Biophysical Factors Associated with Septoria Leaf Blotch

Journal of Environmental and Agricultural Sciences (JEAS). Alamirew et al., 2020. Volume 22(3): 13-22

Open Access – Research Article

Septoria Leaf Blotch (Mycosphaerella graminicola) Incidence on Bread Wheat (Triticum aestivum L.) and its Association with Biophysical Factors of Western Amhara, Ethiopia
Belayneh Alamirew1, 2*, Merkuz Abera3, Ayele Badebo4
1Adet Agricultural Research Center, Bahir Dar, Ethiopia

2Universidad Politècnica de Valencia, Spain
3Bahir Dar University, College of Agriculture and Environmental science, Bahir Dar, Ethiopia
4International Maize and Wheat Improving Center, CIMMYT, Addis Ababa, Ethiopia

Abstract: Autumn Bread wheat (Triticum aestivum L.) is one of the most important small grain cereals produced worldwide. Despite its economic significance, wheat production and productivity is challenged by biotic and abiotic factors. Septoria leaf blotch (Mycosphaerella graminicola) is among the most important pathogens that inflict qualitative and quantitative losses on susceptible wheat varieties in the highlands of Western Amhara, Ethiopia. However, the relative importance of this pathogen has not been well documented to develop a sound management strategy in the studied areas. The survey was conducted during the  main cropping season (2017) to assess the Septoria leaf blotch incidence, severity, and management of the disease with cultural practices in nine districts of western Amhara. During the study, a total of 120 farmers’ fields in nine districts were surveyed. The field survey results revealed that there was96.7% prevalence of Septoria leaf blotch on bread wheat and only four fields (3.3%) were found free of infection. Mean incidence of Septoria leaf blotch varied from 33.7% in Burie to 90.2% in Farta district. The severity also ranged from 5.31 to 30.61% in the 116 fields. Wheat variety, wheat population density, previous crop, soil type, weed density and cropping system were significantly associated with Septoria leaf blotch incidence and severity. However, the impact of altitude on the specific disease incidence remained non-significant. In general, the current study suggests that the disease has become severe and a serious constraint in most assessed fields. So, the findings of the present study indicate the need to undertake different cultural practices, including late sowing, rotating with non-cereal crops (legumes) and mixed cropping system for the integrated management of Septoria leaf blotch. However, germplasm screening and selecting tolerant varieties could be the other option for the management of the Septoria leaf blotch in the future.

Keywords: Disease Associations, Disease Incidence, Importance, Septoria leaf blotch, Disease Severity, Variable.

*Corresponding author: Balayneh Alamirew:

Cite this article as:
Alamirew, B., M. Abera and A. Badebo. 2020. Septoria leaf blotch (Mycosphaerella graminicola) incidence on bread wheat (Triticum aestivum L.) and its association with the agroclimatic conditions of Western Amhara, Ethiopia. Journal of Environmental & Agricultural Sciences. 22(3): 13-22.
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Copyright © Alamirew et al., 2020. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium provided the original author and source are appropriately cited and credited.

1. Introduction
Bread wheat (Triticum aestivum) is among the most important cereal food crops produced worldwide (Curtis, 2002; Miransari and Smith, 2019), which is grown on a large scale in the tropical and subtropical regions of the world (Gibson and Benson, 2002). About 75% of the world’s population consumes wheat as part of the daily diet. Ethiopia is the largest producer in sub-Saharan Africa (SSA) and the second-largest wheat producer next to South Africa in Africa (ATA, 2015). According to recent estimates, there are approximately 4.76 million farmers who are growing wheat on around 1.74 million hectares of land (CSA, 2019). This represents between 15 and 18% of total cropped areas and 20% of cereal consumption. The national average productivity of wheat in Ethiopia has been estimated at 0.2764 t ha-1 (CSA, 2019). Concerning regional contribution to wheat production, Oromia Region produces the highest (57.4%) volume of wheat grain during the main crop season, followed by Amhara Region (27%).  The Amhara Region shared 570,742.91 ha of the total area under wheat cultivation, contributing 1.4 million tons of wheat grain to the national annual production while the productivity is about 0.25 t ha 1(CSA, 2019).

