveyed areas During the survey, all of the assessed fields were free from weeds, and located in the altitude range of 1790 to 2186 m. a. s. l. The crops commonly grown in the districts were rice, potatoes, sorghum, chickpeas, teff , wheat, barley, garlic and chickpeas. Every assessed field were sprayed with fungicide, and planted in rows with varied source of seeds. About 46.15% fields were planted with farmer saved seeds and the remaining 53.85% fields were covered by seeds bought from local market. Depending on the farmer, different fields have different land preparation procedures before planting onions and 55.38% of the fields were plowed more than 3 times. Mainly, onion planting took place from October to December. Different growth (vegetative, bulbing and maturity) stages were encountered during the survey periods, which accounted for 43.07, 32.31 and 24.62% of these growth stages, respectively ( Table 1 ). 3.2. Prevalence, incidence and severity of onion downy mildew The onion downy mildew disease spread in various ways throughout different regions. The disease prevalence was recorded in all surveyed districts. The highest (50.68%) mean disease incidence was observed in Fogera district, followed by Libokemkem (45.16%) and the lowest (35.65%) disease incidence was recorded in Farta district. Similarly, higher disease severity was assessed from Fogera and Libokemkem districts ( Table 3 ). Higher Onion downy mildew disease incidence (44.25%) and severity (22.27%) were assessed at fields located at lower altitudes (≤1800 m. a. s. l.) than higher (>1800 m. a. s. l.) altitudes ( Table 2 ). Regarding seed source, higher downy mildew incidence (42.37%) and severity (23.67%) were noted from fields planted with seeds bought from market than those fields planted with farmer saved seeds. Variability in onion downy mildew intensity was observed among onion growth stages. In this regard, onion fields at maturity growth stage resulted in higher disease incidence (46.91%) and severity (26.97%) than bulbing and vegetative growth stages ( Table 2 ) Land preparation (≤ 3 times) relatively increased onion downy mildew incidence from 39.16 to 41.08% and severity from 11.04 to 12.47 compared to land ploughed more three times. View this table: View inline View popup Download powerpoint Table 2. Incidence and severity (mean ± SE) of onion downy mildew for different independent variables in northwestern Ethiopia, during the 2024/25 growing season. View this table: View inline View popup Download powerpoint Table 3. Logistic regression model for onion downy mildew incidence and severity, and likelihood ratio test on independent variables in northwestern Ethiopia, during the 2024/25 onion growing season. Variable disease intensity was recorded in terms of onion transplanting dates, where late October planting was relatively suitable for downy mildew disease epidemic with 41.17% mean incidence and 12.74% severity. Concerning previous crop, the highest incidence (42.37%) and severity (13.21%) with large lesion symptoms were observed on onion growing previously after blubs than cereals and legumes ( Table 3 ). Fields with fungicide spray (≤4 times) had higher disease incidence and severity than onion fields with spray (> 4 times) ( Table 3 ). 3.3. Association of onion downy mildew incidence and severity with biophysical factors The associations of all biophysical factors with disease incidence and severity are described in Table 4 . The biophysical factors, such as growth stage, land preparation, seed source, previous crop history and fungicide application were significantly (P< 0.05) associated with onion downy mildew disease intensity when entered first and last into the logistic regression model and in addition those associated with intensity, field size, transplanting date and plant population attained its significant association with onion downy