“Some people plant what others tell them to plant, and others plant based on the data and what others are telling them. Everyone has their own process of deciding what to plant, and all we can do is present the numbers to you.

“If you can’t have multiple years of data, then the next best thing is to have multiple locations within the same year. At least you’re comparing similar environments over different soil textures.”

Researchers and Extension specialists in Alabama continue to evaluate conventional cotton varieties, says Monks, and it’s expected there will be some options from public breeders available to growers this year.

Assessing the major cotton pests in Alabama cotton, Monks lists the usual suspects — gylphosate-resistant Palmer amaranth pigweed and stink bugs — and a relatively new threat, corynespora leaf spot.

In contrast to most other field crops, leaf spot and blight diseases have never impacted cotton yield. And leaf spots such as stemphylium, cercospora and alternaria — which are fairly common on cotton grown on lighter Coastal Plain soils — are typically associated with a potassium deficiency, says Monks.

“If it’s a leafspot, we usually say it’s due to low potash on the plant, and that’s the right diagnosis for a lot of these problems. The thing about the corynespora is that the target shape of the spot is really not connected to a fertility problem. It is a true leaf spot, where the plant starts shedding leaves, causing yields to go down.”

Corynespora leaf spot has damaged cotton in southwest Georgia for the past four to five years. With early disease onset, yield losses of 100 to 200 pound per acre have been noted. This past summer, it was found on cotton from the Florida Panhandle up the Tennessee Valley.

Researchers have found that leaf spotting and early leaf shed were heavier in no-till or strip-till cotton, in fields where cotton followed cotton. Frequent showers and/or irrigation, along with high nitrogen fertility levels, also may have contributed to increased disease.

Individual leaf spots have a distinct “zonate” or “target spot” pattern with alternating laight and dark brown bands.

Limited information concerning factors that influence the onset and severity of corynespora leaf spot in cotton complicates the development of effective control strategies.

In fields where significant disease was seen in 2011, cropping corn, peanuts, soybeans or summer grazing should reduce the carryover of inocula.

If rotation isn’t an option, high-risk fields should be deep turned to bury residue from the previous cotton crop and then planted to a less susceptible cotton variety.

For cotton following peanuts or corn, tillage likely will have little or no impact on disease development. In irrigated cotton, growers should water according to crop needs to avoid creating the wet environment that would favor rapid disease spread.

Fungicides have been used with some success to prevent corynespora leaf spot-relate premature defoliation, but their value in protecting cotton yields has been difficult to define.

“In most cases, the fungicide needs to be applied before the disease gets on the plant, which is difficult to do,” says Monks.

Quadris, Headline and Twinline are all labeled for leaf spot diseases on cotton.

phollis@farmpress.com