Field Validation of Fungicides for the Management of Rice Brown Spot Disease (Bipolaris oryzae)

Abstract

Brown spot, caused by Bipolaris oryzae, is an important constraint to rice production under favourable disease conditions. This study evaluated the field efficacy of eight fungicides against brown spot and assessed disease severity among rice genotypes cultivated in farmers’ fields. A randomised complete block design with three replications was established at Rampur, Chitwan, using the susceptible cultivar Sankharika. Eight fungicide treatments and an untreated control were compared following three foliar applications at seven-day intervals. Disease severity and the area under the disease progress curve (AUDPC) were determined, and grain yield was correlated with AUDPC. A roving survey was also conducted at 21 locations in Chitwan and Nawalparasi districts. All fungicide treatments reduced final disease severity relative to the untreated control. Metalaxyl 8% + mancozeb 64% WP recorded the lowest mean AUDPC (436.85), followed by carbendazim (470.50) and hexaconazole (482.20). Hexaconazole recorded the lowest final disease severity (28.90%), whereas the untreated control recorded 45.20%. The association between grain yield and AUDPC was weak and not statistically significant (r = −0.031). The survey identified six genotypes, with maximum severity of 78% in Samba Mansuli and Sabitri and minimum severity of 26% in Kalo Basmati. These findings indicate differential fungicide performance and genotype responses under the evaluated field conditions and support further multilocation and multiseason validation.