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Reduction of bacterial leaf blight of rice caused by Xanthomonas oryzae pv. oryzae by its avirulen strains
Thesis Abstract:
Bacterial leaf blight (BLB) caused by Xanthomonas oryzae pv. oryzae is the most devastating bacterial disease of rice worldwide. The feasible control for BLB once it occurs is resistance, where the effective cultivars are limited. There are numerous examples of prior inoculation of a host with a non-pathogenic bacteria inducing systematic acquired resistance with the same virulent pathogen. Studies were then conducted on the management strategies with avirulent strains of X. oryzaepv. oryzae (Xoo) obtained from natural occurrence and induced by ultraviolet (UV) mutagenesis. The virulent wild strain Xoo-19 was mutated by exposing it to UV radiation (65 W) for 3 min. Two UV-induced biotype strains, M-407 and M-690, that survived the irradiation and reduced the growth rate and pathogenicity were selected and characterized. They differed from the wildtype Xoo-19 in colony morphology,production of bacteriocin, exopolysaccharide and exoenzyme activity, and reduced motility on the surface agar. They were further investigated for the ability to control BLB and disease suppressive mechanisms of action. Competition, bacteriocin production, and induced systemic resistance mechanisms of two mutants were compared with previous ISR-P® uses (Pseudomonas fluorescences SP007s product) and naturally-occuring avirulent strain Xoo-7. Twenty-one-day-old rice plants of two cultivars (susceptible cv. Khao Dawk Mali or KDML 105 and resistant cv. Suphanburi 1) separately pretreated with 1x108 CFU/ml each of four antagonist strains as foliar sprays were challenged inoculated after three days with wild strain Xoo-19. All four antagonist strains (M-690, M-407, Xoo-7, and SP007s) provided significant disease reduction (p = .05) by 53.7, 49.5, 45.2, and 58.1 percent in KDML 105 and 62.1, 51.4, 47.9, and 62.1 percent in Suphanburi 1, respectively, when assessed 14 days post inoculation compared to the non-treated control. Strain M-690 showed equally or nearly effective in disease reduction compared to SP007s with no significant difference (p = .05) on both cultivars tested. In a time course study, it was shown that direct competition for infection sites and nutrient of avirulent strains is unlikely to be main mechanisms in the reduction of BLB. This was probably due to the abundant production of bacteriocin and the result of increase in an accumulation of β-1,3- glucanase, a peroxidase enzyme induced by non-pathogenic mutants of X. oryzae pv. oryzae. Control efficacy of all four antagonist strains also showed the difference on two rice cultivars. Suphanburi 1 was found higher in both disease suppression and defense related to enzymes accumulation than KDML 105. Biological control with avirulent strain may be most effective on cultivar that has some genetic tolerance to BLB. To obtain further confirmation of this research, different cultivars and larger numbers of experiments in commercial farm production should be conducted.