Corynespora leaf spot, more commonly called cotton target leaf spot, is real and can cause problems in cotton fields if left untreated. But how much trouble it can cause is still being debated in the industry. Still, a lot was learned about this new disease in 2012.
"Bottom line is target spot is very real, and 2012 was an important year for it; it is something growers need to be aware of and prepared for (in 2013). … It's an aggressive disease and can cause significant premature defoliation and symptoms can be reduced with a fungicide application," said Bob Kemerait, University of Georgia Cooperative Extension plant pathologist.
New cotton disease
Corynespora leaf spot was unheard of before being suspected in southwest Georgia in 2005. It was positively confirmed in 2009. In 2012, Kemerait submitted evidence of the new disease to the American Phytopathological Society. Unlike other cotton leaf spot diseases, it is not caused or prompted by low fertility, most often low potassium levels.
Overall yield impact from the disease isn't known, or not enough data is in to be conclusive at this point, Kemerait said. But he feels confident that the disease can claim 100 pounds of lint per acre or more if left untreated, depending on when it first enters a field. Some anecdotal reports put the yield lose much higher.
If fungicides are recommended, what is the timing on application? That isn't certain either, but he does believe strobilurin fungicides are more effective in controlling the disease than tebuconazole-based fungicides.
Corynespora leaf spot risk
Not every acre in cotton needs to be sprayed and not every field that gets this disease sees a yield increase by using fungicides, especially if the disease doesn't hit a field until late in the year. In fact, if it hits late, it could provide some good timely defoliation to help cotton development. But then again, that's betting on an untreated disease to do your bidding. "Like I've said, if I could make this disease stop after 20% defoliation, I'd bottle it myself and sell it," Kemerait said. "But we can't predict that."