Selecting the wrong cotton variety in a field with a history of verticillium wilt could cost farmers as much as 750 pounds of lint per acre.

“Variety selection is critical,” says Terry Wheeler, Texas A&M research plant pathologist at Lubbock, who spoke at the Concho Valley Cotton Conference at San Angelo, noting that some cultural management options that reduce disease pressure also reduce yield potential.

Cutting back on irrigation, for instance, may provide a less hospitable environment for the disease, but it also limits yield potential.

Wheeler said the disease began showing up again in West Texas cotton in 2004 after six to seven years without wilt. “We had good conditions for wilt that season,” she said.

Symptoms include chlorosis and yellowing of the leaves, beginning at the bottom and moving up the plant, and the leaves fall.

“With root rot, the plant holds leaves,” Wheeler said. “With verticillium wilt, it loses leaves. And the microsclerotia survive many years in the soil, even without a host, so rotation helps very little. It does help in that the disease organisms do not increase, but it doesn't reduce counts.”

Soil populations indicate infestation levels, but the organism still needs water in the field, Wheeler said.

She's conducted trials with 32 varieties per site, and eight sites in the last two years, looking for the best resistance.

“We count the percentage of wilt infections in August.” Varietal differences are significant, ranging from two to three times the yield, depending on the variety (700 pounds per acre to 1,400 pounds per acre, for example).

“Choosing the right variety is the key,” Wheeler said. “Verticillium is reducing yield and quality; it affects micronaire, uniformity, and loan value.”

The best varieties tended to do well in numerous fields.

“We don't have many Flex varieties that perform well,” she said. “We have only two we would recommend in verticillium wilt fields, AFD 5065B2F and AFD 5064F. We need to look at all the Flex varieties more closely.”

Other varieties with favorable outlooks from Wheeler's research include:

  • Roundup Ready varieties: PM 2326 RR, PM 2379 RR FM 989RR, and FM 960 RR

  • Bollgard Roundup Ready varieties: FM 960 BR, FM 989 BR, and DP 455BR

  • Flex varieties: AFD 5064 RF B2BR and AFD 5065 B2RF

  • B2R varieties: FM 989 B2R and FM 960B2R

  • Widestrike: Phytogen 440W

  • Liberty Link: FM 958LL, FM 832 LL and FM 988LLB2

“We're seeing a pattern for what works well,” Wheeler said

Tom Isakeit, Texas A&M plant pathologist at College Station, discussed another damaging cotton disease, root rot. He said root rot research has been a black hole for research pathologists for years, but deserves more attention.

“We have no plant resistance and we have a long way to go to get resistance from gene manipulation.”

Planting early or late may help farmers escape disease, but is not viable in most areas. “Deep plowing doesn't get rid of the pathogen; it just puts it out of reach from some plant roots. Crop rotation to monocots is the best management practice to date, although it doesn't eliminate the pathogen from the soil.”

Isakeit said some growers have tried soil amendment with sulfur to lower the soil pH, since the disease is suppressed in acid soils. “It takes tons of sulfur per acre to have any effect,” he said. He is aware of one grower who applied 400 pounds per acre. In a lab experiment, he found no pH change after adding the equivalent of 400 pounds of sulfur to the grower's soil, which had a pH of 7.11.

At two tons, pH dropped only slightly, to 6.91. At 20 tons, pH dipped to 3.16.

Expense becomes a factor. Isakeit said the 400-pound per acre treatment cost the grower $74 per acre and achieved nothing.

Organic amendments such as manures are helpful, but several tons per acre are needed.

He said a few biological treatments are currently sold for root rot, but they have not been validated using controls for comparison and by testing over several years and at several locations.

“We are testing fungicides, particularly those applied through drip irrigation. There is no fungicide currently labeled for root rot control and if we identify something effective, we may face issues of cost effectiveness and willingness of the company to support a label. “We're also looking for naturally-occurring biological controls. Some soils don't have this pathogen and it may be related to the microbes that are in these soils. I'd like to understand why, but for now, we have no controls and nothing in the pipeline.”