XANTHOMONAS ORYZAE PV. ORYZAE. AN OVERVIEW OF BACTERIAL BLIGHT DISEASE OF RICE CAUSED BY

Rice is the most widely cultivated food crop of the world.Asia alone accounts for 90 per cent of the world's production coupled with consumption of rice also because of favorable hot and humid climate.Rice accounts for 35-75 per cent of the calories consumed by more than 3 billion Asian (Khush, 2004).It is also expected that by the year 2050, 90 per cent of world's projected 11 billion people will reside in the developing countries (Krattigar, 1996).The production is constrained mainly due to biotic and abiotic stress.Major advances have occurred in food production during the last seven decades due to adoption of improved techniques including high yielding varieties.This has also lead to emergence of new pests and diseases simultaneously.The loss of the yield in all crops due to biotic stress is approximately 500 billion.It has been estimated that in bacterial blight of rice the yield losses are as high as 6 to 60 per cent depending upon location, season, weather condition and cultivars (Srivastava, 1967).Bacterial blight (BB) of rice, caused by Xanthomonas oryzae pv.oryzae (Xoo) reported as economically important disease in tropical Asia also (Mew, 1987 and1989).The disease was known to occur in epidemic proportions in many parts of the world, incurring severe crop loss of up to 50 per cent.Crop loss assessment studies have revealed that this disease reduces grain yield to varying levels, depending on the stage of the crop, degree of cultivar susceptibility and to a great extent, the conduciveness of the environment in which it occurs.Bacterial blight of rice is difficult to control through chemicals.Varietal resistance is considered the most practical and economic way of keeping the disease below the economic injury level.Variability in pathogenicity have now been fully recognized.Resistant varieties development has been targeted in almost all the breeding programmes of varietal improvement.In this context number of isogenic lines, differential having known genes and pyramids with more than two resistant genes have been also developed by different agencies.The literature pertaining to plant resistance and its nature of response in rice to Xanthomonas oryzae pv.oryzae has been reviewed under following heads: