Suppression of sheath blight (Rhizoctonia solani Kuhn) development in rice by bacterial biocontrol agents

Results of the study indicated that the best period for antagonist application was T2 (1 day before), which was significantly different from the others. The sprays applied at 1 day after, as well as simultaneously, gave a good result. Thus, BCA [biological control agent] application should not be later than 2 days after inoculation. The results suggested that the antagonists could be used for biocontrol as a preventive rather than a curative treatment. The same antagonist were also tested for controlling ShB [sheath blight] using different density of the pathogen and biocontrol agent. In stem segment test, significant reduction of ShB severity was achieved with the plants introduced with low or moderate (1 mm; 1-2 mm or 2-3 mm of sclerotial diameter) levels of pathogen density but not with the highest (3 mm of sclerotial diameter) density of pathogen. Similarly, in greenhouse test, at 10 sup 8 CFU/ml concentration of antagonistic bacteria substantially reduced ShB on rice when the plants were inoculated with moderate inoculum density of pathogen (5 gr RHG mixture/hill). However, at doubled inoculum density of pathogen (10 gr RHG mixture/hill), the density of biological agent of approximately 10 sup 12 CFU/ml was required for a significant suppression of ShB in rice. Eighteen bacterial isolates from ShB lesions, and four strains of antagonistic bacteria from rice seedling (P. flourescens strain 7-14) rice field soil (B. subtilis strain B 916), rice seeds (P. cepacia strain P 6858), rice plant (P. putida strain 1821) were used for testing the antagonistic activity against R. solani both in stem segments and intact plants. Among antagonists, more number of antagonists was found in intact rice plant tests as compared to that in rice stem segment tests. The results showed that bacterial antagonist originated from ShB lesions had efficiencies from 12.49 to 25.32 percent. These data indicated that antagonistic bacteria from the ShB lesion were more effective in reducing infection of R. solani and in suppressing ShB