Phenotypic Variation in the in Vitro and in Planta Aggressiveness Traits Among <i>Exserohilum turcicum</i> Strains From South Africa

ABSTRACT Exserohilum turcicum , causal pathogen of northern leaf blight (NLB), infects maize at various growth stages, resulting in economic loss. Characterising strain aggressiveness is essential for understanding disease development and informing management strategies. Using 10 E. turcicum strains, this study evaluated (i) in vitro colony growth (AUMGC), spore production (SP) and morphological variations, and (ii) in planta aggressiveness on the inbred maize line B73 by measuring the incubation period, time to necrotic lesion development (NLD), area under the disease progress curve (AUDPC), disease severity at 28 days post‐inoculation (dpi), and confirming Koch's postulates. Significant differences ( p  < 0.05) were observed among strains, with NLD, AUDPC and disease severity at 28 dpi reliably distinguishing aggressiveness. CMW‐IA:2926 was the most aggressive, with the shortest NLD (12 days), highest AUDPC and disease severity at 28 dpi (99%). CMW‐IA:2904 was the least aggressive, with the longest NLD (20 days), lowest AUDPC (22) and disease severity at 28 dpi (6.3%). A strong negative association ( r  = −0.83, p  < 0.05) was observed between AUMGC and SP, whereas SP showed a positive but non‐significant association with in planta aggressiveness traits AUDPC ( r  = 0.60, p  = 0.064) and disease severity at 28 dpi ( r  = 0.48, p  = 0.164). Although in vitro assays are valuable, in planta assays are essential for accurately evaluating the aggressiveness of E. turcicum strains. These findings highlight variation in the aggressiveness of E. turcicum strains, and provide a valuable foundation for durable NLB management in South Africa.