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Many corn foliar diseases thrive with summer humidity and moisture. Checking fields after a rain event, even during drought-like conditions, can help growers manage these diseases that are waiting to attack the crop.

“Prolonged water on the lower canopy of corn plants is what leads to infection,” said Matt Vandehaar, Pioneer Field Agronomist. “Diseases like gray leaf spot, northern corn leaf blight and tar spot are likely to pop up after these rain events.”

Tar spot, which develops during low temperatures (60°F to 70°F) and high relative humidity (greater than 75%), spreads from wet lower leaves to the upper leaves. It will then make its way to the leaf sheathes and eventually the husks of developing ears, resulting in reduced weight and loose kernels, with some kernels germinating prematurely.

Northern corn leaf blight (NCLB) is a concern for growers with higher-residue fields. Rain splash can spread NCLB spores, which attack lower leaves and progress up the plant. New NCLB lesions can produce spores in as little as one week, allowing the disease to spread much faster than other corn leaf diseases.

Yield losses are most severe when NCLB infects corn plants early and progresses to the upper plant leaves prior to pollination or ear fill.

Gray leaf spot thrives in high temperatures and high humidity. Conducive weather conditions encourage the rapid spread of disease near the end of summer and early fall when corn plants allocate more resources to grain fill.

Entire leaves can be killed when weather conditions are favorable, and rapid disease progression creates leaf lesions. These lesions hinder photosynthetic activity, reducing carbohydrates allocated to grain fill. Damage can be more severe when developing lesions progress past the ear leaf during pollination.

“It’s important to check those leaves as soon as possible,” Vandehaar said. “Rain can cause or spread diseases rapidly. Proper identification can help get a treatment plan in place before too much damage is done.”

We can set up time for you to interview a Pioneer agronomist about these or other topics. Contact Larissa Capriotti at