Despite the economic importance of wheat in the country, its production and productivity are limited by various biotic and abiotic factors (Tesfaye et al., 2001; Merkuz, 2017). Biotic stresses include diseases, lack of improved varieties, insect pests and weeds, and abiotic constraints include drought, soil acidity, alkalinity, extreme temperatures, depleted soil fertility and low adoption of new technologies (Merkuz, 2017). Septoria leaf blotch (SLB) (Mycosphaerella graminicola, anamorph, Septoria tritici) (Allioui et al., 2016), is currently one of the most important damaging disease of bread wheat (Triticum aestivum L.) and durum wheat (Triticum turgidum L. subsp. durum) in many parts of the world (Boukef et al., 2012). This pathogen is rated as one of the top ten economically critical fungal pathogens in the world (Dean et al., 2012) and the most important disease in Ethiopia (Eshetu, 1986; Mengistu et al., 1991).

Septoria leaf blotch can cause grain yield losses ranging from 30 to 70% (Eyal et al., 1987). It has also become very severe disease resulting in up to 82% grain yield loss in all released bread wheat cultivars in Ethiopia (AARC, 2000; KARC, 2005). Similarly, at the West Shewa zone, Ethiopia estimated 41% yield losses of bread wheat have been recorded due to septoria leaf blotch (Abera et al., 2015). Although recently various methods are used for the detection and diagnosis of cop disease (Chouhan et al., 2020; Zhao et al., 2020). However, field surveys are helpful tools in studying the distribution and importance of different crops and their diseases (Ruska et al., 1997). Research information on the epidemiological factors influencing disease development is important to propose appropriate management options. Aange of disease management options is recommended to control Septoria leaf blotch in wheat fields.  Among these, cultural management options designed to reduce inoculum pressure are the first one. These include rotation to non-hosts, field sanitation by deep incorporation of crop debris to decrease the amount of inoculum available to initiate a new disease cycle. Knowledge of the distribution of SLB over time and across locations is essential for the further development of disease management strategies

Therefore, the present study was carried out with the specific objectives to:(1) assess the distribution, relative importance and the intensity of Septoria leaf blotch on bread wheat;(2) determine the association of biophysical factors and cultural practices with the disease intensity.

2. Materials and Methods
2.1. Study area
A field survey was conducted in the field of bread wheat crop in three major wheat-producing zones of western Amhara. Disease survey was carried out in 9 districts, including two in South Gondar, three in East Gojjam and four in West Gojjam, during the main growing season of 2017. The surveyed districts namely Lay Gayint and Farta (South Gondar Zone); Yilmana Densa, Burie and Wonberima (West Gojjam Zone); and Baso Liben, Debre Elias, Goncha Siso Enese and Hulet Eju Enese (East Gojjam Zone). Roadside survey was carried out, and farmers’ fields were randomly assessed depending on the availability of bread wheat fields at an interval of every 5-10 km with odometer readings. A Mixed farming system is the main occupation of the farmhouse holds. The surveyed districts are known for their high wheat yield potential.

2.2. Disease Assessment
For A total of 120 fields were assessed for Septoria leaf blotch incidence and severity. The field survey was conducted once at different growth stages of the wheat crop. In each field, data were recorded by walking along the two diagonals of wheat fields in “X” pattern. Exactly ten randomly selected plants were considered at five spots of 1m×1m (1m2) quadrat sample area to determine the disease severity and incidence and the average of the five quadrats was used for analysis.

During the field survey several parameters, including disease prevalence, incidence, severity, and agronomic data as follows:

Disease prevalence (%) =  ×100

The severity of Septoria tritici blotch was recorded using the double-digit scale (00–99) developed as a modification of Saari and Prescott’s severity scale to assess wheat foliar diseases (Saari and Prescott,1975).

