Indianapolis - October 05, 2012
With the extreme weather experienced throughout the Southeast this season, a leaf spot initially found in southwest Georgia in 2005 was for the first time seen in many Southeast cotton fields.
Corynespora leaf spot was found in southwest Georgia around 2005, tentatively identified by University of Georgia researchers in 2008 and positively confirmed in 2009.
University of Georgia Extension pathologist Dr. Bob Kemerait recalls thinking the new fungal disease was Cercospora leaf spot, a disease common in potassium-deficient cotton.
“It didn’t look exactly right, but I was second-guessing myself,” Kemerait recalls. “This disease wasn’t even in the bible of cotton diseases, the Cotton Compendium.”
That may change. This year Kemerait submitted evidence of the new disease to the American Phytopathological Society, which publishes the Compendium of Cotton Diseases.
“Cotton professionals do not all agree that this is a problem that can or should be controlled,” says Dr. Keith Edmisten, North Carolina State University cotton specialist.
At this point, land grant university researchers and industry agronomists have more questions than answers. However, the PhytoGen cottonseed team is working with researchers and scouting fields for answers.
“The bottom line is that there are no clear guidelines for control of foliar diseases in cotton,” PhytoGen cotton development specialist Dr. Russell Nuti points out. “We are working together to develop a better understanding of how to best manage them.”
The difference between Corynespora leaf spot and other leaf spot diseases is management. Other leaf spot diseases are opportunistic in potassium-deficient cotton so a fungicide treatment is not economically effective. Corynespora can occur in cotton with sufficient potassium levels and will respond to chemical treatment, perhaps profitably.
“In many cases the apparent defoliation that is evident in many fields may be as much a result of shading in rank cotton, perhaps more so than disease,” Edmisten says. “Some of these fields may actually benefit from some early defoliation as this may reduce boll rot.”
Conversely, Edmisten says, fungicide may complicate harvest.
“On some varieties this disease will actually aid in defoliation, whereas fungicide applications may make defoliation more difficult,” Edmisten says.
Positive identification is the first step in managing this disease. One challenge for growers is that positive disease identification requires a laboratory test. But, as growers well know, assumptions and fungicide applications are expensive relative to the minimal costs to have a leaf spot properly identified.
“Leaf spots in cotton commonly cause a target-like lesion and have been broadly named target spot,” Nuti says. “Alternaria, Cercospera and Corynespora all can be misidentified in the field as ‘target spot."
The priority questions for growers, of course, are whether this disease will impact yields, how much treatment will cost and if it will provide economically advantageous disease control.
“How important it’s going to be in year-to-year management, whether it’s going to be an issue every year, are questions still to be answered,” PhytoGen cotton development specialist Dr. Steve Brown says.
At this point, Edmisten, Kemerait and Auburn University Extension pathologist Dr. Austin Hagan do not believe managing the disease will be a significant production factor.
“In observations of varying height and canopy within the same field, defoliation due to Corynespora was significantly greater in taller, dense cotton relative to the shorter, less dense areas. Hopefully, with the arrival of 2012 research results, we will know more about the value and timing of fungicides and what production practices can be employed to best manage this pathogen,” PhytoGen cotton development specialist Dr. Joel Faircloth says.
Hagan says he would not consider Corynespora leaf spot a factor in deciding whether to plant a high-yielding variety.
Land grant scientists are comfortable recommending growers scout for the disease at first bloom and consider applying a fungicide if conditions are conducive to spreading the disease. Aggravating factors to consider include a short cotton rotation, conservation tillage with a short cotton rotation, field history, weather favorable for disease (moisture) and rank growth.
“You don’t necessarily need to spray if you find one spot,” Kemerait says. “I’ve seen too many fields where it just doesn’t develop.”
Several land grant universities, including the University of Georgia and Auburn University, have trials under way this year to examine management options. Researchers also are working to develop a prediction model.
“We’ll know a lot more after this season,” Brown says.
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For Editorial Information:
Pam Golden
Bader Rutter & Associates
(850) 758-8700
pgolden@bader-rutter.com
Megan DeVelvis
Dow AgroSciences LLC
(317) 370-5072
megan.develvis@dow.com