mildew severity when entered first and last into logistic regression model ( Table 3 ). View this table: View inline View popup Download powerpoint Table 4. Analysis of deviance, natural logarithms of odd ratio and standard error of onion downy mildew incidence and likelihood ratio test on independent variables in northwestern Ethiopia, during the 2024/25 growing season. The independent variables that showed significant associations and some variables with higher deviance reduction were tested in a reduced multiple-variable model. The results of the analysis of deviation for each variable and variable class to incidence and severity, parameter estimates, standard error, and odds ratio are given in Tables 4 and 5 . Onion downy mildew incidence and severity of >40% and >20% were highly associated with Fogera, and Libokemkem districts, land preparation (≤3 times), bulbing to maturity growth stage, onion growing after bulb, late October seedling transplanting and fungicide application (≤4 times spray) compared with their respective counterpart class variables ( Tables 4 and 5 ). Among the tested independent variables, growth stage, land preparation, field size, seed source, previous crop fungicide application frequency were the most important variables in their association with onion downy mildew incidence and severity when entered first and last into the regression model respectively ( Tables 4 and 5 ). View this table: View inline View popup Download powerpoint Table 5. Analysis of deviance, natural logarithms of odd ratio and standard error of onion downy mildew severity and likelihood ratio test on independent variables in northwestern Ethiopia, during the 2024/25 growing season. The probability of highest (>40%) onion downy mildew disease incidence was associated with Fogera and Libokemkem districts. The probability of the highest (>40%) disease incidence relation with Fogera and Libokemkem was about 3.25 and 2.79 times greater than the Farta district, respectively. Disease severity >20% showed high relationships with Fogera and Libokemkem compared to Farta district. The probability of highest (>20%) disease severity with Fogera and Libokemkem was about 61.18 and 11.05 times greater than Farta, respectively. maturity growth stage, less than three times plowing, fields covered with bulbs previously, seeds bought from local market, field size greater than 0.25ha showed a greater association with the highest disease incidence and severity. Fungicide spray (≤ 4 times) and transplanting in late October showed a greater association with the highest incidence of the disease. In this regard, land preparation (≤ 3 times) was strongly associated with the highest disease incidence and severity about 6.29 and 97.5 times greater than more than three times plowing, accordingly. However, the lowest (≤ 40%) disease incidence and PSI (≤ 20%) exhibited a strong association with good land preparation, vegetative growth stage, farmer saved seeds, less than 0.25ha field size, cereal previous crop ( Tables 4 and 5 ). Transplanting early December and greater than four times fungicide spraying were less associated with only (≤ 40%) disease incidence ( Table 5 ). 4. DISCUSSION Onion downy mildew is caused by the fungus Peronospora destructor causing serious epidemics in Ethiopia’s humid and temperate onion-growing regions. Regardless of environmental and onion management practices, onion downy mildew was prevalent in all surveyed fields. In northwestern Ethiopia, yield loss due to downy mildew has been recorded up to 80% of the total production. Due to favorable environmental conditions, downy mildew was more abundant and widespread in northwestern districts of Ethiopia [ 27 , 19 ]. Onion downy mildew disease incidence and severity varied across agro-ecological conditions. The highest disease intensity was assessed from the Fogera district than