Impact of seasonal rainfall variability on Rice yield and disease incidence in Bihar State

In rainfed environment, the productivity of cropping systems is primarily determined by the amount and distribution of rainfall. Seasonal rainfall, monthly, weekly, total rainfall during critical stages was analysed. Besides, this variability in rice yield was also worked out based on coefficient of variation. Rainfall data for monsoon season for 34 years (1980–2013) and crop yield data for 16 years (1997–2013) was used. The years 2004, 2005, 2009, 2010 recorded significant rainfall negative departure, so these years were selected for study. Rainfall deficit in year 2005 was more than year 2004 while rice yield in 2005 was higher than that in 2004. This could be attributed to uneven distribution of rainfall as analyzed based on weekly rainfall. The excess rainfall during the Standard Meteorological week (SMW) (8^th-14^th July) in 2004 impacted the normal sowing of rice crop. Also the yield suffered from mid season drought due to long dry spell during the three SMW (22 July25 Aug.), coinciding with booting and heading phase. In 2009 and 2010, rainfall was 40% below the normal and significant yield loss was also found due to the drought occurred at flowering phase. As per observations of the disease; it revealed that the Bacterial Leaf Blight disease and Brown Spot (Bipolaris oryzae) are pre-dominant diseases having its disease pressure followed by Sheat Blight disease. Crop Simulation Model was run to quantify the water stress and related conducive environment for stronger epidemics especially Brown Spot epidemics (as drought was recognized as a key factor) developed in rice crop during 2004 and 2005, 2009 and 2010. The analysis of the simulated results also showed the association between yield and rainfall and occurrence of diseases during various phenological stages and its distribution during the season.