Target spot or Corynespora leaf spot has a history of being an elusive pest in cotton. It was first discovered in Mississippi in 1961, and then didn’t appear again until it was found in Georgia in 2008.
Since then, it has become prevalent in the coastal regions of the Cotton Belt and, more recently, has been found in the Mid-South states, costing growers there 100 to 200 pounds of lint per acre through premature defoliation of leaves in 2016.
While the impact can be costly in these times of low prices, detecting target spot in the cotton canopy can be difficult. Growers in Arkansas first spotted the disease in soybeans in 2015 and then began checking their cotton in 2016 to see if it was causing premature defoliation in the latter.
But, while the target spot disease found in soybeans is the same genus and species as that found in cotton, there is a difference in the isolates found in cotton and in soybeans in the Southeast, according to Bob Kemerait, Extension plant pathologist, University of Georgia.
“There’s been some information generated by the University of Georgia in cooperation with Cotton Incorporated, and the yellow portion of this chart lists the cotton isolates found not only in Georgia but in the Southeast,” said Dr. Kemerait, who spoke on target spot and bacterial blight at the Beltwide Cotton Conferences in Dallas.
Unique to cotton
“What we have found so far is that the Corynespora that causes target spot on cotton is unique to cotton. What’s on soybeans is not the same as on cotton; what’s on cucumbers is not the same as on cotton, and that’s important for two reasons. The first is that suggests it is a single introduction. Where it was between 1961 and 2008, I can’t tell you, but it’s here now.”
Dr. Kemerait said it shouldn’t be surprising that target spot first appeared in the Gulf Coast states and has now expanded into other areas “because that’s what it would do if it was an isolated introduction.”
Corynespora begins as a small spot that develops into whitish lesions with the characteristic “target-like” concentric circles on them.
“Once they reach that larger, target-shaped spot stage it leads to rapid defoliation,” Kemerait said during a presentation to the Consultants Conference at the Beltwide. “By that time the disease has already started.
“There are other diseases that can look similar. It’s important to know that if you’re even considering use of a fungicide, and we make a mistake and treat for Alternaria leaf spot, there’s no chance of success. We have to know what we’re treating.”
90 percent defoliation
As some Mid-South growers learned last fall, the disease can go from “a few spots” in the third week of bloom to 90 percent defoliation six weeks later. “Under the right conditions with the right variety, the disease can cause significant loss from premature defoliation,” he said.
Scientists are continuing to make new discoveries about target spot. “We’ve been focusing on these large, target-shaped lesions,” said Kemerait. “But, when you look at the top of the plant at the bolls, they’re covered with smaller spots. I believe they are becoming more important than we thought.”
Kemerait said bacterial blight was also difficult for producers in Georgia in 2016, a phenomenon reported in other areas by experts speaking at the Beltwide Cotton Conferences.
“Let me say I recognize as much as anyone that this disease can be seed-transmitted,” he noted. “But I would say that for consultants and growers going into 2017, we need to focus on the variety selection; we need to focus on residue management; and we need to be prepared for it.
“I’ll also say that if you are a grower that has zero tolerance for the risk that bacterial blight causes you, then you need to look for the most resistant variety. But, in doing that, there are things you may give up.”
For openers, growers have no nematode-resistant varieties that also have bacterial blight resistance. “And where I come from nematode resistance is critical,” he said. “We have to consider not only bacterial blight resistance, but we also have to consider other diseases, yield potential and varieties.”