the other districts. The initial inoculum, cultural practice, and environmental factors may be the cause of the variance in disease epidemics among the surveyed districts. This suggests that biological, cultural, and environmental factors had a major impact on the development and spread of onion downy mildew disease. Factors including as inoculum load, farming practices and environmental variables might also affect the disease levels in different each districts. Because of several agro-ecological conditions, the main and irrigation onion production systems at Fogera Ethiopia showed the maximum onion downy mildew disease severity [ 41 ]. Onion downy mildew is naturally more common at higher elevations, especially in regions with cool temperatures and moisture but the disease intensity were higher in fields at altitudes ≤ 1800m.a.s.l. at the Fogera and Libokemkem districts respectively than onion fields assessed at altitudes above 800m.a.s.l. Because, these areas during onion growing season had optimum temperature to pathogen, high night, and morning humidity and collaborating with other agro ecological variables ( figure 2 ). These combined conditions were hot spot for pathogen infection leads to disease development and epidemics. The pathogen sporulated during night, with sporangia maturing in the early morning and being released throughout the day. The sporangia were favored by a wide range of temperatures and a lot of moisture [ 42 , 43 , 44 ]. Favorable early morning temperatures and the few hours of high humidity have been attributed for a major mildew outbreak in rainless weather [ 45 , 46 ]. Obviously, the effects of onion downy mildew might vary depending on the stage of growth, from vegetative to maturity. In this study, onion downy mildew disease epidemic was higher at bulbing to maturity stages than the vegetative stage and associated with higher disease incidence and severity. The presence of older leaves, a decrease in the start of new leaves, and a loss of crop resistance to the disease make onions more susceptible to systemic infection at maturity. The bulbing to maturity stage of onion plants was shown to be particularly susceptible with older leaves present because new leaf initiation is stopped and crop resistance to the disease is lost have been reported[ 47 , 19 ]. Regarding to the previous crop, Onion downy mildew outbreaks were significantly associated with the planting of onions following bulbs. The occurrence and severity of onion downy mildew may be significantly influenced by past crop history. Spores that overwinter in crop debris can serve as a source of inoculum for just emerging plantings. Seasonal outbreaks of onion downy mildew may be significantly influenced by the survival of overwintering structures. It is known that Peronospora destructor naturally develops oospores that can persist in the soil for a long time [ 48 , 49 ]. The initial inoculums may distributed in to field through irrigation, wind, plowing and by another mechanisms and increasing the risk of infection in subsequent crops. This could lead to an increase in epidemics and the occurrence of onion infection by the disease. The main source of inoculum for subsequent bulb crops is diseased debris that contains pathogen fruiting bodies in the field of crops and onion seeds [ 50 , 51 , 52 ]. Fields ploughed more than three times related with less disease intensity. Frequent ploughing may disperse the organic matter in the soil that is appropriate for the robust growth of crops, making them less vulnerable to pathogens. It is also used to expose the main inoculum to solar energy, which breaks the initial inoculum population in the soil. This may result in the decrement of disease epidemics. Previous study by [ 53 ], reported that many times plowing reduce relatively disease epidemics than with few times ploughed plots. During the current