For those hoping Asian soybean rust had petered out, the week of Aug. 15 wasn't a good one. While still not the explosive disease many feared, the rust continues to spread in the Southeast. The latest infected sites are the farthest north yet discovered.
On Aug. 16, South Carolina was confirmed as the latest stop for the disease. As the season has progressed, soybean producers in the state watched the disease creep through neighboring Georgia and wondered when it would leap the border.
“No one has to wonder anymore,” said John Mueller, a Clemson University plant pathologist. “We've got it and I doubt anyone is shocked.”
The soybean rust was found in a commercial field in Hampton County. Hampton County is on the Savannah River “almost directly across” from Georgia's Effingham County, where rust was discovered earlier this summer.
The South Carolina soybean crop has both full-season soybeans and those planted after wheat. In the southern half of the state, the full-season beans are about three weeks from finishing.
“Some of the producers are spraying for pests and mixing in a fungicide, mostly a strobilurin. On the early-planted beans — which look really good, incidentally — that should hold them through to harvest.”
Many of South Carolina's soybeans have already been treated with fungicides. “We've had an extremely wet year here — especially in the south-central part. We've had a lot of frogeye, septoria and downy mildew. So there's already been fungicide spraying.”
On the same day South Carolina confirmed rust, Georgia was discovering a new infected sentinel plot — this one in Putnam County. Two days later, on Aug. 18, rust was found in Plains (in Sumter County) in a sentinel plot at the Southwest Georgia Research and Education Center.
Nine Georgia counties are now known to harbor the rust. However, the disease has yet to be found in a commercial field. Few suspect that will remain the case.
“A continuing spread of the rust is a very reasonable expectation, especially if we stay in the current humid, rainy conditions,” said Phil Jost, University of Georgia Extension agronomist.
There have been showers somewhere on the Coastal Plain almost daily for weeks. “Soybeans like a good shower but so does rust. And there are several tropical storms threatening. Our beans' growth stage in conjunction with the weather points to rust building. Those factors are helping this problem get worse.”
The majority of Georgia's crop is just beginning to bloom heavily. Beans are weeks away from finishing. “Obviously the inoculum is building and spreading,” said Jost. “Our beans, in the next few weeks, are going to be in susceptible stages. Producers need to get a fungicide — a combination strobilurin and triazole — out. That combination will provide some curative action along with a couple weeks worth of residual protection.”
In the early morning of Aug. 18, standing in a north Talledega County, Ala., sentinel plot, Ed Sikora found soybean rust lesions on two leaves from R-6 plants. He then drove a mile down the road to a commercial soybean field outside Renfroe, Ala.
“The field is about 18 acres at R-3/R-4,” said Sikora, Auburn University Extension plant pathologist. “The grower allowed me to scout and I found a lesion on one leaf along the tree line.” The lesion later proved to be rust. The field it was found in hadn't been sprayed with a fungicide.
From Talledega County, Sikora traveled north and east to three other counties — DeKalb, Etowah and Calhoun. In them, he checked five commercial fields and three sentinel plots and found nothing suspicious.
Through August, Sikora anticipates new reports of rust movement every couple of days. “Favorable weather conditions are here but the problem doesn't appear to be blowing up. I just see it continuing to spread slowly.”
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