Distribution of enset bacterial wilt in southwest Ethiopia, characterization of the pathgen (Xanthomonas campestris PV. musacearum) and variations in host pathogen interaction

Abstract

Enset bacterial wilt (EBW) caused by Xanthomonas campestris PV. musacearum is one of the highly destructive diseases that affect enset plants in the enset growng areas. However its extent of damage and distribution, the characteristics and variability of the pathogen, and the host-pathogen interactions are little studied in southwest Ethiopia. Thus, objective of this study were to assess the distribution of enset bacterial wilt, to isolate and characterize strains of the causal pathogens, and to study host-pathogen interactions of enset clones and Xanthomonas campestris PV. musacearum strains collected in these areas. Survey and field studies were conducted in three enset growing areas of Sheka, Keffa and Bench-magi zones of southwest Ethiopia and the laboratory and greenhouse experiments were conducted in Jimma University College of Agriculture and Veterinary Medicine. Completely randomized design (CRD) with three replications were used for the laboratory study while randomized complete block design (RCBD) with 3 isolates x 4 enset clones factorial treatment combinations, replicated three times, were used for host-pathogen interaction studies in the field experment. The disease incidence ranged from 25.56% to 59.63% in enset growing areas of Sheka, Keffa and Bench-Magi zones in southwest Ethiopia. EBW symptoms were more severe in two Weredas, Masha and She-bench, each having percentage severity index (PSI) of 55.56% than in other Weredas (<50%). The most important factors responsible for spreading the disease include infected planting materials, contaminated farming and processing tools, and human and animal vectors. In cultural and physiological characterization of Xanthomonas campestris PV. musacearum populations, almost all of the 19 isolates slightly differ in colony color and growths types with similar pathogenicity. In the field experiment, there were highly significant differences (p<0.001) in disease incidence (DI), incubation period (IP), percentage severity index (PSI) and area under disease progress curve (AUDPC) values; and significant differences (p<0.05) in date to complete wilting (DW) in the host-pathogen interactions study. Based on the result, the enset clones Nobo and Gudiro revealed complete and high resistance reaction, respectively, to all isolates with low mean percentage severity index of 0.0% and 6.7%, respectively; whereas enset clones Yeko and Chikaro were susceptible to all the isolates and showed high mean percentage severity index of 100% and 85.2%, respectively. Among the isolates, Sheka (YeLYe) isolate was highly pathogenic to Yeko and Chikaro enset clones as compared to the moderately pathogenic Keffa (GiHSh) and Bench- magi (ShMGe) isolates. In comparing clones by isolate interactions, all isolates caused equal percentage severity index on enset clone Yeko (100%), and isolate YeLYe induced higher percentage severity index on enset clone Chikaro (100%) than the isolate ShMGe (71.2%) and GiHSh (68.9%). In conclusion, enset bacterial wilt is prevalent and important disease of enset in southwest Ethiopia although there exists variation in host reaction to the pathogen. The bacterial population collected in the enset growing areas showed basically similar cultural, physiological and biochemical nature of the species Xanthomonas campestris PV. musacearum although clear difference in some features like colony color, growth nature, salt tolerances and pathogenicity. Variations in isolate-clone interaction are suggestive of the need to evaluate the response of several number of Xanthomonas campestris PV. musacearum isolates and characters of enset clones. Therefore, for further work by including more number of Xanthomonas campestris PV. musacearum isolates and enset clones as well as their genetic backgrounds associated with pathogenicity of the Xanthomonas campestris PV. musacearum strains and resistance of enset clones are important