<p>Alternaria leaf spot diseases threaten global cabbage production, causing yield losses of up to 70%, especially in West Africa. In Ghana, an outbreak resembling Alternaria leaf spot on Oxylus cabbage has been observed in Ejisu Municipal (Besease and Boankra) and Bosome Freho District (Adeito and Kokoben) but remains uninvestigated. This study aimed to identify the pathogen responsible and assess the virulence of the isolates. During the 2022 growing season, diseased leaf samples were collected from Oxylus cabbage fields for <i>Alternaria</i> species isolation. A completely randomized design with four replicates was used to test <i>A. brassicicola</i> and <i>A. alternata</i> in detached leaf and whole plant assays. Disease incidence, severity, infected leaf area and overall disease development were recorded. Results showed <i>A. brassicicola </i>[Schwein] Wiltshire as the primary pathogen, with significant differences (<i>p</i> &lt; 0.05) in disease incidence and severity. The isolate from Adeito caused the most severe infection, with the largest infected leaf area (16.57&#xa0;cm²) in detached leaves and the highest disease incidence (95.99%) and severity (50.71%) in whole plant assays. This study confirms <i>A. brassicicola </i>[Schw.] Wiltsh. as the causal agent of Alternaria leaf spot disease on Oxylus cabbage in the Ashanti Region. Further research is needed to develop effective management strategies for this disease in Ghana.</p>