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

Abstract

Enset (Ensete ventricosum) is an important staple food crop supporting the livelihoods of more than 20 millions of people in Ethiopia. Enset bacterial wilt (EBW), caused by Xanthomonas campestris pv. musacearum (Xcm), is one of the highly destructive diseases of enset. However, its distribution and extent of damage, and biophysical factors associated with its epidemics have not been well studied in southwestern Ethiopia. Similarly, diversity within the bacterial pathogen populations and the reaction of selected enset clones to pathogenic Xcm isolates have not been studied in the areas. Thus, the field survey was carried out in 10 districts in three zones [Bench-Maji (Maji, Semen-Bench, She-Bench), Keffa (Bita, Chena, Decha, Gimbo) and Sheka (Andiracha, Masha, Yeki] in 2017 followed by laboratory and screen house evaluation experiments with the specific objectives 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 virulence of Xcm isolates and identify enset clones resistant to pathogenic Xcm isolates in southwestern Ethiopia. Upon the distribution study, a total of 120 enset fields were inspected across 10 enset-growing districts. One hundred and twenty infected samples were collected from the 120 enset fields in the 10 major enset-growing districts and the pathotypes were isolated by streaking on yeast peptone sucrose agar (YPSA) medium and chemical tests were conducted in the laboratory following standard procedures. Pathogenicity test was also conducted with a total of 30 representative Xcm isolates on a susceptible enset clone, Yeko, after the isolates were clusterd to their respective farmer associaitions. Out of 30 pathogenic isolates, three isolates corresponding to 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 tested for two years (2017 and 2018) under screen house conditions at Tepi National Spice Research Center, southwestern Ethiopia. The treatments were factorially arranged in a completely randomized design (CRD) 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 recorded on incubation period (IP), days to complete wilting/death (DD), disease incidence (DI) and disease severity. The mean disease incidence across districts ranged from 23.67 to 31.92%, and significantly different levels of disease severity were recorded across the surveyed districts. The highest (62.50%) mean disease severity was recorded from Semen-Bench district, whereas Andiracha district had the lowest (49.58%) 13 mean disease severity. Logistic regression analysis indicated EBW incidence of >25% had high probability of association with enset grown on soils with pH of 5.5-7.0, sole cropped enset, susceptible clones, when planting materials were shared among farmers and when enset fields lacked proper weeding and management against EBW. High (>55%) EBW severity showed high probability of association with growing enset in 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. The bacterial 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 a similar manner as it was seen right in the fields. The bacterial 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; produced H2S, and utilized citrate and malate but 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. About 15% of the isolates grew on medium containing 4 and 5% NaCl, while the others did not, and 36 and 30% of the total isolates failed to grow at extreme temperature of 26 and 32 oC, respectively. However, all the 30 selected and tested isolates were found pathogenic. 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 enset clones 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 63 days, while the calculated AUDPC values ranged from 0 (Gudiro, Maziya and Nobo clones) to 3190%-days (Arkia, Ataro, Yeko, Chikaro and Ogisso clones). Disease progress rates also ranged from -0.00165 to 0.04398 units per day for evaluated clones. The enset clones Gudiro, Maziya and Nobo showed a tolerant/resistant reaction to EBW, while the clones Arkia, Ataro, Chikaro, Ogisso and Yeko were the most susceptible enset clones. Findings of this study indicated that EBW is a widely distributed pathogen and could be minimized through integrated management approaches: growing enset preferably on soils out of pH 5.5-7.0 ranges, practicing intercropping systems, proper weeding practices, using disease-free planting materials, disinfecting farm tools before using and sharing, rouging out and burning of infected enset plants, accessing of advisory services, limiting free exchange of planting materials among enset growers, and using tolerant/resistant enset clones, such as Gudiro and Nobo. These approaches are key components in enset bacterial wilt management schemes in the study areas or similar agro-ecologies.