Occurrence and Characterisation of Alternaria Blight on Onion (Allium cepa) in Sokoto State, North West Nigeria

Onion (Allium cepa) production in Sokoto State forms the foundation of a major commercial agricultural enterprise serving both domestic Nigerian markets and trans-border trade with Niger Republic. Alternaria blight, caused by Alternaria porri, is among the most frequently observed foliage diseases of onion and can significantly reduce bulb yield when severe defoliation occurs at critical growth stages. This study examined the occurrence and severity of Alternaria blight on onion in five major onion-producing communities in Sokoto State during the 2023 to 2024 dry season. Seventy-five onion plots were assessed for disease incidence and severity using published scoring criteria. A. porri was confirmed as the causal agent through spore morphology, isolation, pathogenicity testing, and molecular analysis. Overall blight incidence was 62 percent across assessed plots, with severity scores significantly higher in plots planted in high-humidity microclimates along irrigation canal edges. Plots receiving nitrogen fertiliser at recommended rates exhibited marginally lower blight severity compared to over-fertilised plots, consistent with published observations that excessive nitrogen can enhance Alternaria susceptibility. Farmers primarily relied on mancozeb fungicide applications but showed inconsistency in spray intervals. The study recommends targeted fungicide scheduling aligned with rainfall and dew periods, balanced fertiliser management, and promotion of Alternaria-tolerant onion varieties suited to Sokoto State growing conditions. Keywords: onion, Alternaria blight, Alternaria porri, Sokoto State, disease characterisation.