Many questions remain about how to treat target spot on cotton, but researchers do know that under the right conditions, it can lead to significant yield losses.
“Target spot first showed up in Alabama about three or four years ago and has really become an issue in south Alabama primarily on irrigated cotton or on cotton that gets a lot of rain,” said Auburn University Extension Plant Pathologist Austin Hagan during the Wiregrass Crops Field day held in Headland, Ala. “When you get into chest-high cotton in south Alabama you see a lot of this disease.”
There’s some question as to how much yield can be lost to target spot, says Hagan.
“We really haven’t had to deal with leaf spot diseases in cotton in the past. Some of the earlier estimates were around 200 pounds of lint per acre, which right now would be about a $120-per-acre yield loss. Some of the crop consultants in southwest Georgia were talking along the lines of about 600 pounds of lint per acre in some situations. The work we’ve done in Fairhope near the Alabama Gulf Coast suggests the yield loss may be up around 400 pounds of lint per acre, and that’s based on data from several different cotton varieties over the last two years,” he says.
It’s a very noticeable disease in that the lesions are very large, up to ¼ to ½ inch in diameter, explains Hagan. In addition, it also has a prominent ring-spot pattern.
Target spot is a late-season leaf spot disease on cotton, he adds.
“The main thing is that you start to get leaf shedding in the lower canopy, and we’ve seen up to 80-percent defoliation by the second week in September. That’s probably where we’re seeing these 400 to 600-pound-per-acre lint losses. If you’ve got 10 to 20 percent loss in the lower canopy, it’s probably not affecting yield too much,” says Hagan.
Based on what researchers have seen in Alabama, with trials from the Gulf Coast and into the Tennessee Valley, the highest risk of the disease is in south Alabama, he says.
“We have the moisture and temperature conditions that drive this disease. It likes a lot of moisture. And most likely, the higher the temperature, the worse the disease is going to get. It wasn’t as bad in 2013 as in 2012, and weather was warmer in August of 2012 than it was last year.”
This disease, says Hagan, is driven by both moisture and temperature. There also are differences among varieties as to how much leaf spot you’ll see, he adds.
“There’s probably a planting date effect, though we’re still looking at that. Geographically, the risk drops the further north you go. We had four trials in the Tennessee Valley last year, and there was no target spot in any of them. There was some in dryland cotton that became rank, but defoliation never got beyond 10 percent.”
Dryland cotton that has stayed small and where the canopy has stayed opened won’t be affected by target spot, he says. Also, there may be some association between yield potential and the severity of this disease in cotton.
“We usually see symptoms for the first time during the last week in July. It waits until the cotton has already lapped in the middle and has already been in bloom for a couple of weeks before it shows up. We don’t know yet if there’s an association between the timing of bloom set and fruit set and the severity of the disease.”
Leaf shed of up to 25 percent was seen in the canopy of plants in those fields that were affected by target spot this year, says Hagan. In the past two years, a cold front during the first couple of weeks in September along with dry weather has “shut down” the disease, he says.
“We continue to look at the effect of cotton varieties, and there are some differential sensitivities among them.”
While a high-yielding variety may have more leaf shed, it also might have the highest yield potential from a seed-cotton standpoint, he says
“There are several sides to that equation, but there are some differences among the varieties in defoliation, and some are considered more tolerant than others. Even though we have more target spot on some varieties, they still have very high yields. Don’t select a variety based on how much target spot leaf shed there is.”
There are three fungicides labeled for target spot on cotton at this time, says Hagan, and there may be one more next year. Current materials include Headline SC, Quadris and TWINLINE
“It will take a few years to get some more products in the supply chain simply because until a year or two ago, we didn’t have any companies even looking at this disease in cotton, but they are now.”
Researchers are looking at optimum timings of the fungicide treatments, he says. “The labels say to apply at first bloom and two weeks later, but I don’t know if that’s the best timing. We did a study at Fairhope last year that suggested that growers wait until the disease shows up or even when defoliation started. So we do have some options for timings.”
In looking at any differences between broadcast spraying the fungicide versus using drop nozzles, Hagan says the yield response was the same.
“Right now, with a fungicide, you’re looking at yield gains 100 to 300 pounds of lint per acre in the right situation, so a fungicide will more than pay for itself under optimal conditions for the disease. We see larger yield gains in more susceptible varieties. If there’s no disease out there, like we’ve seen in the Tennessee Valley, we’re not going to gain any advantage by using fungicides.
A higher volume spray – a minimum of 10 gpa spray volume per acre – probably is going to help with coverage down in the canopy, he says. Also, on-demand scouting-based programs have proven effective.