The lesions show up first on the lower leaves on the cotyledons and then spread upwards through the canopy, he explains.

“That’s not the pattern you see with what we consider the fertility-related leaf spot diseases where the spots themselves seem to show up in the top and middle canopy at almost any time of the year, particularly after the plant starts to set bolls.”

Corynespora spots are fairly large in comparison with the spotting that you see with fertility-related leafspot, he says. Spots are one-quarter to three-quarters inch in diameter, there’s a good amount of chorosis of the leaf, and the leaves fall off once they get four to six spots on them.

“There clearly is a defoliation issue,” says Hagan.

“Whether it affects yield clearly will depend on when the defoliation occurs. If it occurs late, it could help to control boll rot. What we don’t know is that if the leaf sheds while the bolls are forming, what impact will it have on the yield and quality characteristics of the cotton produced in that boll.”

There haven’t been any studies that specifically addressed the amount of lint loss you could have from this disease, says Hagan.

“We have studies where we’re looking at two varieties and different numbers of applications of Headline, which is one of the two fungicides registered for the control of this disease.

“We’re putting from one to six applications in different plots with the idea of generating different levels of leaf spot, and then we’ll relate that to yield. In those plots, the cotton is rank, and the leaf spot actually got ahead of us. There’s up to 50-percent leaf shed in the cotton.”

There are several varieties, he says, with between 10 and 25 percent defoliation, and other varieties are at 10 percent or less. Potassium-related issues will complicate this test, notes Hagan.

“One of the problems with fungicides is that when we spray over-the-top, we’re putting fungicides only on the top leaves. We may have to look at changing our sprayer systems to try and drive the spray down into the canopy.”

The only two products labeled for corynespora are TwinLine and Headline, he says.

“It looks like the more applications we put on, the better the leafspot control, and that’s not real good. Both of those have the same basic chemistry.

“We’re limited to two applications of either of those products, and that’s it. We might need options because the same fungus goes into tomatoes, and in a couple of years, they had control failures due to resistance using the same product we’re using right now on cotton.”

phollis@farmpress.com