SPATIAL DISTRIBUTION, CHARACTERIZATION AND MANAGEMENT OF ENSET (Ensete ventricosum) BACTERIAL WILT (Xanthomonas campestrispv. musacearum) IN SOUTHWESTERN

Abstract

Enset bacterial wilt (EBW), caused by Xanthomonas campestrispv.musacearum (Xcm), is one of the highly destructive diseases of enset(Ensete ventricosum) in Ethiopia. However, its distribution and extent of damage, and biophysical factors associated with its epidemics were not well studied in southwestern Ethiopia. Similarly, diversity within the bacterial pathogen populations, and the pathogenicity of Xcm isolates and the reaction of selected enset clones to pathogenic Xcm isolates were not studied in the areas.Thus, the objectives of this study were to (i) determine the distribution of EBW and its association with biophysical variables; (ii) characterize isolates of Xcm morphologically, biochemically, physiologically and pathogenically; and (iii) determine the pathogenicity ofXcm isolates and select enset clones resistant to pathogenic Xcm isolates in southwestern Ethiopia.Up on the distribution study, a total of 120 enset fields were inspected across 10 enset growing districts. The mean disease incidence across districts ranged from 23.67 to 31.92%, and significantly different levels of disease severity were recorded across surveyed districts. Among districts, the highest mean disease severity of 62.50% was recorded from Semen bench, whereas Andiracha district showed the lowest (49.58%) mean disease severity. Logistic regression analysis indicated that EBW incidence of >25% had high probability of association with enset grown on soils with pH of 5.5-7, sole cropped enset, susceptible clones, when planting materials are shared among farmers and when enset fields failed to be properly weeded and managed for EBW. High (>55%) EBW severity had high probability of association with growing ensetin Semen-bench and Yeki districts,weed management through machete slashing, growing local susceptible enset clones, vegetative to maturity growth stages, and low to medium levels of farmers‟ awareness about EBW. One hundred twenty isolates were collected and isolated from 120 enset fields in 10 major enset growing districts in southwestern Ethiopia. The isolates were identified as Xcm on the basis of morphology and biochemical tests, and confirmed by pathogencity test, as the pathogen showed wilting and necrosis of leaves in similar manner as it was seen right at the xiii field. The colonies were dome-shaped, circular, their growth ranged from less to highly mucoid with light to deep yellow and creamy colony, gram negative, catalase and oxidase positive; could hydrolyze casein and gelatin; produce H2S, and utilize citrate and malate while could not reduce nitrate to nitrite; could not hydrolyze Tween 80 and starch; and could not produce indole. Variability among isolates was observed in colony appearances, color, growth types and tolerance to different NaCl concentrations (many isolates were capable to grow on medium containing 3-5% NaCl and the others were not) and temperature extremes (36% and 30% of the total isolates failed to grow at 26oC and 32oC, respectively).A total of 30 representative Xcm isolates were subjected to pathogenicity test on a susceptible enset clone,Yeko, and all were found pathogenic. Out of 30 pathogenic isolates, three isolates representing three altitude groups [lowland (1470 m.a.s.l), midland (1938 m.a.s.l) and highland (2360 m.a.s.l)] were used for enset clonal evaluation trials. In the clone evaluation trials, 15 enset clones (13 local and a tolerant and a susceptible check) were evaluated for two years (2017 and 2018)under screen house conditions at Tepi National Spice Research Center, southwestern Ethiopia. The experiments were factorial arranged in a completely randomized design with three replications. An aliquot of 10 ml of the bacterial cell suspension (a concentration of 1x108 cfu ml-1 ) was inoculated into the second innermost leaf petiole of enset using a sterile hypodermic syringe. Starting from 15 days after inoculation (DAI), data were collected on incubation period (IP), disease incidence (DI), percentage severity index (PSI), days to complete wilting/death (DD), area under disease progress curve (AUDPC) and disease progress rate. Analysis of variance for IP, DI, DD and AUDPC revealed significant (P<0.05) differences among tested enset clones, while significant (P<0.05) differences existed among the interaction effect of ensetclones x bacterial isolates for PSI. Disease incidence recorded ranged from 0 to 90% and IP ranged from 0 to 23 days. Similarly, the days to complete wilting of susceptible clone reached up to 63 days, while the calculated AUDPC values ranged from 0 (Gudiro,Maziyaand Nobo clones) to 3190%-days (Arkia, Ataro, Yeko, ChikaroandOgisso clones). Disease progress rates also ranged from -0.00165 to 0.04398 units day-1 for evaluated clones. Clones Gudiro,Maziya and Nobo showed a resistant/tolerant reaction to EBW, while clones Arkia, Ataro, Yeko, ChikaroandOgisso were the most susceptible enset clones.Findings of this study indicated that EBW is widely distributed and could be xiv minimized through the folwlloing integrated management approaches: growing enset preferably on soils out of pH 5.5-7ranges, intercropping systems, proper weeding practices, using disease-free planting materials, disinfecting farm tools before using and sharing, rouging out and burning of infected ensetplants, accessing of advisory services, limiting free exchange of planting material among enset growers, and using resistant/tolerant enset clones such as Gudiro andNobo. These approaches are key components in enset bacterial wilt management schemes in the study areas or similar agro-ecologies.