Know Your Spots - Leaf Diseases On Corn

Several foliar diseases are showing up on corn. Should you spray a fungicide to try to control them?

What disease is this on my corn? Should I spray a fungicide? Those were two popular questions from farmers this summer. Alison Robertson, Iowa State University Extension plant pathologist, spent a lot of time answering them.

Several foliar diseases of corn show up in Iowa, and there's considerable interest in applying foliar fungicides to corn to try to protect potential yield. "It's important to be able to tell the difference between various leaf spots because management options will vary," she says. "Just because it's a leaf disease doesn't necessarily mean a fungicide application will control it."

Not all diseases are caused by fungi. Some leaf spots are caused by bacteria and can't be managed with a foliar fungicide. You also must consider that development of other leaf spots will slow considerably in hot weather. However, there are some leaf spots that favor hot, humid conditions and depending on disease pressure and corn hybrid susceptibility, management with a foliar fungicide may be warranted. Robertson provides the following guidelines.

Know your corn leaf diseases

Common rust. There have been numerous reports of common rust across Iowa in 2008. While some indicate higher common rust pressure than in previous years, most say the disease pressure is about the usual amount.

Common rust development is favored by moderate temperatures. Thus, hot temperatures should slow common rust development down significantly. Foliar fungicides are effective against common rust, but it's rare for common rust disease severity to be high enough to warrant application.

Common rust pustules are brick red in color. They can be found on both the upper and lower leaf surfaces. Southern rust pustules are more orange-brown and are only found on the top surface of the leaf. It can be difficult to distinguish between these two diseases. Common rust pustules tend to be more elongated than the pustules of southern rust which are usually more round.

Southern rust was reported in Nebraska at the beginning of August. This disease caused considerable yield loss in Nebraska in the 2006 growing season. Corn growers in southwest Iowa should watch for southern rust.

Southern rust can develop very rapidly when conditions are hot, humid and wet. A timely fungicide application can protect up to 30% of yield.

Gray leaf spot. As of early August, a few gray leaf spot lesions (GLS) could already be found in the lower part of the crop canopy. Lesions of GLS are light, tan and rectangular in shape because the lesion's width is limited by leaf veins. The lesions expand lengthways (1/4 in. to 2 in.) and become gray in color.

Most reports in early August this year specify that GLS lesions can be found on the fifth or sixth leaf below the ear leaf, although there are reports of GLS occurring as high as the third leaf below the ear leaf. Hot (75 to 95 degree F), very humid (90% humidity for 12 hours or more) conditions favor GLS development.

Management recommendations advise if a susceptible hybrid is being grown, and GLS can be found on the third leaf below the ear leaf of 50% or more of the plants in the field, and hot, humid conditions continue—then a fungicide application may be warranted.

Northern leaf blight. Northern leaf blight has shown up in eastern Iowa. Some hybrids seem particularly susceptible to this disease. Lesions of northern leaf blight are large (1 to 6 inches long), cigar-shaped and tan in color.

Northern leaf blight lesions are very similar to those of Stewart's disease which are caused by a bacterium spread by the corn flea beetle.

Lesions of Stewart's may also be somewhat cigar-shaped, but the margins of the lesion are wavy and the lesion tails off down a vein. Often times a flea beetle feeding scar is evident in the lesion.

Eyespot. Eyespot is another common foliar disease in Iowa. The disease is favored by cool, wet weather and thus is usually more common in Northern Iowa. Hot temperatures slow disease development. Foliar fungicides are effective against eyespot, however the disease is usually not severe enough to justify an application.

Southern rust. Southern rust has recently been reported in Nebraska. This disease caused considerable yield loss in Nebraska in the 2006 growing season. Corn growers in southwest Iowa should watch for southern rust.

Southern rust can develop very rapidly when conditions are hot, humid and wet. A timely fungicide application can protect up to 30% of yield. Unfortunately, southern rust can be easily confused with common rust, although there are a few subtle characteristics that can be used to distinguish southern from common rust. For photos of these diseases see http://www.extension.iastate.edu/CropNews/2008/0805robertson.htm

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