BIOLOGICAL CONTROL OF BACTERIAL BLIGHT OF WALNUT BY ANTAGONISTIC BACTERIA
H. Ozaktan, M. Erdal, A. Akkopru, E. Aslan
The goal of this study was to determine whether bacterial antagonists could be used to control Xanthomonas arboricola pv. juglandis (Xaj), the causal agent of bacterial blight of walnut. A total of 35 potentially bacterial antagonists, 29 of which were isolated as epiphytes from the phylloplane of healthy walnut trees, were screened for their in vitro biological control activity against Xaj. Pantoea vegans strain C9/1 was also tested as reference biological control agent. Eighteen strains, that inhibited the growth of Xaj between 3.0 mm to 13.0 mm on TSA medium, were selected and further tested for their ability to control bacterial blight on immature nuts. Approximately 60% of the antagonistic strains, significantly reduced bacterial blight by 41% to 77% as compared to the positive control. Bacterial antagonistic strains, which effectively inhibited the development of Xaj on immature nuts were selected for further testing on walnut seedlings. Antagonistic strains were sprayed in a suspension of 109 CFU ml–1 on the leaves which were subsequently inoculated with suspension of Xaj strain W7/1 (108 CFU ml-1), 24 h after the antagonist treatments. According to the in vivo biocontrol tests on walnut seedlings (cv. Chandler), 4 putative antagonists significantly reduced the symptoms on walnut leaves (41% to 82%) compared with the positive conttol. Strains WH77/1, WH68 and WH48/1A were the most effective in the seedling test, yielding 55%, 42%, and 82% disease reduction, respectively, in two consecutive years. These strains were tentatively identified as Pseudomonas fluorescens on the basis of biochemical and physiological tests. P. vagans strain C9/1 agent, reduced the severity of bacterial blight on the leaves by 71%. The results obtained both nut and seedling tests agreed, except for P. fluorescens strain WH55/4 which was less effective in inhibiting symptom development in walnut leaves than in the fruits. Prohexadione-Ca (Pro- Ca) treatments of walnut leaves reduced by 66% blight symptoms.