Effect of bacterial blight on growth of four IR varieties under different nitrogen levels.

The reduction of rice crop growth in terms of biomass production and leaf area due to bacterial blight (BB) at different nitrogen levels were studied in the greenhouse at dry season 1989-1990. The reduction of leaf area of susceptible variety (IR56) was greatly increased before 4 WAI. However, after 4 WAI [weeks after inoculation] it did not affect the reduction of total leaf area due to the absence of secondary infection. The leaf area production of resistant and moderately resistant varieties were highly affected after 4 WAI. Different levels of nitrogen on all varieties affected the leaf area production and increase of nitrogen application maintained active leaf area at later stage. However, it did not increase the dry matter of BB infected rice. The reduction of dry weight due to BB under proper nitrogen application (100 kg N/ha) were not much different compared to that of other applications. The higher reductions of dry weight under low and high level of nitrogen might be due to lack of source and to competition of sinks or to consumption of maintenance of leaf dry, respectively. The nitrogen levels, disease and the parameters of crop growth were highly correlated with reduction of leaf area on varieties inoculated at maximum tillering stage. However, dry weight and percentage filled grain were negatively correlated with the disease development on susceptible varieties. The applications on increased nitrogen highly affected the leaf area production and dry weight of all varieties, and there was significant difference in the number of filled grains and 1000 grain weight on susceptible varieties only. However, nitrogen did not affect grain weight and percentage of filled grain under diseased condition. Nitrogen levels significantly affected the lesion length and diseased area at early stage of BB on susceptible variety and at later stage on moderately resistant varieties.