field survey, Fields with a high population density had the highest mean disease intensity. This may mainly be related to a microenvironment that promotes disease outbreaks due to higher humidity, decreased air circulation, and high plant nutrient completeness. Previously scholars [ 54 ] reported that the development and outbreak of plant diseases are influenced by the morphology, canopy, and density of plants. Regarding field size, onion growing in small areas (≤ 0.25ha) had less disease intensity than large cultivated area ( > 0.25ha). The association of higher disease epidemic in large field size might be create a more favorable environment for downy mildew because the fields are more humid, have less air circulation, and are more susceptible to sporangia spreading from one area of infection to another due to several factors, particularly through irrigation and wind. Due to it’s easily dissemination, the outbreak of diseases was abundant in large onion fields [ 55 , 56 ] showed how easily downy mildew sporangia may be distributed and moved from one field to another through wind. High disease incidence and severity were observed with onion fields transplanting in late October, particularly when combined with other factors. This is a result of having an environment that is conducive to the establishment of pathogens and the development of disease. During this season, these studied sites had low temperatures and intermittent rainfall, which may have played a significant role in the development of disease. The periods of rainy season, high moisture and humidity promotes the disease development [ 57 , 58 , 59 ]. Fungicide spraying (≤ 4 times) on onion fields resulted in greater disease development compared with fungicide application (> 4 times), The application that occurs the least frequently Fungicides might not disrupt the life cycle of the pathogen and spread unevenly in order to prevent the development of diseases and this may lead pathogen adaptation against chemicals resulted in diseases outbreak. Therefore, frequent application Fungicides are often used to prevent the germination, growth, or multiplication of the pathogen, which has suppressed the spread of onion downy mildew. High spraying frequency is the primary factor that determines the success of chemical control [ 60 ]. Plots with frequent scheduled fungicide application to ensure good yields and high crop quality showed the lowest average disease severity and highest yield [ 61 , 62 ]. The present study shown that downy mildew damages bulb crops in areas with high humidity and moisture content; as a result, the disease was more affected in the Fogera district. 5. CONCLUSION Onion downy mildew disease was distributed variably in each of the districts that were surveyed. The disease was widely distributed and prevalent in all surveyed areas with high incidence and severity in the Fogera, followed by Libokemkem district. The independent variables, such as transplanting of onion until late October, maturity growth stages, poor land preparation, seeds bought from market, large field size, more than four times fungicide applications and bulb previous crop were shown to be crucial variables that contributed to the downy mildew disease outbreak and exhibited a significant association with the highest diseases intensity. The multiple-reduced regression model analysis identified significant associations between district, crop growth stage, land preparation, plant population, field size and previous crop history, and downy mildew incidence and severity. Additionally, the model assessed the relative relevance of the biophysical parameters under study, which may indicate that some variables, either alone or in combination, contributed to the development of the downy mildew epidemic in the study areas. In this regard, among the highly onion producing areas, Because of downy mildew diseases, the areas of Fogera and Libokemkem were the most problematic for growing onions. The study indicated that transplanting on early December, growing of onion in previously cereal cultivated land, good land preparation, and 4-6 times frequent fungicide applications may be regarded as useful options for management to mitigate the disease’s effect on onion crops. 