Yield Loss Due to Septoria tritici Blotch (Septoria Tritici) of Bread Wheat (Triticum aestivum L.) in the Central Highlands of Ethiopia

Septoria tritici blotch (STB) is an economically important foliar disease on wheat. The current research was conducted to determine the yield loss of STB on wheat at Holeta and Kulumsa in 2017. Maximum relative grain yield loss (36%) was recorded on unsprayed plots of the variety Kekeba at Holetta. Whereas; at Kulumsa, maximum relative grain yield loss (39%) was on unsprayed plots of the variety Madawalabu. Disease severity, incidence and AUDPC showed highly significant and negative correlations with grain yield, spike length, plant height, and weight of kernel per spike, kernel number per spike, thousand seed weight and hectoliter weight. The present findings confirmed the importance of STB in Ethiopia and the role fungicides play in managing the disease on partially resistant varieties.

Overall STB has remained an important constraint to wheat production all over the world including in Ethiopia (Randhawa et al., 2019). However, effective and sustainable managing of the disease is yet to be achieved under Ethiopian condition. Septoria tritici blotch (STB) is one of the major diseases of wheat around the world and across wheat growing regions of Ethiopia. Yield loss assessment studies have been carried out in fewer areas and they are largely based on data from field surveys. As a result there it is very crucial to develop disease management option and recommended in areas, where the disease is prevalent and economically important. Thus, our study was designed with the objectives of determine the yield loss of each cultivars due to STB and determine the correlation between yield components and disease parameters.

Materials and Methods Study areas
Our study conducted at Holetta and Kulumsa Agricultural Research Centers, Ethiopia. Both sites are suitable for bread wheat production, and STB pressure is generally high during the rainy season (Birhan, 2011;Beyene et al., 2016).

Treatments and Experimental Design
The treatment arrangement of our experiment was (6 different spray schedules of propiconazole (Tilt 250 EC) and Mancozeb (unsprayed plot, Tilt, Mancozeb, Mancozeb-Tilt-mancozeb-Tilt, Tilt-mancozeb-Tilt-mancozeb and mancozeb-mancozeb-Tilt-Tilt) were combined with three wheat varieties (Alidoro, Kekeba and Madawalabu). Our treatments were arranged in randomized complete block design (RCBD) with three replications. The fungicide Tilt was applied at a rate of 0.5lt/ha and Mancozeb at a rate of 3kg/ha based on recommendation.

Data collected
Incidence of STB was assessed by counting the number of infected plants in the middle four rows and was expressed as percentage of total plants infected as shown below.
. * The severity of Septoria tritici blotch was recorded using the double-digit scale (00-99) (Saari and Prescott, 1975;Eyal et al., 1987). Percent disease severity is estimated based on the formula: % = ! * " " ! * Where D1 and D2 represent the score recorded (00-99 scale) and Y1 and Y2 represent the maximum score on the scale (9 and 9) (Sharma and Duveiller, 2007). #$%&' = ∑ )*+)*+ " ,* + − ,* /*0 *1 Where, Xi= the cumulative disease severity expressed as a proportion at the i th observation, ti = the time (days after planting) at the i th observation and n= total number of observations. AUDPC values are used in analysis of variance to compare amount of disease among different treatments. Relative losses in yield and yield component of each variety were determined as a percentage of that of the protected plots of the respective variety. Losses were calculated separately for each of the treatments as: 23 % = − " * % Where, RL -relative loss (reduction of the parameters yield, yield component), Y1 -mean of the respective parameter on protected plots (plots with maximum protection) and Y2 -mean of the respective parameter in unprotected plots (i.e. unsprayed plots or sprayed plots with varying level of disease). Percent yield recovery was also calculated to compare the yield difference among fungicide treatments using the formula: Where, YR is yield recovery in percent, YUP is yield of unsprayed plot and YSP is yield of sprayed plots. All agronomic, yield and yield related data were recorded on the middle four rows of each experimental plot.

