Effects of Planting Date on Grain Yield Components and Northern Leaf Blight Severity in Two Contrasting Maize (Zea mays L.) Cultivars Under Rainfed and Irrigated Conditions

Adverse meteorological conditions often force farmers to plant maize ( Zea mays L.) outside its optimal planting window. Early or delayed planting can significantly influence crop phenology, yield, and disease severity. This study evaluated two contrasting cultivars, PAN4R-872BR (early-maturing and NLB-susceptible) and DKC75-65BR (medium-maturing and NLB-resistant), across multiple planting dates and assessed the effect of planting date on Northern Leaf Blight (NLB) severity. Delayed planting increased days to flowering and the anthesis–silking interval (ASI), while reducing dry matter production, grain yield, and yield components. The area under the disease progress curve (AUDPC) showed that NLB severity increased with later planting under both irrigated (110% increase from planting date 1 to planting date 4 for both cultivars) and rainfed conditions (29% and 44% increase in PAN4R-872BR and DKC75-65BR, respectively). PAN4R-872BR consistently exhibited greater disease severity (59% higher AUDPC across all treatments) than DKC75-65BR, although disease severity also increased in DKC75-65BR (47%) under late planting. Planting after mid-January reduced grain yield by 40–50%, associated with increased NLB pressure, less favourable solar radiation and temperature conditions during grain filling. PAN4R-872BR did not provide a yield advantage under late planting, as maximum yields were achieved within the optimum planting window. Because only one genotype represented each maturity class and the cultivars differed in NLB resistance, these cultivar-specific responses should not be generalized as effects of maturity class. These findings highlight timely planting as an important strategy for maintaining maize productivity in the Highveld and underscore the need for integrated cultivar, disease, and water management under increasingly variable climatic conditions.