Any number of pathogens may cause leaf spot in cotton, and for many years this bit of speckling was usually not much of a problem. Some leaf spot problems were also found to stem from a nutritional problem. But now Georgia producers are facing a new leaf spot disease, and this one is for real.
“Corynespora leaf spot seems to be a more typical, traditional disease tied to the pathogen and the weather,” says Bob Kemerait, University of Georgia Extension plant pathologist. “This disease is causing significant mid-season defoliation. It is attacking the leaves, bracts and the bolls as well.”
More Traditional Disease
“Since around 2004 and 2005, consultants in southwest Georgia have been talking about leaf spot getting on bolls, bracts and leaves of their crops. We’ve always assumed it was Stemphyllium leaf spot or Cercospora gossypina,” Kemerait says. “However, we are seeing a completely different aspect: Corynespora leaf spot disease.”
Kemerait says the wetter conditions this year have likely played a role in the expression of this leaf spot.
“To demonstrate what it can do, across varieties you will see most of the upper canopy intact, but much of the underneath will be missing. In some fields that were five or 10 weeks away from harvest, the bottom 60 percent of the canopy was missing. One consultant pulled up a cotton plant and counted 21 nodes, and the bottom 11 nodes were void of any leaves.”
More Questions Than Answers
For Kemerait and others, there are many more questions than answers with this disease.
“Will we see it again? I think we will,” Kemerait says. “What will it do to yield? We’ll have to find out, but at this stage in the season having that much defoliation can’t be good.”
The last question involves managing the disease. Prior to 2006, no fungicides were labeled on cotton.
“Now we have at least two: Quadris and Headline, and we know that spraying these products has helped us control some other diseases,” Kemerait says. But there is a lot to learn about Corynespora leaf spot.
“Could you control this disease with a fungicide? How would you time the fungicide? Is a single application good enough or would you need two applications?”
These are questions Kemerait and others will be working to answer as quickly as possible. However, it is best to rule out the expression of the other types of leaf spot caused by a nutritional problem.
Pour On The Potash
“We know that some of these leaf spot occurrences are based on a deficiency of potassium,” says Glen Harris, University of Georgia Extension agronomist. “If you keep potassium deficiency away, you can prevent that.”
Harris is concerned that potassium deficiency may be more prevalent because of the high cost of this input.
“Potash remains high, and other fertilizers have come down significantly,” he says. “Leaf spot or no leaf spot, it is best to soil test and apply all recommended nutrients at planting.”
Harris adds, “A sidedress of potash does not work as well. Put it all on at planting.”
Contact Amanda Huber at (352) 486-7006 or firstname.lastname@example.org.
What Is Corynespora Leaf Spot?
• Typical disease tied to pathogen and weather pattern.
• Does not appear to be nutrient related like other leaf spots.
• Can cause significant mid-season defoliation.
• More questions than answers currently.