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Foliar diseases threatening corn unlike anything in the past

Agriculture.com Staff 07/22/2009 @ 11:27am

High humidity, scattered rain showers, overcast skies, and below-normal temperatures are brewing up a huge threat for corn in the form of disease, warns Tamra Jackson.

The University of Nebraska plant pathologist warns that diseases not normally seen on corn this time of year are rapidly forming and to such an extent that farmers need to be aggressively scouting their fields now to detect their onset. The weather conditions have already spawn a "rapid spread of gray leaf spot and common rust," Jackson points out.

Jackson sounded the alarm on gray leaf spot last week. This week she warns farmers to be the look out other diseases and, in particular, common rust.

Common rust is NOT the same as southern rust, which ravaged corn fields in some parts of the country in 2006 and 2007. Common rust occurs to some extent every but is normally not a major concern in western cornfield. "However, in 2008 when we experienced cooler than normal temperatures accompanied by frequent rainfall and high humidity, it became a major concern and was worse than in recent history in some parts of Nebraska," Jackson adds.

Common rust prefers temperatures that are cooler (61 to 77 degrees F) than the optimal temperatures preferred by southern rust (77 to 82 degrees F), which is why we saw more common rust last year, and will likely see it increase in severity and incidence this week as long as conditions persist.

Although the spores of the common rust and southern rust pathogens may differ somewhat in color, this is not a reliable way to differentiate them. Instead, look on both leaf surfaces to determine where pustules and spores are being produced. If there are just as many (or almost as many) pustules on both the upper and lower leaf surfaces, then the disease is more likely common rust. However, if the disease is southern rust, most of the spores and pustules will be limited to only the upper leaf surface.

Jackson also warns of other disease on the the increase including:

  • Physoderma Brown Spot. This disease can be easily confused with southern rust or other foliar diseases of corn. The pathogen causing this disease is Physoderma maydis, which requires free-standing moisture for its swimming spores to infect the leaf. Infection frequently occurs in the whorl, which can act as a cup and capture and hold rain/irrigation water for several hours, giving the spores time to infect the leaf. The alternating cycles of wet and dry that occur inside the whorl while the leaf is emerging eventually appear as bands of lesions across the leaf. These tan/brown lesions on the blade may be mistaken for other more severe diseases, such as southern rust.
  • Goss's Bacterial Wilt and Blight. This disease is caused by bacteria instead of fungi. Thus foliar fungicides are not expected to be effective controlling it, making disease identification critical for management and preventing unnecessary fungicide applications.

For more information about these and other disease and their control go to the University of Nebraska's Plant Disease Central.

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