Table 1. Categorization of variables used for logistic regression analysis of SLB (n=120) in western Amhara, Ethiopia, during main cropping season (2017)
Categorization of variables used for logistic regression analysis of Septoria leaf blotch

Disease Severity (%)= (D1/9 ×D2/9) ×100

where the first digit (D1) indicates disease progress in plant height (the relative height of the disease) and the second digit (D2) refers to severity measured as the diseased leaf area.

Besides the disease intensity data, agronomic (cultural) practices were recorded on each field to analyze their association with Septoria leaf blotch. Variety(ies) grown whether local or improved, previous crop (barley, maize, tef, wheat/triticale, legumes, oil crops, pepper and potato) and planting date (late May, early June, late June, early July, or late July) were collected through interview with farmers, filling structured questionnaires and visual assessment. The average of five quadrats (1m2) of wheat population [high (>210 plants/m2), medium (140-210 plants/m2and low (<140 plants/m2)] and weed density [low (<40/m2), medium (41-60/m2) and high(>61/m2)] (Getaneh et al., 2006; Bogale and Amare, 2014) were taken. Altitudes: high altitude (>2500) or mid-altitude (2500-1500) m.a.s.l. of each field was measured using Global Positioning System (GPS). Soil types (whether black, light sandy or red soil) were recorded in each field and the cropping system (whether sole crop or mixed cropping) was recorded in each farmer’s field..

2.3. Statistical Data Analysis
In Field survey data were analyzed using SAS 9.0 version software (SAS. 2002) for the disease intensity levels, and the incidence and severity were associated with the independent variables, using the logistic regression model by forming binomial data. The logistic regression model assesses the importance of multiple independent variables that affect the response variable. It calculates the probability of a given binary outcome as a function of the independent variables. The binary outcome was the probability that SLB severity exceeds 25% and incidence exceeds 50% (Table 1) in each wheat field based on Bogale and Amare (2014). Exponentiation of the parameter estimates of each variable class results the odds ratio, indicating the relative risk. The SAS procedures GENMOND and logistic were used to estimate the parameter estimates (McCullagh and Nelder, 1989).  The chi-square test was used to detect significant differences among the treatments.

3. Results and Discussion
3.1. Prevalence and distribution of Septoria leaf botch in Western Amhara
Septoria leaf blotch infected fields attributed about 96.7% out of the total 120 fields assessed, at different crop growth stages across the nine districts. Only four fields (3.33%) were found free from Septoria leaf blotch infection. This result implicated that Septoria leaf blotch is found to be the most distributed diseases during 2017 main cropping season. The reason for the high prevalence of the disease might be due to the frequent cultivation of early sown susceptible varieties, like Kakaba and local varieties, which served as a source of initial inocula, and favorable environmental conditions for the disease development and spread. Similarly, Wallelign et al. (2014), indicated that Septoria leaf blotch was the most aggressive disease in all assessed bread wheat fields of South Gondar, East Gojjam, Awi and West Gojjam. These results are also in line with the observation of Endale and Getaneh (2015). They reported that there was 100% SLB distribution/prevalence in Southwest and West Shewa Zones of Oromia Regional State, Ethiopia. Similar studies in Bale wheat-producing areas, Oromia regional state, Ethiopia indicated that Septoria leaf blotch found to be the first distributed disease across the surveyed regions (Kasa et al., 2015).

3.2. Septoria leaf botch Incidence and Severity
The incidence of Septoria leaf blotch in western Amhara ranged from 0 to 100% (Table 2). Of the surveyed 120 fields, 46 fields (38.33%) had less than or equal to 50% incidence and 74 fields (61.67%) had more significant disease incidence (> 50%). Only four fields had no infection by Septoria leaf blotch. Overall mean incidence over local and improved bread wheat varieties varied from 82.23% in the local varieties to 64.90% in improved varieties (Table 3).

In South Gondar Zone the mean incidence was 49.66%, while 90.17%) mean incidence was recorded in Farta. In Western Gojjam, the mean incidence of Septoria leaf blotch was 33.69,84.30 and 69.93%, in Burie, Wonberima, and Yilmana Densa, respectively.  

