Rice-associated-microorganisms with potential for biological control of sheath blight of rice caused by Rhizoctonia solani Kuhn [Thanatephorus cucumeris (Frak.) Donk.]

One hundred bacterial isolates were isolated from sheath blight lesion and then screened for inhibition of Rhizoctonia solani. Thirty bacterial isolates showed activity against R. solani on PPM (Pigment Production Medium) and KMB (King's medium B), while 26 and 3 bacterial isolates expressed activity against R. solani on NA and PDA, respectively. Interaction of 24 bacterial antagonists and 13 isolates of R. solani from different locations in Philippines were tested by dual culture method. Stable antagonists, namely, In-b-5351, In-b-7-14, In-b-6854 and In-b-6858 were obtained on PPM, while In-b-5422, In-b-5423, In-b-5424 and In-b-5425 were determined as stable antagonists when tested on NA. There was a negative correlation between the inhibition zone width and percent incidence of disease. Isolates In-b-5423 and In-b-5351 significantly promoted root and shoot length and dry weight. Bacterized rice seed were sown in the soil infested with inoculum of R. solani, the results indicated that most of antagonistic bacteria effectively reduced percent incidence of disease. After second planting with non-bacterized seed in the same box, the detected sclerotia were reduced. However, percent relative lesion height could not be reduced as compared to control at one month after sowing. Introduction of antagonistic bacteria to rice plant by seed bacterization plus two spray applications after inoculation with R. solani at maximum tillering stage could minimized the disease and caused sclerotia germination losses due to bacterial colonization and degradation of sclerotia, therefore caused reduction in the inoculum level of R. solani. The mixture isolates of In-b-7-14 and In-b-5423 gave the best result and In-b-7-14 was ranked the second in controlling sheath blight disease as compared to the control (water spray). Some isolates provided efficacy with non significantly different from the fungicide (validamycin) usage. Sclerotia soaked in high concentration of bacterial suspension isolate In-b-6854 were completely lost in germination. At lower concentration, the bacteria reduced and delayed germination of sclerotia, caused mycelial malformation and reduction in mycelial length. The PPM was a suitable medium for isolate In-b-6854 to produce active secondary metabolite or antibiotic. Antibiotic extracted by methanol and 80 percent acetone successfully inhibited R. solani. Crude acetone extract was spotted on TLC plate and developed with acetonitrile/methanol/water (1:1:1). The substance eluted from TLC plate at the Rf value of 0.25 inhibited mycelial growth of R. solani. Isolates In-b-6854, In-b-7-14 and In-b-5351 were identified by Biolog test method as Pseudomonas (Burkholderia) cepacia, Pseudomonas fluorescens, and Enterobacter gergoviae, respectively.