Higher disease incidences were observed during 65-75 DAS with half nitrogen rate.• Weed control led reduction of disease incidence during 50-80 DAS in Yala season 2018.• Planting density (seeding rate) had no significant impact on disease incidence index.

Narrow Brown Leaf Spot (NBLS) caused by Cercospora janseana is a common disease of rice causing severe yield loss. In order to identify the factors favourable for disease development under field conditions, a study was carried out during the 2017/2018 Maha and 2018 Yala seasons. Effects of mineral N fertilizer, weed control, and seed rate on the incidence and severity of NBLS disease were determined. Two levels of mineral N, i.e., Department of Agriculture, Sri Lanka (DOF) recommended level and half of the DOF level were used with weedy and weed-free conditions, under four different seed rates, 100 kg/ha, 125 kg/ha, 150 kg/ha and 175 kg/ha. NBLS incidence was significantly higher (p<0.0001) in the Maha season compared to the Yala season. In the Maha season, mineral N fertilizer by weed interaction was significant on disease severity. In the Yala season, the disease incidence was significantly (p<0.05) higher in weedy conditions and fertilizer, weed and seed rate interaction and fertilizer and seed rate interaction were significantly (p<0.05) higher. The lowest disease severity was recorded in both seasons with the 100kg/ha seed rate, 100% fertilizer and weed-free conditions. When the weedy conditions prevailed in the field in Maha season, a disease severity scale value of 2 was observed at the highest frequency. Although only the weed condition affects NBLS incidence, it was found that the seed rate, fertilizer and, weed condition interaction was critical to control the severity of NBLS in paddy cultivation.