Population Structure of Fusarium oxysporum f. sp. lycopersici in Irrigated Tomato Production Zones of Kano and Kaduna States, Nigeria

Fusarium wilt of tomato caused by Fusarium oxysporum f. sp. lycopersici (Fol) is a destructive soilborne disease in Nigeria's irrigated vegetable belts, with virulence structure of local populations remaining poorly understood. The presence and distribution of pathogenic races determine which resistance genes remain effective, making population characterisation directly relevant to breeding and management decisions. This study characterised the race structure and genetic diversity of 82 Fol isolates collected from wilted tomato plants across nine irrigation schemes in Kano and Kaduna States. Race determination was performed using a standard differential host set including tomato varieties carrying I, I-2, and I-3 resistance genes. Molecular diversity was assessed using amplified fragment length polymorphism and translation elongation factor sequencing. Race 1 was the predominant race, accounting for 62 percent of isolates, followed by Race 3 at 29 percent, with Race 2 observed in 9 percent of samples. Farms previously planted with Race 1-resistant varieties showed a significantly higher proportion of Race 3 isolates, suggesting directional selection pressure from resistance deployment. Genetic diversity was higher in Kano State populations, consistent with longer irrigated tomato history in that region. Phylogeographic analysis indicated that Race 3 isolates from Kaduna may have been introduced from a single source population, with subsequent clonal spread. These findings directly inform the selection of I-3 gene-based resistance varieties as the priority deployment strategy in the study zones and caution against Race 2 complacency in Kano irrigation schemes. Keywords: Fusarium oxysporum, tomato wilt, race structure, irrigated production, North West Nigeria.