Agricultural Reviews

Sheath blight (ShB) of rice caused by Rhizoctonia solani Kuhn. is a major biotic constraint of rice in most of the rice growing countries of Asia. The pathogen is polyphagous competitive saprophyte and has a wide host range. A crop with a high plant density and closed canopy associated with high nitrogen management favours disease build-up from panicle initiation onwards. Crop losses generally vary from 0 to 50% depending on severity of the disease and the stage at which the crop is infected and environmental conditions. Sheath blight infection increases peroxidase, chitinase and polyphenoloxidase activity but decreases catalase activity. Infected tissues contain higher levels of phenols than in healthy tissues. Initial symptoms of sheath blight appear in the form of circular, oblong or ellipsoid, greenish, grey, watersoaked spots of about 1 cm long that occur on leaf sheaths near the water line. To date, only partial resistance to sheath blight has been identified, as evidenced by a survey of 6000 rice cultivars from 40 countries, frim which no cultivar exhibiting a major gene for rice sheath blight resistance was identified. Recently, quantitative trait loci (QTL's) analysis identified six QTLs associated with sheath blight resistance. Many bacteria and fungi from rice field soils are antagonistic to sheath blight. Since no commercial variety resistant to sheath blight is available, the strategy of disease management will be to destroy weeds; use of need-based effective chemicals, use of herbicides and balanced fertilizer and nutrient application. More studies will be necessary to evolve a technology using antagonists as biological control measure particularly in upland rice.