Genetic Diversity and Pathogenicity of Cercospora zeae-maydis Isolates from Maize Farms in South West and South East Nigeria

Grey leaf spot of maize caused by Cercospora zeae-maydis is a foliar disease of increasing importance in Nigerian maize production, particularly in zones with extended leaf wetness periods. Its genetic diversity and pathogenicity across different agroecological zones have not been characterised at the molecular level in Nigeria, creating a knowledge gap that limits resistance breeding and regional management planning. This study collected 65 C. zeae-maydis isolates from maize farms in Ogun, Osun, and Oyo States in the South West and from Anambra, Enugu, and Imo States in the South East during the 2022 and 2023 wet seasons. Genetic diversity was assessed using microsatellite markers and partial sequencing of the cercosporin toxin biosynthesis gene cluster. Pathogenicity characterisation involved inoculation of eight maize inbred lines representing major genetic groups, with disease severity measured at 21 and 28 days post-inoculation. Significant genetic differentiation was detected between South West and South East populations, with FST of 0.28 suggesting moderate gene flow restriction between zones. South West isolates showed higher cercosporin production capacity in vitro, and pathogenicity assays confirmed their greater aggressiveness on all tested inbred lines. Inbred line B73 showed consistently lower disease ratings across both isolate populations, identifying it as a useful resistance source. No isolates were pathogenic on sorghum controls, confirming host specificity. The study provides foundational genetic and pathogenicity data supporting region-specific resistance breeding for grey leaf spot in Nigerian maize. Keywords: Cercospora zeae-maydis, grey leaf spot, maize, genetic diversity, pathogenicity.