SPATIAL DISTRIBUTION, AND CHARACTERIZATION OF MUNGBEAN HALO BLIGHT (Pseudomonas savastanoi pv. phaseolicola) AND ITS MANAGEMENT IN THE RIFT VALLEY OF ETHIOPIA

Abstract

Mungbean (Vigna radiata L.) is an important legume crops, grown throughout the world including in Ethiopia. However, halo blight (HB) caused by Pseudomonas savastanoi pv. phaseolicola is one of the major biotic factors that threaten mungbean production in the Rift- Valley of Ethiopia. Nevertheless, there was scant information on the disease distribution and importance, associated risk factors, genotypes reaction, and other management options. Thus, the present study was conducted to assess the distribution of halo blight and associated factors, characterize P.savastanoi pv. phaseolicola isolates and evaluate the efficacy of mungbean genotypes,seed treatment and foliar spray bactericides against the disease in major mungbean growing areas of Rift Valley. Field survey was conducted across four mungbean growing districts and 60 fields were selected to determine the distribution and importance of HB intensity and its associated factors across the targeted districts during the 2021 main cropping season. P.savastanoi pv. phaseolicola isolates collected from the study areas were identified based on morphological,biochemical and pathogenicity tests. Twenty-five mungbean genotypes were evaluated for HB resistance in glasshouse. The treatments were laid out in a completely randomized design with five replications. Seed treatments followed by foliar sprays were evualated as management options. The overall, survey revealed that, the mean disease prevelance, incidence and severity were found to be 93.33, 62.35 and 22.61%, respectvely. Among districts, the maximum mean diseases incidence (88.48%) and severity (36.56%), and minimum (28.23 and 6.3%) were recorded from Adama and Kucha districts, respectively. Logistic regression model was used to analyze the association of biophysical factors with HB incidence and severity. Regarding the characterization of P.savastanoi pv. phaseolicola isolates, 45 were identified as P.savastanoi pv. phaseolicola based on colony morophology and biochemical tests. All isolates were morphological white, flat, round, entire and mucoid colonies on the NA medium and biochemically a positive response in the KOH test, levan production, tobacco HR, and sucrose where as a negative results in the oxidase test, arginine and esculin hydrolysis, dulictol, sorbitol, maltose, lactose, and inositol. All isolates were found pathogenic to the susceptible Shewarobit variety, of which MG4 was the most virulent isolate and was used to evaluate the level of disease resistance in 25 genotypes. Halo blight mean severity revealed that, there were significant differences in disease resistance among the genotypes. Thus, Black Bean and NM94 found resistant, while NLLP-MGC-24, VC6368, and MB614805-12 were moderately resistant, the remaining were susceptible or highly susceptible to HB. Among all treatments, seed treated with streptomycin sulfate followed by foliar spray of Kocide were most effective in reducing the HB intensity. Overall, this study proved that P.savastanoi pv. phaseolicola is found in diverse populations with different degree of virulence in the Rift Valley of Ethiopia and can be considered as a threat of mungbean production. However, mungbean HB could be minimized by using resistant variety and seed treatment followed by foliar sprays. Future work should focus on genotype screening under mullti-location; seed treatment with diverse antibiotic and identification at molecular level.