Mid-October Sowing: An Eco-Friendly Strategy for Managing Alternaria Blight and Improving Yield in Rapeseed (Brassica napus L.) under Nawalpur Sarlahi Conditions of Nepal

Abstract

Alternaria blight, caused by Alternaria spp., is a major constrain to rapeseed production and Often leads to reliance on chemical pesticides, which may cause health and environmental problems. A two-year field study (2023-2024) was conducted under natural epiphytotic conditions at the Oilseed Research Program, Sarlahi, Nepal to evaluate sowing date adjustment as an eco-friendly approach for disease and yield management. The experiment was conducted in a randomized complete block design using rapeseed variety Nawalpur Tori-4 and nine sowing dates from mid-September to mid-November. Disease scores and yield attributes were recorded and analyzed. Sowing date significantly affected disease severity and grain yield. Sowing on October 15 resulted in the lowest percent disease index (PDL 28%) and the highest grain yield (1424.50 kg,ha), followed by October 23 (32% PDI and grain yield 1319.17 kg/ha). Early sowing in September resulted in lower disease severity but comparatively lower yield (1165 kg/ha), whereas late sowing from late October to November resulted in severe disease severity and reduced yield (841.67 kg/ha). Regression analysis showed that weather conditions 35 days after sowing explained 90% of the variation in disease severity. Higher temperature and relative humidity during this critical period increased disease severity (p < 0.01). Mid-October sowing therefore offers an effective strategy for reducing Alternaria blight and improving yield without fungicides.