6. COMPETING INTEREST The authors declare that they have no known competing financial interests. 7. ETHICAL STATEMENT This material is the authors own original work, which has not been previously published elsewhere and is not currently being considered. 8. DATA AVAILABILITY STATEMENT The datasets generated and analyzed during the current study are available from the corresponding author on reasonable request. 9. FUNDING There was no fund for this manuscript. 10. AUTHOR CONTRIBUTIONS Mitku Bitew: Conceptualization, investigation, formal analysis, writing-original draft and writing-review. Addisu Mandafro: Conceptualization; supervision; validation; writing-review and editing. Yehizbalem Azmeraw: Conceptualization; supervision; validation; writing-review and editing. REFERENCES 1. ↵ FAOSTAT (Food and Agricultural Organization ). World Food and Agriculture . Food and Agriculture Organization, Rome, Italy. Statistical Year 2021 . 2. ↵ Joosten F. , Boselie D. , Bekele W. , Lemma , D. Exporting Fruit and Vegetables from Ethiopia: Assessment of the Development Potentials and Investment Options in the Export oriented Fruit and Vegetable Sector . Ethiopia-Netherlands Horticulture Partnership Programme . 2011 : 51 3. ↵ Ketema S. , Dessalegn L. , Tesfaye , B. Effect of Planting Methods on Maturity and Yield of Onion (Allium cepa var cepa) in the Central Rift Valley of Ethiopia . Journal of Agricultural Science . 2013 . 4. ↵ CSA (Central Statistical Agency ). The Federal Democratic Republic of Ethiopia Central Statistical Agency . Agricultural sample survey in 2021/2022 volume I report on area and production of major crops (Private Peasant Holdings, Meher season). Stat. Bull. 590. Addis Ababa, Ethiopia . 2022 . 5. ↵ Nikus O. , Mulugeta , F. Onion Seed Production Techniques: A Manual for extension agents and seed producers, Asella, Ethiopia . 2010 . 6. ↵ Ravichandra S. Epidemiology and Management of Purple Blotch of onion caused by Alternaria porri . UAS, Dharwad publisher . 2012 . 7. ↵ Smith C. , Lombard K.A. , Peffley E. B. , Liu , W. Genetic analysis of quercetin in onion (Allium cepa l.) ‘Lady Raider’ . Texas Journal of Agriculture and Natural Resources . 2016 ; 16 , 24 – 28 . OpenUrl 9. ↵ Getaneh A. Comparison of different irrigation scheduling methods: for onion seed production under fertilized and unfertilized conditions at melkasa . MSc Thesis, Haramaya University, Haramaya, Ethiopia . 2007 ; 67 . 10. ↵ Lemma Dessalegn and Shimeles Aklilu . Research experience in onion production . Research Report No. 55, EARO, Addis Ababa, Ethiopia . 2003 . 11. ↵ Baghizadeh A.F. , Baniasadi G.H.S. Bonjar G.R.S. Sirchi H. Massumi M. Jorjandi P.R. Aghighi S. Bio control of Botrytis allii Munn the causal agent of neck rot, the post-harvest disease in onion, by use of a new Iranian isolate of Streptomyces . Am. J. Agric. Biol. Sci . 2009 ; 4 : 72 – 78 . OpenUrl 12. ↵ Goldman IL . Molecular breeding of healthy vegetables . EMBO Rep . 2011 ; 12 : 96 – 102 . OpenUrl FREE Full Text 13. ↵ Getachew T. , Selamawit K. , Yosef A. , Mohammed Y. , Gashaw Beza A. Guide to major vegetable crops production and protection . Amharic Version. Addis Ababa, Ethiopia . 2014 . 14. ↵ Mulatu G. , Gemechu T. Technical efficiency of onion production: The case of smallholder farmers in Dallo Mena district, Bale zone, Oromia national regional state, Ethiopia . Cogent Business & Management . 2023 ; 10 ( 3 ): 2265092 . OpenUrl 15. ↵ Jayakumar M. , Ponnuswamy K. , Amanullah MM . Influence of nitrogen and intercropping on pest incidence, yield attributes and yield of cotton . Journal of applied scientific research . 