Auburn University Plant Pathologist Austin Hagan has worked with the disease pathogen for the past couple of years in cotton and says he still isn’t sure how big a problem it can be.

“I work primarily with peanuts, and when I see 40-50 percent defoliation, I’m expecting big yield losses.

“Even on varieties with some tolerance or resistance to the disease, we saw 50 percent defoliation from target spot on cotton. So, from that standpoint, it is a scary disease,” Hagan says.

“However, when you look at the impact the disease has on cotton yield, it’s a little like a beagle — its bark can be much worse than its bite,” he adds.

Target spot occurrence and severity is definitely different among different cotton varieties, the Auburn researcher adds.

In tests in 2011 and last year, he says PhytoGen varieties, in particular Phytogen 499, were more susceptible to target spot than Deltapine varieties, especially DPL 1050, which appears to be somewhat tolerant or resistant to the disease.

“In untreated plots at two different locations in the southern part of Alabama, we consistently saw 80 percent defoliation in Phytogen varieties, compared to 50 percent on Deltapine varieties.”

Hagan says this increased defoliation did not equate to lower yields in the PhytoGen varieties.

“By applying fungicides, we were able to see a 120 pound per acre increase in lint on PhytoGen 499, versus a 50-60 pound increase for DPL-1050, per application of Headline, and we can only use two applications of any of the strobilurin fungicides,” he says.

“Still, there was no statistical difference in yield between the two varieties in these tests,” he adds.

“Exactly how much target spot affects cotton yields is just not clear. We know heavy pressure and subsequent defoliation on PhytoGen 499 caused a 200 pound per acre loss.

“However, the yield potential of this variety is so high, maybe 7,000 pounds per acre, so that 200 pound loss could be more like 1,200-1,400 pound loss from the true yield potential of the variety, Hagan says.

Likewise, the plant pathologist says the efficacy of strobilurin fungicides for managing target spot on cotton is not clear.

“I’m used to working with peanuts, and when I put out 4-5 fungicide applications, I expect to see a clean field.

“With this disease, we were still getting 50 percent and higher defoliation of cotton plants,” he says.

For growers in the Upper Southeast, the jury is still out on target spot. At the very least growers should keep an eye out for the disease and make cost effective decisions whether to spray fungicides, if it occurs.

rroberson@farmpress.com

 

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