Dispersal of airborne pathogenic conidia of Bipolaris oryzae inciting brown spot disease of paddy in West Bengal, India

Brown spot (BS) is one of the most damaging diseases of paddy observed from seedling stage in nursery to milky stage in main field, caused by the pathogenic fungus Bipolaris oryzae (BO, Syn: Drechslera/Helminthosporium oryzae; Sexual stage: Cochliobolus miyabeyanus). A two-year (2016 and 2017) aerobiological survey was carried out continuously in a paddy field of 24 Parganas (North) district in the lower Gangetic basin of West Bengal using Burkard 7-day volumetric sampler. The seasonal variation, diurnal periodic pattern and vertical profile (in peak period upto 5 m height using rotorod sampler) of airborne conidia of pathogenic conidia were recorded. The BS disease progress in the crop was recorded with time in field condition. Conidia of Bipolaris oryzae were found throughout the year in the air with two peaks, one during April-May (<24 conidia/d/m3 air) and another in October-November (<28 conidia/d/m3 air), just after the harvest of dry and wet season crops respectively. The occurrence of BO conidia was compared with airborne fungal conidia of Nigrospora oryzae (NO), which causes minor opportunistic infection in rice, producing minute leaf spots. Airborne BO conidia showed no correlation with meteorological parameters, whereas relative humidity had significant correlation with airborne NO conidia. Disease development analyses of both the dry and wet season crops revealed that both the leaf and grain infections had positive and significant correlation with airborne BO conidia (p<0.1-0.001), which could be expressed with fitted regression models. In case of NO conidia, no such regression models could be fitted. Comparative study with airborne small-sized NO conidia, with only <4 per cent leaf infection, clearly indicated that airborne BO conidia were potential threats to rice crop, which might also induce primary infection in field condition. Airborne BO conidia depicted a bimodal diurnal pattern with two maxima at morning (8.00 AM, 5.5 conidia/h/m3 air) and afternoon (15.00 PM, 4.0 conidia/h/m3 air). It had the maximum level of concentration at one metre height (2.5-10 conidia/h/m3 air) over crop canopy.