Screening of potential bacterial antagonists for control of sheath blight in rice and development of suitable bacterial formulations for effective application

Soil samples were taken from paddy rice fields in 14 provinces in the southern part of Thailand. Bacteria were isolated from these soils using the soil dilution plate method on King's B medium and Thornton's standardised medium. Isolation yielded 323 bacterial isolates which were subsequently tested for their effectiveness in inhibiting mycelial growth of Rhizoctonia solani, the causal agent of sheath blight of rice. Eight isolates were selected for their ability to create a clear zone in a dual culture test. Further tests evaluated the effect of selected bacteria on sclerotial germination and subsequent mycelial growth, and also on the development of sheath blight lesions on excised rice stems. Three bacterial isolates (16, 26 and 29) provided the greatest inhibition of sclerotial germination and mycelial growth and maximum suppression of sheath blight lesions. Isolate 26 and subsequently isolate 29 were chosen for formulation studies. Granulated formulations of these bacterial isolates were developed using the wet granulation technique. The main components of these bacterial formulations were bacterial cells, hydrogenated vegetable oil, monohydrate lactose, polyvinyl pyrrolidone, and crosslinked sodium carboxymethyl cellulose. The efficacy of the formulation of isolate 29 in suppressing sheath blight symptoms was similar to that of fresh cells of isolate 29.