ROCKVILLE, Md. (DTN) -- Defying its name once again, southern corn rust has invaded Midwestern cornfields earlier than ever -- and this time, it arrived in time to do economic damage.
"This is the earliest I've ever seen it -- and that's the third year in a row that I've said that," said Kansas State University Extension plant pathologist Doug Jardine.
The disease has been found in Kansas, Nebraska, Iowa, Indiana and Illinois. You can see a map tracking its spread here: http://bit.ly/….
Later-planted cornfields will be most at risk from the disease, and most corn hybrids are susceptible to it, said University of Nebraska Extension plant pathologist Tamra Jackson-Ziems.
Both Jackson-Ziems and Jardine estimate that most corn plants that are not at the end of dent stage could benefit from a fungicide application if infested.
That accounts for a lot of U.S. corn right now. As of July 17, USDA estimated that 60% of the nation's corn was not yet silking.
Fortunately, most of the rust found in Kansas and Nebraska are low-level detections, Jardine and Jackson-Ziems said. That gives growers time to begin to scout diligently and keep an eye on crop stage and weather conditions.
Currently, conditions are favorable for the disease in most of the Midwest, the plant pathologists noted.
"Southern rust is a tropical disease," Jardine said. "The combination of high nighttime humidity over 90% and hot daytime weather -- that's what it loves."
Because the disease often starts in hotspots within a field, scout multiple places within your fields.
DISTINCTIONS IN RUST DISEASES
Southern corn rust looks very similar to common rust, but there are some important distinctions. Common rust prefers cooler weather, which is why its progress has slowed in many Nebraska fields, as southern corn rust picks up, Jackson-Ziems said.
Common rust pustules tend to be dark red or brown, and are found primarily on the bottom of corn leaves. In contrast, southern corn rust pustules are more orange and tan in color and more often spotted on the tops of corn leaves.
"If they're not side by side, it's darn hard to tell them apart," Jackson-Ziems admitted. "Pustule location is probably the more important difference for identification, and by all means, get a sample for the diagnostic clinic."
Perhaps most importantly, southern corn rust can be far more economically damaging. Both Jackson-Ziems and Jardine said they have seen up to 30% yield loss in research fields from the disease in the past.
The disease is well-controlled by most fungicides, but knowing when to spray is tricky. Because southern rust's spread and damage potential depend so heavily on weather conditions and crop stage, plant pathologists do not publish specific thresholds at which to treat.
Jackson-Ziems puts it this way: "If I started seeing it in several locations in my own field -- not just in one or two spots -- and the weather conditions were favorable, and the corn was still early in the grainfill stage, I would consider a fungicide application."
Jardine agreed and noted that fields starting to dent are probably beyond much economic damage. Milk to soft-dough cornfields would be well-worth scouting in the next two weeks, however.
"Then there are the fields that are just pollinating -- those are the ones I'm most concerned about," he said. "There's not enough rust out there yet to spray today, but there might be by next week."
Most fungicides will only provide a 21- to 28-day window of protection, so growers with delayed fields should keep in mind that if they spray too early, they may have to re-apply fungicides later in the season, both plant pathologists warned.
Finally, don't forget to tweet any discoveries or questions about the disease in your field, along with your location, to the Twitter account @corndisease.
Emily Unglesbee can be reached at Emily.email@example.com.
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