Table 2. Logistic regression modeling of wheat Septoria leaf blotch incidence and severity for eight independent variables as single predictor of disease outcome
Logistic regression modeling of wheat Septoria leaf blotch incidence and severity

Table 3. Minimum, maximum and mean incidence (%) and severity (%) of septoria leaf blotch for different independent variables in western Amhara in 2017 cropping season
Minimum, maximum and mean incidence (%) and severity (%) of septoria leaf blotch
Min: Minimum, Max: Maximum, SD: Standard Deviation

The mean incidence in East Gojjam zone ranged from 60.00% in Hulet Eju Enese to 82.66% in Baso Liben. High Septoria leaf blotch incidence was recorded from Baso Liben,Farta and Wonberima districts as compared to other districts, which had lower Septoria leaf blotch incidence (Table 3). This variation could be probably due to the prevailing favorable temperature and rainfall and the use of monocultural system of wheat after wheat production without rotation, which is conducive and a source of inocula for Septoria leaf blotch development. Similarly, Shaw and Royle (1993 and Lovell et al. (2004) indicated that variable periods of rainfall are critical factors in disease initiation and development.

The highest incidence of the disease was recorded from fields at higher altitudes (>2500 masl.) than on mid altitudes (1500-2500 m.a.s.l.) (Table2). Among the wheat fields surveyed for Septoria leaf blotch, the lowest (62.8%) incidence was recorded from fields sown in late July. Bailey et al. (2001) also reported that late sowing was found to be associated with increased grain yield and reduced Septoria leaf blotch infection levels.

Fields with red soil types, wheat/ triticale rotated fields, high weed density, high wheat population density, and sole cropping wheat fields as compared with their counter variable classes (Table 2). Similarly, Fakhfakh etal. (2009) indicated that susceptible cultivars along with favorable climatic conditions and common cultural practices are among the principal aggravating factors causing variable but significant grain yield losses attributed to Septoria leaf blotch. Burdon and Chilvers (1982) reported that dense stands contributed to severe Septoria leaf blotch epidemics and they argued that higher plant densities (population densities) would provide a micro-environment more conducive to the disease development than optimal planting densities.

Among the wheat fields surveyed, the minimum (17.85%) mean severity was recorded in improved bread wheat varieties and the highest (20.74%) mean severity was recorded in local or farmers’ varieties. Among the surveyed districts, the minimum mean severities of 5.31 and 10.12% were recorded in Burie and Hulet Eju Enese districts, respectively. In contrast, the highest mean severities of 25.13 and 30.61% were observed in Farta and Baso Liben districts, respectively (Table 2). This might be because these districts used the same variety (Kakaba) every year without change and perhaps the variety might has lost its resistance. Similarly, the use of crop rotation with other non-cereals is poorly practised in these districts; and in Farta, the rainfall and RH conditions were optimum for SLB epidemics (Table 3).

The least (17.23%) mean Septoria leaf blotch severity was recorded in wheat fields sown at the mid (1500-2500 m.a.s.l.) altitudes and wheat fields planted in late June (14.25%) and late July (13.82). Contrarily, the highest SLB severity was recorded from fields sown in late May and early June (Table 2). Wheat crop grown in red soils, fields rotated with barley and wheat/triticale, with high weed population (>61/m2), increased plant population (>210/m2) and sole cropping system had the highest severity as compared to their respective variable classes in the surveyed districts in the cropping season (Table 2). Similarly, Shaner et al. (1975) and Shaw and Royle (1993) reported that early sowing gives more time for infection and multiplication or development of SLB. Gladers et al. (2001) also reported that the high risk of Septoria leaf blotch severity periods was greatly affected by early sowing in England. Lebon et al. (2012) also reported that mixed cropping system served as a physical barrier to spore dispersal or microclimatic changes. Simón et al. (2003, 2005) reported that higher SLB incidence and severity had been attributed to an increased built up of primary inoculum of the Septoria leaf blotch from previous cereal crops, mainly when favorable conditions are prevalent.

