SIOUX FALLS, S.D. (AP) -- A late-summer string of hot, humid weather has brought an unwelcome visitor to South Dakota's corn crop.
Southern rust, a leaf disease caused by spores blowing in from southern states, has been detected in fields in Turner, Lincoln and Brookings counties, said Emmanuel Byamukama, a South Dakota State University Extension plant pathology specialist.
When it hits a corn crop in its early stages, the disease can negatively impact the number of bushels that can be pulled from an acre, but the southern visitor's arrival should not have a significant impact on yields as much of South Dakota's corn kernels have already turn from doughy to dented, Byamukama said.
"This is really a surprise," he said. "Either we're looking a little more cautiously or it never occurred and is starting to reach South Dakota."
Southern rust differs from common rust, which can be seen in most South Dakota corn fields.
In August, plant pathologists in Nebraska announced the confirmation of southern rust in 11 counties.
Sam Markell, a North Dakota State University Extension plant pathologist, said trace amounts of common rust have been detected in North Dakota but southern rust has never reached so far north. He said it's surprising and a little alarming that the disease has found its way into South Dakota.
"Usually it doesn't really come across the Mason-Dixon line all that much," Markell said.
In its latest South Dakota crop report, the U.S. Department of Agriculture estimated that 41 percent of the state's crop had reached the dented stage as of Sept. 1, a pace that trails the five-year average of 49 percent but is far behind last year's 78 percent.
Southern rust's arrival in South Dakota should not have a significant effect on yield for crops that have passed the dent growth stage, Byamukama said.
"The risk for this rust to become an issue is really limited," Byamukama said.
But many South Dakota cornfields were planted late because of the cool, wet spring, he said, and those growers should survey their fields to check for the presence of southern rust. Although most corn hybrids have resistant genes to common rust, the plants are susceptible to southern rust so a fungicide should be applied as soon as southern rust is detected, he said.
Common rust, a typical sight each year on tasseled corn rust appears as elongated dark red to brown spores in a random pattern on both the top and lower sides of leaves.
Southern rust shows itself as clusters of light brown or orange bumps, round or elliptical in shape, mainly on the top sides of leaves.
"If you hold the leaf against the light, for southern rust you're going to see a yellow halo around each pustule," Byamukama said. "For common rust, there's usually no halo."
Southern rust favors hot, humid conditions, as spores can develop quickly in warm nighttime dew and reproduce in high temperatures. The spores can't survive a South Dakota winter, so new ones would have to blow in next summer to affect the 2014 crop, Byamukama said.
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