“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.”
Where was cotton target spot between 1961 and 2005?
In 2006, says Kemerait, consultants and county agents in southwest Georgia began telling him there was a disease causing tremendous defoliation. “I told them it was stemphylium leafspot. In frustration, they told me that it could not all be stemphylium leafspot because there wasn’t a potassium deficiency, and they were absolutely correct.”
It soon became clear, he says, that while stemphylium leafspot was widespread in the state, a second disease was causing a different type of defoliation, from the lower older leaves up, progressing up the plant. There was a loss of leaves and bolls.
“With this new disease, we had green leaves and large, marble-sized, target-shaped spots and rapid defoliation without the reddening nutritional deficiency associated with stemphylium. We quickly learned that the causal agent was corynespora leafspot – a common pathogen of other crops as well," he said.
Target spot is not a new disease, says Kemerait. “It’s new to us, and it is not reported in the compendium, but in 1961, it was reported in Mississippi. It’s found on soybeans, tomatoes, cucumbers and hydrangeas, but we don’t know where it has been on cotton since 1961. If we haven’t seen it between 1961 and 2005, where has it been? It is an emerging pathogen around the world.”
You will never lose the entire canopy to target spot, but you’ll lose the interior of the canopy, he says.
“My first trial looking at target spot was in 2010. We used applications of Headline at 6 to 12 ounces versus the untreated. This is the first time we had a statistical reduction in disease where we were specifically looking at target spot," he said. "There were slight trends in yield differences, but they were not statistically different. The difference in lint was about 40 pounds between where we did and did not treat. We could reduce the severity of the disease but see no difference in yield.
“In 2010, we had a large-scale trial in Thomas County, Ga. The percent of defoliation was statistically reduced, and we did see a statistical difference in yield, up to 123 pounds where our best cotton was being grown, and about 65 pounds on average. It was the first indication that we also were protecting yield by treating with a fungicide," he said.
Since 2012, small-plot trials have been conducted to determine the best timing, best fungicide and whether a fungicide matters in the management of the disease, says Kemerait.
“Many times, the Phytogen 499 cotton variety is thought to be more susceptible to target spot. Some have even said that this is a 499 problem. But if you want to say that it’s a 499 problem alone, I’ll say you’re mistaken," he said. "We want to make the point that it’s not just 499 that’s being affected, but it’s cotton that is currently grown in the Southeastern U.S. Trials have shown that clearly the amount of rainfall and irrigation drive this disease. We reduced the disease with irrigation and managed it more efficiently.”
First and third week of bloom critical for control
Progressing through the season, the most effective timing of applications to reduce target spot was either late in the season where there was regrowth or at first and third or third week of bloom, says Kemerait.
“The first point is that this is not a Phytogen 499 problem alone. And in 2012 and then again in 2013, the most appropriate time to make a fungicide application would have been sometime between the first and third bloom, with the third being most critical.”
One of the most frustrating problems of working with target spot are not so much defoliation and disease control but the variability of yield, he says.
“The amount of yield we can protect with our current programs is about 200 pounds of lint in a worst-case situation. That’s a bold statement to make and we don’t have the statistics to back it up," he said. "But when we look at trials over and over again, the best recommendation we can make is that the largest differences we’ve seen is about 200 pounds of lint.
“What we can absolutely say is that the disease is out there, and it causes significant defoliation. We can reduce that severity and typically the best treatments have been with the strobilurin chemistry at the first and third, or centered around the third week of bloom. Late applications have protected new growth but have not resulted in yield gains. We can’t expect a 200-pound yield gain every time, but I do believe it’s the best we can hope for with our current programs.
We certainly aren’t calling for every cotton grower in Georgia to spray a fungicide, but we do believe there are fields that are at higher risk – those that have rank growth, better growth, irrigation or plentiful rainfall.”
Going into 2014, Kemerait recommends that growers strongly consider using a fungicide. “For growers at lower risk it may be an insurance policy. If you’re at high risk and have had the disease in the past, there are good reasons for spraying at least once in that critical time period.
We’re looking at formulating a risk index for this disease, considering factors like location and history of the field, irrigation, weather, crop rotation and other factors.”