A leaf-spot diseases related to nutrition deficiency and one that eventually may be controlled with fungicides are showing up in cotton fields throughout Georgia, according to a report by the University of Georgia Cotton Team.
"The dramatic appearance of these spots has caused concern to many producers," according to an item in the Georgia Cotton Newsletter. "The majority of the leaf spots sent to the UGA Diagnostic Clinic in Tifton have been diagnosed as Stemphylium leaf spot thus far."
Potassium deficiency greatly increases the cotton plant's susceptibility to Stemphylium leaf spot. Typical symptoms include numerous medium-sized spots on the leaves, often with a purplish margin, and a tan-to-gray center that becomes brittle and may give the leaf a "shot hole" appearance. Rapid and complete premature defoliation is possible in severe cases.
"Much of the Stemphylium leaf spot currently plaguing our cotton fields is a result of extremely hot and dry weather that is affecting the uptake of potassium in the cotton," according to the report.
To manage Stemphylium leaf spot growers must ensure sufficient potassium is available in the soil and that the potassium also is adequately transported throughout the plant. A fungicide such a Headline may help manage Stemphylium leaf spot.
"However," the UGA team notes, "we still struggle to develop a recommendation that provides a grower with reasonable expectation of successful control with the use of a fungicide."
Corynespora leaf spot also is showing up, primarily in Southwest Georgia.
"This disease was first diagnosed in Georgia in 2009, though it was likely reported by consultants and agents for several years before that," according to the UGA cotton team.
This disease closely resembles Stemphylium leaf spot but likely is not related to a nutritional deficiency. "Therefore," the team reports, "there is optimism that growers may one day be able to manage this disease that seems to be of increasing importance with the judicious use of fungicides."
Research is underway to determine how fungicides can be used to manage Corynespora.