What is in this article?:
- Target spot disease continues its march into Southeast cotton
- Drop nozzle advantage
- Target spot first began showing up on Alabama cotton in 2011.
- There are differences in the sensitivities of cotton varieties to the disease.
- Cotton with a high yield potential is more susceptible to target spot.
With an absence of pressing issues in cotton insects, other pest problems are moving to the forefront of producer concerns, including one that was given little thought until recently — target spot disease.
“There has been a lot of interest in target spot on cotton since it was first detected in southwest Georgia eight or nine years ago,” says Austin Hagan, Auburn University Extension plant pathologist. “It started showing up in Alabama in 2011, and in 2012, it jumped into South Carolina, North Carolina and Virginia. This year, it has expanded into Mississippi and into Arkansas and possibly Louisiana.”
Target spot is definitely a wet weather disease, said Hagan during the recent Central Alabama Crops Tour. “You see a great variation in the amount of disease in a field, and it has a lot to do with canopy architecture. If you have a small plant, and they don’t lap or don’t lap very quickly, the odds are that you won’t see much target spot in those fields, regardless of the variety. If you get fast top growth and rapid canopy coverage, that’s an ideal situation for target spot,” he says.
To some degree, says Hagan, target spot is a disease of high-yield cotton. “The higher the yield potential, with a lot more growth, the more likely you are to see it, and the more severe it’ll be.
There also are differences in the sensitivity of cotton varieties to target spot. There are some that are very susceptible and some that are simply susceptible. There are no resistant varieties, and there’s no variety that you can plant where you won’t under ideal conditions see a moderate amount of defoliation.”
The issue at this point in time is which varieties are having substantial yield loss and which are not, says Hagan. Based on work from last year, varieties that are more susceptible may be losing around 200 pounds of lint per acre.
Big losses reported
“Some consultants in southwest Georgia are saying the losses are up to about 600 pounds of lint, but we haven’t been able to document that so far.”
In the central Alabama trial, researchers are putting out six applications of Headline at 9 fluid ounces per acre with the intention of reducing the amount of target spot to a bare minimum. “We’re still going to see some leaf shed in some of these varieties. Hopefully, it’ll be reduced to the point to where it’s not greatly affecting yields.”
In trials being conducted near the Gulf Coast, two varieties — Phytogen 499 and DPL 1252 were getting five and six applications of fungicide, says Hagan.
“That was the test we used last year to illustrate how much yield loss to expect from target spot. Last year, we used DPL 1050 instead of 1252, and it was harder to show a yield loss in that variety compared to Phytogen 499.”
Symptoms of target spot include prominent yellow leaf spots, following by defoliation, says Hagan.
“We have normal leaf shed in all cotton. Particularly when it becomes really rank, you’ll have some leaf shed in the bottom of the plants. The issue with target spot is how much faster we’re getting leaf shed with target spot than just normal leaf shed in the lower canopy. We can’t answer that question at this time – there are a lot of unknowns as far as target spot in cotton.”
Researchers also are looking at over-the-top sprays versus using a nozzle arrangement to deliver fungicides, he says. “Instead of going directly over the top, we’re trying to spray into the side of the canopy to see if we can get better coverage for better target spot control.”
The two available fungicide programs are not particularly effective, says Hagan. “If I saw a 50-percent or more defoliation on fungicide-treated peanuts, I’d be suggesting that you try something else. Part of the issue on cotton is that we’re generally making only two applications. We’re trying to slow down the disease in the middle of the summer so that we have a little less defoliation, or at least so it’s occurring at a time when the bolls have already matured in the area where defoliation has occurred. In that case, it won’t affect yields if we do lose leaves.”