What is in this article?:
- Stuck between a rock and a hard spot with target spot on cotton
- Where was cotton target spot between 1961 and 2005?
- First and third week of bloom critical for control
- University of Georgia Extension specialists are reluctantly making specific fungicide recommendations for controlling target spot on cotton.
- The disease appears to affect all cotton varieties grown in the Southeastern United States.
- The best time for making fungicide applications is sometime between the first and third bloom.
RANK GROWTH AND irrigation are thought to put cotton producers at higher risks to corynespora leafspot, or target spot.
“Between a rock and a hard spot.”
That’s where many Extension agents and consultants feel they’re stuck as far as target spot on cotton in Georgia.
“As Extension specialists, agents and consultants, this is really where we are,” said University of Georgia Extension plant pathologist Bob Kemerait, speaking at the recent Beltwide Cotton Conferences in New Orleans.
“If we want to be comfortable with recommendations, we have to have at least three years of replicated studies, and we like to have statistics, statistics that work out 95 percent of the time," he said. "We also would like to have complete agreement with our colleagues, so if I recommend something in Georgia, my colleagues in adjoining states are recommending the same thing.”
However, he adds, with an issue like target spot on cotton, there are fewer than three years of replicated data. Also, the data can be variable, especially concerning yield impact.
“So the quandary we find ourselves in as specialists, consultants and Extension agents, is what do we do? We have recommendations, but it’s certainly not within our comfort zone to do this.
We’re looking through a glass darkly with this topic. This is a disease that as recently as 2006 wasn’t recognized by anyone but a few consultants in southwest Georgia and had not been reported in nearly 40 years. Now it’s something that we’re seeing more of. We don’t have all the answers, but we can tell you what we think the best answers are at this time,” says Kemerait.
In the Southeastern U.S., there are two major leafspot diseases, he explains. The first one is stemphylium leafspot. It is typically associated with a deficiency in nutrients, primarily potassium. Fungicides are largely inadequate for managing it.
“We’re very confident in what does and does not work with this disease. If you manage your potassium, you’ll manage stemphylium leafspot,” he says.
What is known about target spot is that disease severity and defoliation are reduced with the application of appropriate fungicides. “We know that yields can be increased, but we’re very frustrated by the fact that yield increases can be variable. We know that even most aggressive current fungicide programs – typically a strobilurin type of chemistry – do not always control it as we would like," he said.
Simply keeping leaves on the plant is not necessarily the best thing for growers, he says.
Corynespora leafspot or target spot tends to occur in drought years, in sandy fields and in the same fields year after year. “You reach a point to where you have rapid defoliation. Within two weeks, you can go from having a perfect canopy to a loss of nearly complete defoliation, and yields suffer as a result. The main consequence of this rapid defoliation is that the bolls don’t open," he said.
There are good, effective fungicides for target spot, but they’re also more expensive and most are in the strobilurin class, including Headline, TwinLine and Quadris.
“The first time I saw a benefit in disease control with a fungicide was in 2007 in Appling County, Ga. It kept the leaves on and gave us better crop health, but in the end, there were only slight, non-significant differences in yield where the fungicides were used.”