Epidemiology and Management of Ascochyta Blight Disease (Ascochyta fabae Speg.) of Faba Bean (Vicia faba L.) in North Western Ethiopia

Abstract

Faba bean is an economically important and main pulse crop farmed in Ethiopia's mid- and high-land regions and is used both as a staple food and as a cash crop to manage seasonal food shortages. However, Ascochyta blight is an economically important disease that causes 90%–100% yield loss on faba beans.Therefore, this study was conducted to examine the importance, distribution, epidemiology, and development of management options against Ascochyta blight disease for the improvement of faba bean production in North Western Ethiopia. Survey assessment, field trial evaluation, bioassay, and laboratory experimental study methods were used to achieve the general and specific objectives. In the survey assessment study, Ascochyta blight disease intensity and agro-ecological and cultural agronomic practice data were collected. The field experiment contained three sets, namely fungicide efficacy, rate, and spray frequency, of disease management trials that were conducted. The first set of the field experiment contained twelve treatments (three faba bean varieties, three fungicides, and untreated). The second set of the field trial also contained twenty treatments (three fungicides, three rates, two varieties, and untreated). Similarly, the third set of field experiment contained twenty treatments (three fungicides, three spray frequencies, two varieties, and untreated). Randomized complete block design (RCBD) and factorial combination of faba bean varieties with fungicide treatments in three replications were applied for each field trial evaluation study. In the field trial evaluation study, the Ascochyta blight disease intensity, plant growth, and yield data of the faba bean were collected.The laboratory experimental study was comprised of the morphological and molecular study methods. In a laboratory experimental study, the genera and species of Ascochyta blight pathogen isolates, the Bacillus and Trichoderma isolates, were identified. The bioassay study was comprised of detached leaf and dual growth media and laboratory study methods. In the bioassay study, the pathogenic variability of Ascochyta blight pathogen isolates on Faba bean biotypes was evaluated. In addition, the potential antagonistic activities of Bacillus and Trichoderma isolates against the pathogen isolates were evaluated. The inhibition zone, lysis activity, percent area of leaf necrosis reduction effect, and percent mycelia growth inhibition effect of the biocontrol agents against the pathogen isolates were collected during the bioassay study. The faba bean Ascochyta blight disease incidence and severity were analyzed using a logistic regression model. ANOVA was also performed using SAS software to separate treatment means at 5% using LSD. The cause-and-effect relationship of the pattern of disease epidemics (AUDPC) with average grain yield (AGY) was determined by correlation and simple linear regression analyses. A partial-budget analysis was undertaken to analyze the cost-benefit associated with each set of field trial treatments using input costs and output sales at current harvesting season prices. According to the survey study results, the mean disease incidence and severity were 99.5% and 76.9%, respectively. The agro-ecological factors and farm agronomic practices had high significant probability association with Ascochyta blight epidemics. The spatial disease severity distribution in Debark, Janamora, and Laygayint districts was 70–80%; in Dembiya, Gondar Zuriya, and Libokemkem districts, it was 80–90%; and in Dabat, Wogera, and Farta districts, it was greater than 90%. The spatial disease incidence distribution in all districts surveyed was >95%.Therefoe, Ascochyta blight was found to be common and destructive disease in nine major faba bean producing districts of North western Ethiopia, which was predominantly caused by the virulent pathogen, A.fabae, while, A.fabae var. vicia was also widespread and virulent for faba bean production in the region.The pathogenic variability evaluation results of the study indicated that Ascochyta fabae was more virulent on both faba bean varieties in all the study areas. Whereas, the Ascochyta fabae var. vicia strain was less virulent in an altitude range of 1884 to 2312 masl but became more virulent as the altitude increased from 2312 to 3214 masl on both faba bean varieties. Inaddition, Ascochyta blight severity was highly restricted by the tolerant variety and reasonably contained on local susceptible check. Therefore, A.fabae was the abundant and predominant pathogen of faba bean Ascochyta blight disease complex, while, A.fabae var. vicia was also widespread in north-west Ethiopia. By using 18S rDNA sequencing and the BLAST search, F1H, F2H, F3H, F4H, and F5H isolates were identified as Ascochyta fabae, Ascochyta fabae var. vicia, Trichoderma lixii, Trichoderma viride, and Penicillium oxalicum strains, as mentioned. In addition, by using 16S rRNA sequencing and the BLAST search, the B1H and B2H isolates were identified as Bacillus megaterium and Bacillus megaterium var. aryabhattai strains, respectively. The present study for exploring the biological control agents for Ascochyta blight disease of faba bean investigated that Trichoderma lixii, Trichoderma viride, Penicillium oxalicum, Bacillus megaterium, and Bacillus megaterium var. aryabhattai strains were the natural residents of faba bean leaves in North Western Ethiopia. In addition, these biocontrol isolates showed different promising antagonistic potential to control Ascochyta blight disease. As a result, T.lixii, T.viride, B.megaterium and B.megaterium var aryabhattai strains had promising antagonistic potential for Ascochyta blight disease control, on both varieties, under detached leaf and dual growth media tests. Among the three sets of fungicide treatments applied in three field trials, Mancolaxyl (3 kg/ha) and Mancozeb (2kg/ha) in twice spray frequency levels combined with both faba bean varieties were significantly (P< 0.01) effective and economically feasible for Ascocchyta blight disease management,with low disease intensity and relative yield loss, as well as high plant growth, grain yield, total return, net return, and net benefit values, besides MRR>100%. Demonstration (on farm training) for faba bean growers, on agroecollogical variables and cultural crop management practices, which had strong probability associations with both disease intensity outcomes, is very important to adopt effective and integrated disease management practices. Further differential test and population genetic study by collecting more pathogen isolates from different faba bean farm areas is very important to determine the race composition and virulence disparity. The tolerant variety combined with T.lixii or B.megaterium strains are best to apply for integrated and sustainable management of Ascochyta blight under field conditions. Inaddition, further resistant breeding study on the reactions of different varieties with Ascochyta blight severity under field experiment is very important. In general, to control the disease and improve faba bean production in North Western Ethiopia, Mancolaxyl 3 kg/ha with twice spray frequency levels integrated with the tolerant (Hachalu) variety, in a seven-day interval, starting from the onset of the disease, in the early growth stage of the crop, is best suggested, as a first choise, to be applied by the local farmers of the region, followed by Mancozeb. Furthermore, Trichoderma lixii and Bacillus megaterium strains are also best suggested to be used as biocontrol agents for the management of faba bean Ascochyta blight disease under in vitro conditions. However, further study should be conducted on their rhizosphere association and their plant growthpromoting properties under field conditions.