2008 ; 4 : 224 – 28 . OpenUrl 16. ↵ Wondirad M. , Eshetu A. , Mohammed Y. , Alemu L. , Yaynu H. , Meki S. , Fekede A. , Temam H. , Adane A. Review of Vegetable Diseases Research in Ethiopia . Increasing Crop production through Improved Plant Protection, Volume II, Plant Protection Society of Ethiopia (PPSE), PPSE and EIAR, Addis Ababa . 2009 : 203 – 230 . 17. ↵ Makelo M. N. Determination of seed borne fungal pathogens of onion (Allium cepa L.), spread and control of purple blotch caused by Alternariaporr (Ellis.) Cif.M.Sc . Thesis, University of Nairobi Kenya . 2013 . 18. ↵ Muniappan R. , Heinrichs E. Gajendran G. , Dinakaran D. , Mohankumar S. , Karthikeyan G. , Muniappan R. Integrated pest management for onion in India . Muniappan R. , Heinrichs E. (eds). Integrated Pest Management of Tropical Vegetable Crops . 2016 ; 179 – 207 . 19. ↵ Bitew M. , Yitayih G. , Aragaw , G. Distribution and association of factors affecting onion purple blotch disease epidemics in northwestern Ethiopia . Agrosystems, Geosciences & Environment . 2025 ; 8 , doi: 10.1002/agg2.70144 OpenUrl CrossRef 20. ↵ Surviliené E. , Valiskaité A. , Raudonis L. The effect of fungicides on the development of downy mildew of onions . Zemdirbyste Agriculture . 2008 ; 95 : 171 – 179 . OpenUrl 21. ↵ Cook H. T. Downy mildew disease of onion . N.Y. Agric. Exp. Station. Ithaca, New York. Mem . 2015 ; 143 : 1 – 40 . OpenUrl 22. ↵ Viranyi F. Downy mildew of onion . Novenyvdlem . 2015 ; 10 : 205 – 209 . OpenUrl 23. ↵ Rondomanski , W. Final Technical representative research institute Vegetable Crops Skierniewice . 2013 ; 59 . 24. ↵ Ankita Sunita , Chandel Rajender , Sharma Vijay , Kamal Meena . Epidemiological Studies of Downy Mildew of Onion . International Journal of Current Microbiology and Applied Sciences . 2020 ; 9 ( 05 ): 01 – 12 . doi: 10.20546/ijcmas . OpenUrl CrossRef 25. ↵ Sharma R.C. , Gill S.S. , Kohli , N. Pathological problems in production and storage of onion seed in Punjab and their remedial measures . Seed Research , 2002 ; 30 : 134 – 141 . OpenUrl 26. ↵ Pablo H. , Colnago P. Quantitative studies on downy mildew.Peronospora destuctor Berk. Casp.) Affecting onion seed production in southern Uruguay . European Journal of Plant Pathology . 2011 ; 129 : 303 – 314 . OpenUrl 27. ↵ Adina G , Desalegn Y , Muluadam B. Survey on major diseases of Horticultural crops in South Gondar Zone, Ethiopia . International Journal of Novel Research in Engineering and Science . 2021 ; 8 ( 2 ): 1 – 4 . OpenUrl 28. ↵ Schwartz H. F. , Mohan S. K. Compendium of Onion and Garlic Diseases . American Phytopathological Society, St. Paul, MN . 1995 ; 70 . 29. ↵ Praveen Kumar Gupta , Anwesha Sinha , Kandi Arvind Reddy , Shreya Choudhary . Management of plant diseases by using of host resistance mechanism . R.V College of Engineering, Bangalore . 2021 ; 560059 , India. 30. ↵ Hoffmann M.P. , Petzoldt C.H. , Frodsham , A.C. Integrated Pest Management for Onions . New York State IPM Program Publication . 1996 ; 119 , 78 . OpenUrl 31. ↵ Develash R. K. , Sugha S. K. Incidence of downy mildew and its impact on yield, Indian Phytopathology . 1997 ; 50 : 127 – 129 OpenUrl 32. ↵ Krauthausen H. J. , Richter E. , Hagner S. , Hommes M. Epidemiology and control (based on thresholds) of leaf diseases (Peronospora destructor, Botrytis spp.) and thrips (Thrips tabaci) in onion Acta Horticulturae . 2001 ; 25 , 137 – 140 OpenUrl 33. ↵ Raziq F. , Alam I. Naz I. , Khan H. Evaluation of fungicides for controlling downy mildew of onion under field conditions . Sarhad Journal of Agriculture . 2008 ; 24 ( 1 ): 46 – 53 . OpenUrl 34. ↵ Mohibullah A. Studies on major diseases of bulb vegetables (onion and garlic) in NWFP Province, Pakistan . Peshawar, NWP Province, Pakistan, Agricultural Research institute Tarnab, Final Technical Report . 