Why Do My Corn Leaves Have Long Grey-Tan Lesions?

Long, elliptical or cigar-shaped lesions running parallel to the leaf veins — greyish-green to tan-brown, sometimes with darker borders and a slightly water-soaked appearance when young — are the signature symptom of northern corn leaf blight (NCLB), caused by the fungus Exserohilum turcicum. It is one of the most widespread corn foliar diseases in temperate and humid climates and appears most commonly during cool, wet summers.

Recognising northern leaf blight

The lesions are distinctive: they are long — typically 2.5 to 15 centimetres — elliptical, and run parallel to the leaf veins rather than crossing them. Mature lesions are pale grey-tan with a slightly grayish cast, and under humid conditions a dark grey-green powdery spore layer may form on the lesion surface. Lesions typically first appear on lower leaves and progress upward. In severe infections, multiple lesions merge and the leaves die prematurely, reducing photosynthesis and kernel fill.

When yield impact is significant

NCLB affects yield primarily through canopy defoliation that reduces the photosynthetic capacity during kernel fill — the six weeks after silking when kernels are developing and accumulating dry matter. Infection that occurs after the cobs have reached maturity causes minimal yield loss. The critical threshold is whether the disease is defoliating the upper canopy before or during silking. Light infection on lower leaves only is rarely economically significant. Whole-plant infection spreading rapidly before tasselling in a cool wet summer is the scenario that causes meaningful yield reduction.

Management

In the home garden, the most practical responses are: choosing varieties with partial resistance to NCLB (many modern sweetcorn varieties carry Ht genes for resistance, which are worth selecting for in areas with humid summers); rotating corn to different ground each year to reduce the inoculum load; and removing and destroying heavily infected leaves and crop debris after harvest. Fungicide application at the whorl stage or at tassel emergence is practiced commercially but is rarely practical or necessary at garden scale.