Aerobiological dynamics of Bipolaris oryzae in the rice agroecosystem and its impact on rice brown spot disease severity

Rice brown spot disease (RBS), caused by seed-borne and air-borne pathogen Bipolaris oryzae, appears throughout the rice growing season. Considering its polycyclic nature and its significance in yield loss, a one-year (2019) aerobiological study was carried out at Thiruthani (13°11′N 79°38′E) village in Thiruvallur district during two seasons viz., kharif (April 2019 to July 2019) and rabi (August 2019-December 2019). B. oryzae airspora contributed to an average of 20.7% annually. Amongst the barren land conditions, it contributed 11.83%, whereas during rabi 32.52% and kharif 18.04%. The average length (range 40.0-129.8) and average breadth (range 13.17-23.92) of B. oryzae spores were used to calculate dvolume (range 20.54-42.03) and aspect ratio (2.29-6.85). The intra-diurnal behaviour pattern indicated that B. oryzae airspora percentage contribution was highest at 12 noon-1 p.m. (55.64%) in rabi crop and two peaks at 10-11 a.m. (19.56%) and at 12 noon-1 p.m. (19.09%) in kharif crop. The calculated correlation coefficient R value revealed that relative humidity (kharif R=0.9879, rabi R=0.9977) and precipitation in kharif (0.9279) were positively correlated, while the temperature in rabi was negatively correlated with the RBS pathogen airspora percentage. Both the airspora percentage contributions of the RBS pathogen and the PDI were highly correlated (R=0.9002 in kharif, R=0.9874 in rabi). In the Step-Down regression analysis, R2 value (regression coefficient) unveiled that relative humidity (R2 value=0.99760 in kharif, 0.9954 in rabi), precipitation (R2=0.8480) and wind speed (R2=0.7319) in kharif, and temperature in rabi (R2=0.8862) had a greater impact on the RBS pathogen airspora per cent contribution. The airspora B. oryzae per cent contribution aggravated disease occurrence and reflected in the PDI (R2 = 0.8103 in kharif, R2=0.9748 in rabi). The calculated P value and OD ratio supported the significance of the experimental results.