Results and Discussion
Disease incidence in percent STB incidences were significantly different among varieties at both locations. The highest disease incidence was recorded on unsprayed plots of Kekeba variety, while the lowest disease incidence was recorded on Alidoro variety sprayed with Tilt fungicide (Table 1). This result strongly agreed with the finding of (Said et al., 2016), who reported maximum STB incidence value from unsprayed plots of each varieties.

Percent disease severity
At Holeta the highest (97%) severity was recorded on unsprayed plots of variety Kekeba. While the lowest (45%) disease severity was recorded from Alidoro variety sprayed with Tilt. At Kulumsa the highest (85%) disease severity was recorded from Madawalabu variety unsprayed plots, while the lowest (22%) disease severity was recorded from Alidoro variety sprayed with different fungicides. This indicates that the level of disease development is considerably affected by level of fungicide application or varietal resistance to STB.
In general, STB was severe in both locations; however, it was more severe at Holeta than at Kulumsa. According to results of the present study, the currently grown high yielding wheat variety, Kekeba, was the most susceptible to STB suggesting the need to prioritize the deployment of resistance genes. Use of resistant variety is the best control strategy to Septoria tritici blotch for resource poor farmers in developing countries and the most environmentally friendly and profitable strategy for commercial farmers (Tekelay et al., 2015).

Area under disease progress curve
STB AUDPC values across treatments ranged from 866 to 3879 at Holeta and from 592 to 2057 at Kulumsa (Table1). AUDPCs were generally higher on unsprayed plots than on sprayed plots. The maximum AUDPC values recorded on unsprayed plots were 3879%-days on wheat variety Kekeba, 2890%-days on Madawalabu and 1734%-days on Alidoro, at Holeta respectively. At Kulumsa, AUDPC values recorded as follows: 2057%-days, 1699%-days and 762 %-days, on Madawalabu, Kekeba and Alidoro varieties, respectively. Wheat variety Alidoro sprayed with MTMT fungicides schedules had the lowest (866%-days) at Holeta and variety Alidoro treated with MTMT fungicide had the lowest AUDPC (591%-days) at Kulumsa. All fungicide spray schedules have reduced AUDPC compared to the unsprayed plots. This agrees with that of Takele et al., (2015) who reported maximum AUDPC values (2275%-days) from unsprayed plots.

Yield and Yield Components Loss Yield loss
At Holeta, maximum relative grain yield loss was 36%, and this was recorded on unsprayed plots of the variety Journal of Biology, Agriculture and Healthcare www.iiste.org ISSN 2224-3208 (Paper) ISSN 2225-093X (Online) Vol.10, No.10, 2020 Kekeba (Table 2). On Madawalabu variety, grain yield loss of about 34% was recorded when STB was allowed to develop naturally. For Alidoro variety, grain yield reduction of about 35% was recorded on unsprayed plots.
At Kulumsa, maximum relative grain yield loss was 39% on unsprayed plots of the variety Madawalabu. On the variety Kekeba, grain yield loss of about 16.1% was recorded from unprotected plots, while grain yield reduction of about 21% was recorded on unsprayed plots of the variety Alidoro (Table 2).
Our result agrees with the findings of Said et al. (2016) in which STB was reported causing grain yield loss of up to 41%. It confirmed that susceptible genotypes showed higher yield losses as compared to resistant genotypes. Our results also confirm the findings of Goodwin et al. (2011) in which wheat grain yield losses of 30-50% were attributed to STB. Severe yield losses can occur in crops when the top two or three leaves (flag, second and third leaf of wheat plants) become infected (Ayele et al., 2019).

Loss in thousand kernel weight
At Holeta, thousand kernel weight (TKW) was most affected on wheat variety Kekeba. The loss in TKW on this variety reached 25% on unsprayed plots (Table 2). On the other hand, a 7% loss in TKW was recorded on unsprayed plots of variety Alidoro, which is considered the lowest loss in TKW.
At Kulumsa, thousand kernel weight (TKW) was most affected on wheat variety Kekeba. The loss in TKW on this variety reached 18% on unsprayed plots (Table 2). On the other hand, 6.8% loss in TKW was recorded on unsprayed plots of variety Alidoro, which is considered as the lowest loss in TKW. Current results are in agreement with previous reports of Takele et al. (2015) in which yield, thousand seed weight, hectoliter weight, weight of kernel per spike, losses, due to STB were recorded as 41, 36, 5 and 44%, respectively. The reduction in TKW is mainly due to the effect of the disease on the size and mass of the seed (shriveling of the kernels). Dill-Macky et al. (1990) reported up to 45% reduction in 1000-kernel weight of wheat variety Oxley in Australia. Infection of wheat head and leaf sheaths by septoria affects the transport of assimilates to the developing kernel and results in shriveled kernel (Sharma et al., 2002).

