Author : Amy Thein

Summary : The virulent wild strain Xoo-19 was mutated by exposing it to ultraviolet (UV) radiation (65 watt) for 3 min. Two UV induced biotype strains, M-407 and M-690 that survived the irradiation and reduced in growth rate and pathogenicity were selected and characterized. They differed from 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 fluorescens SP007s product) and naturally-occurring avirulent strain Xoo-7. Twenty-one day old rice plants of 2-cultivars (susceptible cv. Khao Dauk Mali or KDML105 and resistant cv. Suphanburi 1) separately pretreated with 1x10 to the 8th power cfu/ml each of 4-antagonist strains mentioned above, as foliar sprays were challenged inoculated after 3 days with wild strain Xoo-19. All 4-antagonist strains including M-690, M-407, Xoo-7, and SP007s provided significant disease reduction (P=0.05) by 53.7, 49.5, 45.2, and 58.1% in KDML105 and 62.1, 51.4, 47.9, and 62.1% in Suphanburi 1 respectively when assessed at 14 days post inoculation compared to nontreated control. Strain M-690 showed equally or nearly effective in disease reduction compared to SP007s with no significant difference (P=0.05) 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.