Gray Leaf Spot

Early symptoms are small, pinpoint spots that expand into long, narrow, rectangular lesions running parallel to the leaf and sharply limited by the veins, giving them straight sides. Mature lesions are grey to tan and, when numerous, can merge to blight large areas of leaf. Holding a leaf up to the light shows the veins clearly bounding each lesion, a key diagnostic feature.

Cercospora zeae-maydis survives between seasons on infected maize residue at the soil surface. Under warm, humid conditions it produces spores (conidia) that are carried by wind and rain splash onto the lower leaves of a new crop. Repeating cycles of infection then move the disease up the plant, with heavy dew and high humidity strongly favouring spore production and infection.

• Infected maize residue at the surface is the main source of inoculum
• Wind- and rain-dispersed conidia infect the lower leaves first
• Warm, humid nights with heavy dew favour sporulation and infection
• Repeating cycles carry lesions up toward the ear leaf during grain fill

Maize, including sweetcorn, is the host of this disease. Continuous maize with reduced tillage keeps large amounts of infected residue at the surface, and warm, humid, dew-prone weather then drives infection. Fields in river bottoms, sheltered sites, and areas with long periods of leaf wetness tend to see the most severe development.

• Continuous maize with surface residue retained under reduced tillage
• Warm temperatures with high humidity and prolonged heavy dew
• Sheltered, low-lying fields where leaf wetness persists
• Susceptible hybrids that allow rapid lesion development

Management combines resistant hybrids with practices that reduce residue-borne inoculum. Because the disease is driven by overwintering residue, rotation and residue handling are particularly effective in continuous-maize systems.

• Select hybrids with good gray-leaf-spot resistance ratings
• Rotate away from maize and manage residue to reduce carry-over inoculum
• Scout the ear leaf and leaves above it from tasselling through grain fill
• Favour practices that reduce prolonged leaf wetness where feasible
• Prioritise higher-risk fields such as sheltered or river-bottom sites