Bacterial biocontrol and soil solarization strategies for suppression of Rhizoctonia solani on rice
Use of known antagonists and antagonistic, endophytic bacteria was investigated to screen a successful antagonist for the fungal strains R. solani AG 2-1, R. solani AG 4 and R. solani AGSL01. Reputed antagonists used in the study; Bacillus subtilis MBI 205, B. subtilis MBI 600, Pseudomonas fluorescens B5 and P. corrugate R. 117 were capable of suppressing the fungus in dual cultures. Manipulation of soil factors to reduce pre-plant density of R. solani sclerotia involved a laboratory experiment followed by two field experiments. In the laboratory study, the effect of constant soil temperatures was investigated by incubating sclerotia at 30, 35, 40, 45 and 50°C. Sclerotia were counted into lots of 100, placed in polyester mesh bags (85 μm pore size, 10 x 10 cm dimension) at depths of 5 and 10 cm in rice field soils contained in plastic containers (20x20x18 cm). Total loss of viability was observed on day 1, day 8 and day 28 at 45°C, 40°C and 35°C, respectively. Loss was even detected within 6.00 h at 50oC. Field experiments of soil solarization (carried out at The Regional Agricultural Research Institute (RARI), Bombuwela, Sri Lanka) were conducted during the fallow periods between the two main cropping seasons in 2003. During the trials, the effect of polythene mulching, straw incorporation and their combined effect on the viability of sclerotia were investigated. Sclerotia (lots of 100) were buried at depths of 5 and 10 cm and the soil temperature was recorded at 8.00, 11.00, 14.00 and 17.00h daily at both depths. During the field trials, at both depths, the effect of treatment over time was noted on percentage recovery and viability of sclerotia (p<0.001). The results showed a drop in mean percentage recovery of less than 50% during the first week of both trials. Germination was markedly reduced to less than 10% by the first week in all the treatments. Depth of burial and straw incorporation had no effect. In solarized plots, a significant increase in soil temperature at both soil depths was observed between 14.00 and 17.00h. Average temperatures of 40°C and 34°C were observed for solarized and non-solarized plots, respectively. The study therefore has identified a practical, low cost and environmentally friendly method of control, and the use of polythene five weeks prior to rice seed sowing is recommended to minimise sheath blight and other diseases caused by similar soil-borne fungal pathogens.