Researchers continue to look at the time frame that the leafspot and defoliation develop, says Hagan.

“I had two trials where I rated cotton over an extended period of time. In the central Alabama study, the spotting probably started about mid-July, and the cotton was about 55 days old at that time.

“On the Gulf Coast, the leafspotting got started at the end of July, and this cotton was planted in early May, making it 75 to 80 days old, so we really haven’t pinned down the time frame of when the leafspotting begins.”

Studies conducted this past year were intended to help determine the type of yield loss that might occur due to target spot in cotton, says Hagan.

“We used Phytogen 499 and Deltapine 1050, which we identified in 2011 as being susceptible and maybe a little less susceptible, respectively, to this disease in a variety trial. At the end of the season, just before the cotton was sprayed with a defoliant, we had about 80 percent leaf shed in the untreated Phytogen 499 versus about 50 percent in the Deltapine.”

It’s very hard to define how much yield loss is occurring, and it’s hard to set up a trial in order to get the high level of disease control needed to show that, he says.

The estimate was made of 250 pounds of lint per acre, on average. Some consultants in Georgia suggested that the yield loss in some cases was up to 600 pounds per acre. These, however, are rough estimates, says Hagan.

“We do know that target spot does reduce the yield of cotton,” he says.

“On Phytogen 499, as the disease intensified, yield went down. We need to more specifically determine what the yield loss might be. On the Deltapine variety, as the disease intensified, yield did not go down.

“As we increased the number of Headline applications on Phytogen 499, we got a 100-pound increase in yield with each successive Headline application. By the time we got done with five Headline applications, we came up with a 600-pound increase in seed cotton yield.

 

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 “With DPL 1050, we also got a significant yield gain as the number of Headline applications increased. With that variety, it was an increase of about 60 pounds of seed cotton per application.”

There are risk factors to be considered in target spot incidence, says Hagan.

“There is some difference in sensitivity among cotton varieties to this disease. You’d think that crop rotation would have a major impact with this pathogen, but that’s still up in the air.

“When we’ve gone out and looked at problems, it seems like the worst incidence of the disease has come in fields where cotton followed cotton in strip- or no-till production. We also need to look at the influence of tillage on this disease.

“The other factor is canopy moisture. You can’t look at higher defoliation and say that will be the higher or lower yielding variety.”

In fungicide trials, the only material that significantly increased yield over the non-treated control was Headline at 9 fluid ounces, says Hagan. Cotton was sprayed at first bloom and then 14 days later, and that’s a standard preventative treatment, he says.