Abstract:Pseudomonas fluorescens strain Pf7-14 and Bacillus subitlis strain B5423, as biological control agents (BCA) of rice sheath blight, suppressed effectively Rhizoctonia solani AG 1, its causal pathogen. The average population sizes of strain Pf7-14, a strain naturally resistant to nalidixic acid, and strain B5423-R, a spontaneous rifampicin-resistant mutant of strain B5423 as well as native bacterial population (NBP) on rice plants, were monitored under greenhouse conditions by spraying the suspensions of strains Pf7-14 and B5423-R onto both healthy and R. solani AG1 inoculated rice plants. Strain B5423-R was as suppressive to R. solani AG1 and functioned as a biological control agent of rice sheath blight as its wild type. Based on periodically sampling of rice leaves and culms, and bacterial population estimated by method of dilution plate counting, the following results were obtained. On healthy leaves, the population sizes of strains Pf7-14 were higher than those of strain B5423-R when the initial population of the two was similar. The population sizes on the culms of the rice plant, strains Pf7-14 declined over time regardless of initial concentration. In contrast, the population of strain B5423-R declined over time when the initial concentration was low (4.0×107 cfu/ml or less), but remained constant or increased slightly from milky to ripening stage of plant growth when a higher concentration of the bacterium (2.0×108 cfu/ml or more) was applied to the rice plants. In the batch of rice plants inoculated with R. solani AG1, at level of sheath blight lesions of 20%-35%, the average population sizes of strain Pf7-14 were 6 and 2 times significantly lower than those on healthy culms, 1 day and 14 days after BCA application(P=0.05), respectively. Contrarily, the population size of strain B5423-R was about 1 time lower on diseased than on healthy culms at one day after its application. Henceforth however, the population of strain B5423-R increased, and it was 6 times significantly higher on diseased than on healthy culms measured at 14 days after its application. NBP was about 6 to 7 times significantly higher on diseased than on healthy culms. The ability to colonize healthy and diseased culms appears to be different between the two antagonistic bacteria. The results suggested that although both strains are originated from the same rice ecosystem, but differ in capacity of colonizing rice plant, that strain B5423-R, a bacillus, seems to be more competitive in diseased plants than strain Pf7-14,and that the introduced antagonistic bacteria (IAB) and NBP are competitive in nutrient and space each other, and NBP has an advantage over IAB in competition.
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