Sometimes, something different or unexplained just appears – something that leaves even experts scratching their heads.
Simply put, that’s target spot. The defoliating disease appeared in South Georgia cotton fields eight years ago as a complete mystery to growers, consultants and county agents. Initially misdiagnosed as Stemphylium leaf spot, it took several years before target spot (Corynespora cassiicola) was correctly identified. And for good reason.
“Target spot was first identified in the early 1960s in Mississippi,” stated Bob Kemerait, Extension plant pathologist for the University of Georgia. “Then, it disappeared. It became so forgotten that it was no longer included in our compendium of cotton diseases.
“Even though it’s been reported to me as common in South America and that cotton growers there spray multiple times for it each year, many of us never knew target spot ever existed in the U.S. We do now.”
In 2012, target spot was confirmed in cotton fields throughout the Southeast. Left untreated, the disease can leave growers with fields that are prematurely defoliated, plants with severe leaf spotting and lesions on bolls and bracts, and a high potential for significant yield loss.
“If you have dryland cotton with a very open canopy, you don’t need to worry about it very much,” explained Kemerait. “Where you will see it is in aggressive growth, thick canopy cotton under irrigation or in areas of substantial rainfall.
“This disease moves from the ground up. Growers may see the upper 20 percent of the canopy. But what they can’t see from a distance are plants that may have lost 80 percent of their leaves, which we see associated with poor fruit development and fruit loss. It can happen very quickly.”
To help provide management solutions, the University of Georgia conducted field studies on both irrigated and dryland plots during 2012 to help determine the impact of irrigation on disease intensity, the effect target spot has on yield and the best timing for fungicide applications. Not surprisingly, the disease was more prevalent and led to more defoliation and yield loss in overhead irrigated trials. In side-by-side comparisons, dryland trials outyielded irrigated trials, with the severity of target spot seemingly being the difference.
Single and multiple fungicide applications were evaluated at different stages of plant growth. Results showed the critical application timing is the third week of bloom, with optimal timing being both the first and third weeks of bloom, depending on a field’s level of risk for the disease. In his studies, Kemerait assessed applications of Headline at six ounces per acre (oz/A) and Twinline at 8.5 oz/A. Quadris is also labeled for target spot, with other fungicides likely to follow.
“It’s important to get the fungicide out before the disease becomes established,” said Kemerait. “To get good coverage, it’s going to require the canopy to be somewhat open. If growers wait until the canopy has gotten thick, they may get the top of the plant but not down where they need the product to go.
“From our studies, I believe we can protect 200 pounds of lint with current fungicide application strategies,” he said. “More than that is lost in a bad situation.”
To help growers assess their chances of finding target spot in their fields, Kemerait has developed a risk management tool for evaluating factors that can impact disease development. The tool is available online at www.ugacotton.edu.
Kemerait reported that target spot was the number one disease topic at cotton meetings throughout Georgia this winter. Although some cotton varieties have been singled out as being more susceptible to target spot, he is quick to point out that any cotton variety can contract the disease under the right conditions.