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.”
Drop nozzle advantage
Headline and Quadris are getting being tested for their effectiveness against target spot, he says. “In Brewton, there was a definite advantage to having a drop nozzle versus a broadcast over-the-top nozzle arrangement. You have less target spot with a drop nozzle, regardless of the fungicide. In Shorter in central Alabama, it doesn’t look like nozzle placement has any effect on disease, and there’s no difference in the performance of the fungicide. The only difference is that there’s less target spot on DPL 1252 than on Phytogen 499.
Target spot is not an easy disease to control, notes Hagan. “Last year, the only treatment that increased yield above that of the non-treated control was 9 fluid ounces of Headline. It is a more serious issue the closer you get to the Gulf Coast and on really good cotton.”
As for cotton insect control, Extension Entomologist Ron Smith says work is continuing to fine-tune thrips control.
“The seed treatment just doesn’t last long enough — it runs out a week or so before our cotton outgrows the thrips injury stage,” says Smith.
This year, researchers looked at what growers can do at planting time to avoid the need to come back with an over-spray, he adds. “If you plant early, and particularly if you use conventional-tillage and other factors, the thrips situation can be made worse. If you plant in reduced-tillage, the more litter you’ve got on the ground, the less thrips pressure you’ll have.
“We’re looking at adding an in-furrow insecticide on top of the seed treatment at planting. This year, we looked at Admire Pro. In central Alabama and in the Wiregrass, it really didn’t add enough to justify the cost. But my counterparts in Virginia and North Carolina are pleased with the results they’ve seen from that product.”
Also getting a look is a new class of chemistry that has been registered for thrips control — Radiant by Dow, says Smith. “It doesn’t flare spider mites. Most everything else you put in that early season window will aggravate spider mites, and that’s a major concern in some areas. The farther we get away from Temik, the greater the spider mite problems will be.
“Of course, spider mites are associated with dry weather, and we didn’t see that this year. But we’ll have to be concerned about spider mites on seedling cotton in the future.”
Plant bugs were very light in Alabama this season, he says, and aphids didn’t amount to anything. “We did have more stink bugs than we’ve had in several years now, but they were not in every field. They’ve been in some fields at treatable levels at multiple times, and they’ve been in some fields that haven’t been sprayed all year.”
We’ve got a new product that’s being sold and promoted for stink bug control called Belay, and we’ve had pretty good success with it. We had good success with it last year on soybeans. In the Wiregrass, it’s not measuring up to Bidrin on cotton.”
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