Hectoliter weight loss
Only the Tilt fungicide treatments showed significant effect (P< 0.05) over the control in terms of hectoliter weight of the variety Kekeba and Madawalabu. None of the spray treatments showed significant effect on hectoliter weight of Alidoro (Table 2). Reduction in test (hectoliter) weight is largely attributed to reduction in seed size. With small size more seeds would fit into a given volume. Even though not quantified, Ayele et al. (2019) reported pronounced effect of septoria on hectoliter weight and kernel plumpness of some winter Australian wheat cultivars. However, the effect may vary depending on the variety and location (Dill-Macky et al., 1990). Table 2

Yield and Yield Components Recovery
The highest yield recovery over unsprayed plot, 55% and 65%, were recorded from Kekeba variety sprayed with MTMT at Holeta and Tilt sprayed Madawalabu variety at Kulumsa (Table 3). These results were higher than those obtained from the work of Tari et al. (2009), which resulted in up to 42% yield loss control by applying foliar fungicides to winter wheat. In general, on all varieties, STB developed after growth stage of Z70 (kernel and milk development stage) was found to be important for grain yield loss. So managing the disease before reaching this growth stage might provide reasonable recovery of yield. This might be related with protecting the top three leaves, especially flag leaf of the crop that contributes most to the grain yield (Vrapi et al., 2009).

Correlation between Disease Parameters, Yield and Yield Components
At both locations correlations among disease parameters were positive and highly significant (p ≤ 0.01), suggesting the possibility of using any of the parameters for STB assessment. However, cautions should be taken as each parameter is worth considering having a complete understanding of disease intensity. STB severity, incidence and AUDPC also showed highly significant and negative correlations with grain yield, spike length, plant height, and weight of kernel per spike, kernel number per spike, thousand seed weight and hectoliter weight (Tables 4 & 5). The highest value of correlation coefficient indicated strong relationships between and within disease parameters. Forrer and Zadoks (1983) also observed that the greatest risk to wheat crop occurs, when conducive environmental factors favor spore dispersal during and shortly after flag leaf emergence, and the crop losses have been related to total leaf area infected including necrotic lesions and chlorotic flakes.   0.97** 0.92** 0.99** -0.56** -0.45** -0.31* -0.31* -0.19 ns -0.19 ns PDI=Percent disease incidence, PDS= Percent disease severity, AUDPC= Area under disease progress curve, SL= Spike length, PH= Plant height, NKPS= Number of kernels per spike, YLD= Yield(t/ha), TKW= Thousand kernel weight, HLW= Hecto litter weight and MDS= Mean disease severity.*:refers to mean square values significant at α=0.05, **: refers to mean square values significant at α=0.01, ns: refers to mean square values not significant at α=0.05

Conclusion and recommendation
A field experiment was conducted at Holetta and Kulumsa in 2016 main cropping season to determine the effect of STB disease on yield and yield components loss and recovery of bread wheat varieties. At Holetta, maximum relative grain yield loss was 36%, and this was recorded on unsprayed plots of the variety Kekeba whereas at Kulumsa, maximum relative grain yield loss was 39% on unsprayed plots of the variety Madawalabu. The highest yield recovery over unsprayed plot, 55% and 65%, were recorded from Kekeba variety sprayed with MTMT at Holeta and Tilt sprayed Madawalabu variety at Kulumsa. STB resulted in significant yield loss of bread wheat varieties, when left unchecked. STB severity, incidence and AUDPC also showed highly significant and negative correlations with grain yield and yield components.