Rust Alert

ROCKVILLE, Md. (DTN) -- Southern corn rust is working its way north earlier than ever this year. Late-planted corn will be most at risk for yield losses from this fast-moving pathogen, plant pathologists told DTN.

"Rust diseases produce so many spores and have such a quick turnaround in the disease cycle that they always have the potential to blow up," explained University of Kentucky plant pathologist Carl Bradley. "It will be really important for people to scout if their corn is prior to R3 [milk stage] and especially if it is silking."

The disease has pushed the geographic limits of its name in the past few years. It overwinters in the tropics and depends on storms to move it northward into the southern states. Last year it reached Georgia in early June, was in Kansas by the end of the month, and by August it was racing through corn fields as far north as Illinois, Indiana, Missouri and Nebraska.

This year, the disease set a record early arrival date of June 1 in Georgia. It has now surfaced in Kentucky, and Kansas plant pathologist Doug Jardine recently spotted it the corn fields of southeast Kansas.

Southern corn rust thrives in tropical conditions -- high relative humidity and high temperatures.

"Weather conditions this season have certainly helped the rust problem to form," said DTN Senior Ag Meteorologist Bryce Anderson. "In contrast to the past two seasons, 2016 has been very warm, and this has been great for the rust development. Add to it the frequency of rain that we saw during June and early July, and the opportunity is there for rust to thrive and move."

With a heat wave descending upon the Midwest and Deep South this week, the disease is likely to spread quickly, Jardine and Bradley said.

Corn fields that were delayed due to a wet spring are most at risk, they said. Growers whose corn is silking or in the milk stages should scout carefully and consider fungicide treatment if they find the disease in their fields.

Left untreated, corn can suffer serious yield losses even with a midsummer arrival of the disease, Jardine said. Last year, corn test plots planted in late April in southeast Kansas lost 10% of their yield on average to the disease, he said.

Southern fields routinely lose 25 bushels per acre when susceptible hybrids aren't treated, according to a Purdue disease guide. Losses of up to 80 bushels per acre have been documented in severe cases, University of Georgia entomologist Bob Kemerait told DTN.

The disease's symptoms, orange pustules, may be confused with common rust, Jardine said. Common rust prefers cooler temperatures.

Southern corn rust pustules are smaller and lighter colored than common rust. The yellow-orange spores move readily -- a light-colored shirt can quickly turn orange in an infected field, Jardine added. Southern rust also tends to appear on the top side of the corn leaves only, unless the disease presence is especially heavy.

There aren't published thresholds for southern corn rust because the disease moves so quickly, Bradley said. If you find the disease in a field at or before the milk stages and conditions are right for further disease development, spraying would be wise, he said.

Even cornfields entering hard dough could benefit from treatment if the southern rust is bad enough, Jardine said.

Most hybrids are pretty susceptible to the disease, although that isn't always noted in seed descriptions, Jardine and Bradley noted. Bradley recommended calling your seed supplier to check on your hybrids' resistance ratings.

For a Midwestern perspective on finding and managing the disease, see this Purdue guide: http://bit.ly/….

Emily Unglesbee can be reached at Emily.unglesbee@dtn.com

Follow Emily Unglesbee on Twitter @Emily_Unglesbee

(GH/CZ)

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