3.3. Association of Independent Variables with the Disease Intensity
The association of the independent variables, namely variety, altitude, sowing date, population density, weed density, soil type, previous crop and cropping system with incidence and severity of Septoria leaf blotch was highly significant as Pr> χ2 is <0.001. However, altitude for SLB incidence was not highly significant as Pr> χ2is >0.001for SLB incidence (Table 2). This implies that all the described independent variables were risk factors and the different variable classes showed different risk levels for Septoria leaf blotch incidence and severity. All variables were tested in reduced multiple variable models with severity and incidence as the dependent variable. For added variables analysis of deviance, parameter estimates, standard errors resulting from the reduced regression model are given (Table 4and 5).This study revealed that Septoria leaf blotch severity was highly associated with higher (>2500 m.a.s.l.) altitudes than lower elevation. As the odd ratio indicates, there were about 1.44 times higher probabilities that SLB severity exceeds 25% in higher altitudes (>2500m.a.s.l.) than lower altitudes (Table 4). The finding in this current study agrees with another investigation made by Abera et al. (2015) who indicated that the intensity of disease increased in higher altitudes than in mid altitudes.

There were also high associations of early sowing dates with Septoria leaf blotch incidence and severity. There were 9.80- and 6.89-times higher probabilities that SLB incidence exceeds 50% in late May and early June sown fields and 10.11- and 3.32-times higher probabilities that SLB severity exceeds 25% in late May and early June sown fields than other planting dates. (Table 4 and 5). The result of this current study corresponds to the previous investigation by Shaw and Royle (1993) who showed that early sown plants produced more leaves, resulting in a greater presence of inocula and also Shaneret al. (1975) and Lovell et al. (1997 2004) showed similar situations that early sowing gives more time for the infection and multiplication to move between leaf layers from older to younger leaves. On the other hand, Eshetu and Zerihun (2003) also supports that late sowing significantly reduces Septoria development, though severity may not affect on kernel weight and grain yield.

Soil types also found to influence incidence of Septoria leaf blotch. The incidence and severity of Septoria leaf blotch were positively associated with red soil types compared to light sandy and black soils (Table 4 and 5). The parameter estimates resulting from the reduced regression model, indicating high SLB incidence (>50%), had a high probability of association to red soils. There were 17.50 times more chances that SLB incidence exceed 50% (Table 4).  Similarly, SLB severity was significantly associated with soil types, 1.82 times higher probabilities that the SLB severities exceeds 25% in red soils than in black and light sandy soils (Table 2). In this regard, Walters and Bingham (2007) discussed that soil factors, such as organic matter, nutrients, soil pH and similar others, have an impact on grain yield, plant disease and their interactions. It is also in line with the finding of Shaw, and Royle (1993) who reported that the availability of moisture in red soils due to high water holding capacity favors disease development.

Septoria leaf blight intensities had direct correlation with preceding or previous crops (Table 4). Disease severities were higher on wheat fields where wheat fields were preceded by cereals (such as wheat and barley). This might be due to infected stables or residues of previous wheat crops that served as source of ascospores for infection. There were 3.11- and 2.08-times higher probabilities in wheat fields that SLB severity exceeds 25% in fields rotated with barley and wheat, respectively, as compared to fields rotated with legume and oil crops (Table 4). These results were consistent with finding of Shaw and Royle (1989) who indicated that infected crop residues and volunteer wheat plants were important sources of primary inoculum pycnidiospores that survive in pycnidia on infected stubble for several months. Abera et al. (2015) have also reported high (100%) disease incidence and severity (70%) on wheat sown after cereals, while low (45%) severity on wheat fields sown after oil crops.

