Alternaria brassicicola is a major necrotrophic pathogen causing black leaf spot in cabbage and other cruciferous crops. This study aimed to characterize 19 pathogenic isolates of Alternaria spp. collected from three cabbage-growing regions in Egypt based on morphology, pathogenicity and genetic diversity. Molecular identification based on ITS sequence clustered the collected isolates along with A. brassicicola confirming their identity. Deep morphological analysis delineated the variations in conidial size and septation among the isolates. Pathogenicity test was performed on detached and attached leaves, showing wide variations in lesion size and disease severity among isolates. Genetic diversity analysis used five ISSR primers, generating 46 bands with 72.61% polymorphism. Cluster analysis grouped the isolates into two main clusters, although no clear correlation appeared between genetic patterns, geographic origin, or virulence. Antifungal in vitro assays showed differential sensitivity to difenoconazole, azoxystrobin, mancozeb, and copper oxychloride, with difenoconazole being the most effective at the recommended doses per each. The combined variation in morphology, virulence, genetic sequences, and fungicide sensitivity suggests high adaptive potential in local A. brassicicola population. These findings highlight the need for integrated disease management using diverse resistance sources and fungicide rotation strategies for sustainable control
Alternaria brassicicola, genetic characterization, leaf spot disease, ITS sequencing, molecular markers, disease severity assessment, cluster analysis, resistance breeding
High genetic, pathogenic, and fungicide-response variability among Alternaria brassicicola isolates underscores their capacity for rapid adaptation in cabbage fields. Accordingly, resistance-breeding programs should pyramid multiple resistance sources and screen candidate cultivars against a broad, representative panel of isolates to secure durable, wide-spectrum control of the leaf-spot disease.