Distribution and Relationship of Brown Spot Disease of Rice Caused by Bipolaris oryzae with Weather Parameters Across Kerala, India

Abstract

Brown spot, caused by Bipolaris oryzae, is one of the most prevalent diseases of rice and can assume epiphytotic proportions. A survey was conducted during August–October 2024 to determine the distribution, incidence, severity and epidemiological factors associated with brown spot across nine major rice-growing agro-ecological units (AEUs) of Kerala, India. Three locations were surveyed in each AEU, covering 27 locations. Disease incidence (DI), per cent disease index (PDI), crop characteristics and weather parameters such as maximum temperature (°C), minimum temperature (°C), relative humidity (%) and rainfall (mm/day) were recorded. Brown spot occurred in all surveyed AEUs, confirming its widespread occurrence in Kerala. Disease incidence ranged from 24.20 to 75.40%, while PDI ranged from 15.00 to 60.00%. The highest DI (75.40%) was recorded at Kollad in cv. Uma, whereas the highest PDI (60.00%) occurred at Mullassery and Panayara. The lowest DI (24.20%) and PDI (15.00%) were recorded at Aingoth in cv. Shreyas. Correlation analysis revealed significant positive associations between relative humidity and DI (r = 0.604) and between relative humidity and PDI (r = 0.426). The study confirms the widespread occurrence of B. oryzae in Kerala and provides a basis for further studies on pathogen variability and disease management.