1992 . 35. ↵ Getahun Y. Analysis of climate variability (ENSO) and vegetation dynamics in Gojjam. Ethiopia . Journal of Earth Science and Climate Change . 2015 ; 6 ( 10 ): 1 – 15 . OpenUrl 36. ↵ McKinney HH . Influence of soil temperature and moisture on infection of wheat seedling by Helminthosporium sativum . Journal of Agricultural Research . 1923 ; 26 : 195 – 217 . OpenUrl 37. ↵ Agrios GN . Plant pathology , 5th edn. Elsevier Academic Press , London . 2005 . 38. ↵ Degu T. , Yaregal W. , Gudisa , T. Status of common bean (Phaseolus vulgaris L.) diseases in Metekel Zone, North West Ethiopia . Journal of Plant Pathology Microbial . 2020 ; 11 : 494 . doi: 10.35248/2157-7471.20.11.494 OpenUrl CrossRef 39. ↵ Yuen J. Deriving decision rules . Plant Health Instructor . doi: 10.1094/PHI-A2006-0517-01 . OpenUrl CrossRef 40. ↵ McCullagh P. , Nelder JA . Generalized linear models , 2nd edition. In: Chapman and Hall, London . 1989 : 511 . 41. ↵ Getinet A. , Yalew D. Demonstration of Recommended Fungicides against Onion Purple Blotch Disease in Fogera Plains, Ethiopia . Journal of Plant Biology and Crop Research . 2022 ; 5 ( 1 ): 1046 . OpenUrl 42. ↵ Cook H.T. Studies on the downy mildew of onions . Journal of the Department of Agriculture of Western Australia . 1932 ; 3 :( 4 ) 313 - 314 , 317-313. OpenUrl 43. ↵ Yarwood C.E. Relation of light to the diurnal cycle of sporuiation of Peronospora destructor , Journal of Agricultural Research . 1937b ; 54 : 365 – 373 . OpenUrl 44. ↵ Farahanaz Rasool , Nighat Mushtaq , Divya Slathia , Sharafat HussainDiseases of Major Bulbous Vegetable Crops and Their Management . 2019 . 45. ↵ Palti J. Epidemiology, prediction and control of onion downy mildew caused by Peronospora destructor Phytoparasitica . 1989 ; 17 : 31 – 48 . OpenUrl 46. ↵ Gilles T. , Phelps K. , Clarkson JP. , Kennedy R. Development of milioncast, an improved model for predicting downy mildew sporulation on onions . Plant Disease . 2004 ; 88 : 695 – 702 . OpenUrl PubMed 47. ↵ Priyanka J , Mahendra K.M. , Mali B.L. Impact of abiotic factors and age of host plant on purple blotch of onion caused by Alternaria porri (Ellis) and estimation of yield losses . Journal of Plant Development Sciences . 2017 ; 9 ( 5 ): 447 – 451 . OpenUrl 48. ↵ McKay R. The longevity of the oospores of onion downy mildew Peronospora destructor (Berk.) Casp . In The Scientific Proceedings of the Royal Dublin Society; Royal Dublin Society: Dublin, Ireland . 1957 ; 295 – 307 . 49. ↵ Abd-Elrazik A.A , Lorbeer J.W. A procedure for isolation and maintenance of Peronospora destructor on onion . Phytopathology . 1980 ; 70 : 780 – 782 . OpenUrl 49. Abd-Elrazik A.A , Lorbeer J.W. A procedure for isolation and maintenance of Peronospora destructor on onion . Phytopathology . 1980 ; 70 : 780 – 782 . OpenUrl 50. ↵ Pandotra VR . Purple blotch disease of onion in Punjab . Proceeding Indian Academic Science . 1964 ; 60 : 336 −340. OpenUrl 51. ↵ Hoepting C. A. Efficacy of fungicide treatments for control of Stemphylium leaf blight on onion, 2017 . Plant Disease Management Report: Report No. 12 . 2018 ; 146 . 52. ↵ Muimba-Kankolongo A. Vegetable production . Food crop production by smallholder farmers in Southern Africa, Academic Press, London . 2018 : 205 −274. 53. ↵ Gupta RBL. , Pathak VN . Morphological, cultural and pathogenic variations in Alternaria porri (Ellis) Cif, the incident of purple blotch of onion . Zentralblatt Mikrobiologie . 1987 ; 142 : 155 – 162 . OpenUrl 54. ↵ Vidal T. , Gigot C. , Vallavieille-Pope C. , Huber L. , Saint S. Contrasting plant height can improve the control of rain-borne diseases in wheat cultivar mixture: Modelling splash dispersal in 3-D canopies . Annals of Botany . 2018 ; 121 ( 7 ): 1299 – 1308 . DOI: 10.1093/aob/mcy024 . OpenUrl CrossRef PubMed 55. ↵ Newhall A.G. The spread of onion mildew by wind borne conidia of Peronospora . destructor. Phytopathology . 