SPATIAL DISTRIBUTION OF COMMON BACTERIAL BLIGHT (Xanthomonas axonopodis pv. phaseoli), AND EVALUATION OF COMMON BEAN GENOTYPES RESISTANCE REACTION AGAINST THE DISEASE IN SOUTH OMO, SOUTHERN ETHIOPIA

Abstract

Common bacterial blight (Xanthomonas axonopodis pv. phaseoli) is an economically important disease of common beans and causes significant yield losses in Ethiopia. However, there is limited empirical research information about the distribution and relative importance of CBB, and possible management options in South Omo, Southern Ethiopia. Thus, the study was conducted to determine the distribution, importance and intensity of CBB epidemics in association with agro-ecological factors, and to evaluate common bean genotypes resistance reaction against the disease in South Omo, Southern Ethiopia. A total of 85 farmer fields were surveyed in four major common bean growing districts during 2018 main cropping season and the disease was found prevalent in all districts. The highest mean incidence (77.6%) and severity (52.7%) were recorded at Debub Ari while the lowest incidence (38.5%) and severity (28.0%) were from Male district. Well weed managed and NPS fertilized fields reduced severity by 45.8% and 38.5%, respectively. Intercropped fields also reduced disease severity by 41.3% in comparison with sole cropped. The associations of CBB incidence and severity with independent variables were analyzed in logistic regression model. Variables like districts, NPS fertilization, crop growth stage, previous crops, weed management and plant population were very highly significantly (p<0.0001) associated with both incidence and severity of CBB. Low incidence (≤50) had high probability of association with Male district, more than twice ploughed farms, recommended rate of NPS fertilization, flowering growth stage, groundnut + maize preceding crops, weeded and less populated fields in row planting pattern. Likewise, low severity (<40) had high probability of association with Male district, low(<1200 m.a.s.l.) altitudinal ranges, recommended rate of NPS fertilization, flowering to podding growth stages, weeded and less populated fields in intercropping system preceded by groundnut + maize crops. The field survey assured that CBB was a major production constraint implying that special attention should be given to weed management, adequate NPS fertilizer application, removal of previous crop debris, selection of planting date to avoid coincidence with disease occurrence, frequent tillage and bean intercropping with other non host crops in row planting pattern; plant population, seed source and altitudinal ranges should also be considered to manage the disease. In addition, field experiments were conducted with 20 common bean genotypes with a randomized complete block design with three replications. The percent severity ranges 19.8% to 74.5% at JARC and 24.1 to 66.9% in Debub Ari and variation in disease progress rate ranged from 0.017 to 0.137 and 0.03 to 0.147unit day-1 in that order. Tested genotypes revealed different reaction type in which, 20% resistant, 55% moderately resistant and 25% susceptible at JARC, and 15% resistant, 80% moderately resistant and only 5% susceptible in Debub Ari. Across fields, SER 119 and MALB 135 showed consistent resistance reaction against CBB and SER119 also gave high yield in both locations. Thus, it is better to use SER119 with proper management practices in production to reduce CBB epidemics in South Omo and related agro-ecologies and MALB135 as parent for CBB resistance development in the plant breeding programs of the country.