When you check corn fields this summer you may find an impostor … corn leaves that only look diseased. It's called “disease lesion mimic” because it looks like typical corn disease damage, but so far plant pathologists haven't been able to isolate a pathogen that causes it.

“Typically, it starts with yellow spots on the lower leaves that develop into necrotic dead tissue. The damaged areas usually coalesce until they touch (see photo),” says University of Nebraska-Lincoln (UNL) plant pathologist Tamra Jackson. “We first saw what we believe is disease lesion mimic in Nebraska in 1999, again in 2005 and again now in 2007,” she says. “Sometimes it's just spots in the field, sometimes it affects the entire field. Along with the leaf damage, we've seen stunted plants and some plants without ears.

“Research from Purdue University tells us the environmental factors that seem to trigger disease lesion mimic are bright light, like on a cloudless day after a rain; low humidity; and low nighttime temperatures,” Jackson says. “Right now there's nothing we know of to treat it.”

Since there's no pathogen, the problem may be genetic, according to Jackson. “We've identified the problem on three different hybrids from three different companies,” she says. “But we really don't have enough data to be able to draw any conclusions at this point.”

For more information check the July 13 issue of Crop Watch at www.cropwatch.unl.edu.