1936 ; 28 : 257 – 269 . OpenUrl 56. ↵ Yarwood C.E. Onion downy mildew . Hilgardia . 1945 ; 14 : 595 – 691 . OpenUrl 57. ↵ Shrivastav P.K. , Bharadwaj B.S. , Gupta P.P. Status of field diseases and selected pest of onion in India . National Horticulture Research Development Found newsletter . 1994 ; 14 : 11 – 14 . OpenUrl 58. ↵ Mishra R.K. , Jaiswal R.K. , Kumar D. , Saabale P.R. , Singh A. Management of major diseases and insect pests of onion and garlic: A comprehensive review . International Journal of Plant Breeding and Crop Science . 2014 ; 6 ( 11 ): 160 – 170 . OpenUrl 59. ↵ Rashid M.H. , Haque M.M. , Bakr M.A. , Islam M.R. Effect of chemicals and environment friendly components on growth parameters and yield contributing character of onion (Allium cepa L .). Journal of Agriculture and Food Technology . 2015 ; 585 ( 2 ): 8 – 14 . OpenUrl 60. ↵ Kucharek T. Florida plant disease management guide: Onion . Plant Pathology Department Document PDMG-V3-41, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville, FL . 2013 . 61. ↵ Wright P.J. , Chynoweth R.W. , Beresford R.M. , Henshall W.R. Comparison of strategies for timing protective and curative fungicides for control of onion downy mildew (Peronospora destructor) in New Zealand . Proceeding of BCPC conference Pests and Diseases . 2002 : 207 – 212 . 62. ↵ Gianessi L.P. , Reigner N. The value of fungicides in U S crop production . Crop Life foundation crop protection research institute. Washington, DC . 2005 : 243 . View the discussion thread. Back to top Previous Next Posted September 01, 2025. Download PDF Email Thank you for your interest in spreading the word about bioRxiv. NOTE: Your email address is requested solely to identify you as the sender of this article. Your Email * Your Name * Send To * Enter multiple addresses on separate lines or separate them with commas. You are going to email the following Distribution and association of biophysical factors with onion (Allium cepa L.) downy mildew (Peronospora destructor) disease epidemics in northwestern Ethiopia Message Subject (Your Name) has forwarded a page to you from bioRxiv Message Body (Your Name) thought you would like to see this page from the bioRxiv website. Your Personal Message CAPTCHA This question is for testing whether or not you are a human visitor and to prevent automated spam submissions. Share Distribution and association of biophysical factors with onion ( Allium cepa L.) downy mildew ( Peronospora destructor ) disease epidemics in northwestern Ethiopia Mitku Bitew , Addisu Mandefro , Yehizbalem Azmeraw bioRxiv 2025.08.26.672276; doi: https://doi.org/10.1101/2025.08.26.672276 Share This Article: Copy Citation Tools Distribution and association of biophysical factors with onion ( Allium cepa L.) downy mildew ( Peronospora destructor ) disease epidemics in northwestern Ethiopia Mitku Bitew , Addisu Mandefro , Yehizbalem Azmeraw bioRxiv 2025.08.26.672276; doi: https://doi.org/10.1101/2025.08.26.672276 Citation Manager Formats BibTeX Bookends EasyBib EndNote (tagged) EndNote 8 (xml) Medlars Mendeley Papers RefWorks Tagged Ref Manager RIS Zotero Tweet Widget Facebook Like Google Plus One Subject Area Pathology Subject Areas All Articles Animal Behavior and Cognition (7635) Biochemistry (17691) Bioengineering (13892) Bioinformatics (41936) Biophysics (21452) Cancer Biology (18588) Cell Biology (25504) Clinical Trials (138) Developmental Biology (13378) Ecology (19899) Epidemiology (2067) Evolutionary Biology (24320) Genetics (15609) Genomics (22506) Immunology (17736) Microbiology (40394) Molecular Biology (17181) Neuroscience (88605) Paleontology (666) Pathology (2832) Pharmacology and Toxicology (4824) Physiology (7641) Plant Biology (15153) Scientific Communication and Education (2045) Synthetic Biology (4294) Systems Biology (9825) Zoology (2271)