Weed population had an influence on the intensity of SLB. Septoria leaf blotch that had a high association with high (>61/m2) weed density. This could be due to an increase of relative humidity on the crop microclimate, which favors the pathogen and their competition for available soil nutrients. This agrees with the observation of Samuel et al. (2008) who reported high weed density resulted in high level of chocolate spot of faba bean. The result of this present study is also consistent with the finding of Duczek et al. (1996) who discussed that dense population of weeds in afield may contribute to disease development by increasing the density of plant canopy or serving as an inoculum reservoir in the absence of the susceptible host.

According to the result of the present study, SLB incidence and severity had a direct association with plant population (Table 4 and 5). There were 2.61 and 1.46 times greater SLB incidence and severity that exceed 50 and 25%, respectively, as compared to the medium (optimum) population. More significant disease development at higher plant density might be due to a more favorable microclimate produced within the leaf canopy than that produced at the lower densities. This is in line with a previous observation made by Chemeda and Yuen (2008) who reported that low maize rust incidence is associated with low maize population. Similarly, Tompkins et al. (1993) reported that high seedling rates and narrow spacing creates a favorable microclimate for the development of Stagonospora nodorum blotch and Septoria tritici blotch. In contrast, Rodgers and Shaw (2000) showed that reduced plant density in the field increased nutrient availability and aggravating the disease. Similarly, Shaw and Royle (1993) reported that reduced plant density increased the dispersal ability of SLB spores by rain splash.

Table 4. Logistic regression modeling of wheat Septoria leaf blotch incidence and severity for eight independent variables as single predictor of disease outcome
Logistic regression modeling of wheat Septoria leaf blotch incidence and severity

The present study demonstrates the effect of cropping system on SLB disease incidence. Mixed cropping system had shown less probability of being infested by SLB (Table 4). There were 1.96- and 1.41-times higher probabilities that SLB incidence exceeds 50% and severity exceeds 25% respectively in sole cropping fields than mixed wheat fields. The reason for reduction of disease severity in mixed cropping systems may be attributed to either the barrier effect or microclimatic changes. Similarly, Lebon et al. (2012) reported that a substantial reduction of severity of SLB was assessed on wheat-pea intercrops. Similarly Malezieux et al (2009) indicated that mixed species cropping, are important tool for pest and disease management particularly row intercropping and strip intercropping, by physical barrier to spore dispersal or microclimatic changes within the intercrop canopy.

4. Conclusion
Despite the importance of bread wheat in Ethiopia, particularly in Western Amhara, its production and productivity are constrained by biotic and abiotic factors. Septoria leaf blotch (Mycosphaerella graminicola) is one of the foliar pathogens that constrained wheat production in the study areas. The goal of this study was to generate information on the importance of Septoria leaf blotch and its association with biophysical and cultural practices and to draw possible recommendations for further Septoria leaf blotch integrated management. According to the data generated by main season disease survey during 2017, revealed that Septoria leaf blotch is an important potential disease constraining rainfed wheat production in western Amhara in wheat producing areas. The disease was widely distributed in all surveyed districts of western Amhara. Different independent variables (cultural practices) were observed to influence Septoria leaf blotch incidence and severity. According to the logistic regression model, varieties, plant populations, previous crops, soil types, weed densities and cropping systems significantly affected disease incidence and severity however, altitude was not significantly associated with the disease incidence. So, the findings of the current study indicate the need to undertake different cultural practices, including late sowing, rotating with non-cereal crops (legumes) and mixed cropping system for the integrated management of Septoria leaf blotch. However, detailed field surveys and due research attention should be given to find the mechanisms of variability of the pathogen and its existing physiological races.

List of Abbreviations: CSA: Central Statistical Authority; RH: Relative Humidity; SLB: Septoria Leaf Blotch.

Competing Interest Statement: The authors declare that there are no competing interests.

Author’s Contribution: All the authors have equal contribution to the planning, conduction and writing of this review article. All the authors have read and approved the final manuscript.

Acknowledgements: The Authors would like to acknowledge the Amhara Agricultural Research Institute (AARI) for the financial and technical support during the entire period of the study. Our gratitude also goes to the Delivering Genetic Gain in Wheat (DGGW) for the financial support rendering during the survey periods.

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