When It Comes to Cotton Diseases, Know What You’re Looking For
Extension pathologists across the Cotton Belt agree on three things:
- If rhizoctonia shows up in your stand, consider a different seed treatment next year or rotate that field to a different crop.
- If any other disease shows up between getting a stand and counting to five nodes above white flower, consider whether to treat.
- If a disease shows up after that, ride it out.
That’s how the clock on cotton disease runs. The position of that clock when cotton disease strikes is a primary factor in whether to attempt control.
Areolate Mildew
As of 2022, pathologists no longer believe areolate mildew is geographically limited.
Mississippi State University Extension Pathologist Tom Allen and University of Georgia Extension Pathologist Bob Kemerait recommend scouting for areolate mildew, but they have slightly different approaches to treatment.
For Allen, treatment should be made only after it’s found in the field, and only when the crop is staged below five nodes above white flower.
Kemerait encourages growers to consider treatment when areolate mildew is found in the area, the crop is more than a month away from harvest, and the cotton has lost less than 25% of its foliage. He also urges growers to confirm that it’s truly the disease. “Whiteflies and areolate mildew can look similar,” Kemerait says.
Bacterial Blight
Bacterial blight may have been a vanishing disease, given that most cotton seed varieties are bred for resistance. For growers who choose a variety susceptible to bacterial blight, Allen says to “manage it like you don’t have the disease” because you may not. Or, he says, bacterial blight may show up and environmental conditions may give it room to damage yield opportunity. In that case, plan how to manage the field the next season.
“You can rotate to a non-host, plant a resistant variety, or till to break up the residue,” Allen says. “There’s nothing you can do to treat it in-season.”
Boll Rot
Growers do not have a recourse with fungal boll rot, Kemerait says. To manage bacterial boll rot, he recommends choosing a resistant cottonseed variety.
Cotton Leafroll Dwarf Disease
Since 2019, cotton leafroll dwarf disease has been a regular visitor to fields in the lower Southeast. However, yield loss to the disease is unusual, says University of Georgia Pathologist Sudeep Bag.
“The virus is still here,” Bag says. “But we are not seeing its impact.”
Bag continues to research the virus, the subsequent disease, and the risk of yield impact.
“We have to be prepared.”
Fusarium Wilt
Growers who manage nematodes control fusarium wilt. Fusarium wilt and nematodes “go hand-in-hand,” Kemerait says. “Nematodes will not kill a plant. Fusarium wilt in combination with nematodes will kill your cotton.”
Target Spot
Extension pathologists recommend scouting for target spot starting the first week of bloom.
If you find it before five nodes above white flower, Allen says, consider treating with an appropriately labeled fungicide. “Target spot and areolate mildew can be more problematic and greater yield limiters if you see them shortly after canopy closure,” he says.
For target spot, Kemerait again suggest growers consider treating for the disease in fields with a history of pressure from target spot or when the disease is reported in the area.
“We have never seen a situation where you don’t make at least 100 lb/A of lint by spraying a $20/A fungicide,” Kemerait says. Trials show Miravis Top and Priaxor provide the highest yield impact when applied between the third and fifth week of bloom.
Achieving adequate treatment coverage for diseases that start low in the plant is difficult, the Extension pathologists note. The most essential step for disease is scouting.
“It’s always best to scout rather than make a blanket fungicide application in the absence of disease,” Allen says.
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