Many diseases can lower lint yields of cotton crops, but Verticillium wilt typically causes the greatest financial loss to producers.
It is widespread on the Texas High Plains, and presently farmers have no feasible means of eliminating the fungus. Planting varieties tolerant to Verticillium wilt can minimize financial losses, according to Terry Wheeler, research pathologist, AgriLife Research, Lubbock.
Since 2005, Wheeler and associates have been evaluating commercial varieties for Verticillium wilt tolerance in six infected areas in the upper, middle, and lower sections of the High Plains.
“We plant the tests at the optimum time and grow them under normal conditions. In early August we begin counting infected plants every two weeks. When the Verticillium wilt incidence increases to a predetermined level we begin checking every week and continue to do so until senescence begins,” Wheeler said.
“Subsequently we determine the final percent of plants infected by Verticillium wilt, lint yields, fiber properties, turnout, loan value (dollars per pound), value per acre (yield times loan value, minus costs of seed and technology fees), and plants per foot of row. This information is posted on http://lubbock.tamu.edu/  in February each year and can be of significant benefit to producers in the variety selection process.
“Significant differences always occur among varieties for lint yields. For example, in the 2007 tests we had a 926 pounds-per-acre yield difference between the highest and lowest yielding varieties,” Wheeler said.
She also works closely with the cotton geneticist at Texas Agricultural Experiment Station in developing breeding lines which have greater tolerance to the fungus.
Potential breeding lines from diverse sources are screened for tolerance in a nursery having a high level of Verticillium wilt. Plants that show the highest levels of tolerance are harvested and their seed stored for greenhouse plantings.
In preparation for the greenhouse tests, Wheeler cultures large batches of “hot” Verticillium wilt inoculum in the laboratory under controlled conditions. These batches are taken to the greenhouse and mixed with dry, disease-free soil in a pharmaceutical-grade mixer to obtain a homogenous media.
This media is poured into 3-inch-diameter by 24-inch long PVC tubes, held upright in specially-designed benches.
“We plant seeds from each individual plant selected from the Verticillium wilt nursery in 10 tubes randomized over the greenhouse, and each tube is appropriately labeled,” Wheeler said.
The tubes are carefully watered and fertilized to ensure seed germination and normal plant growth. The greenhouse temperature is kept in a range conducive to Verticillium wilt development.
“We examine the individual plants frequently for Verticillium wilt symptoms. When a plant shows symptoms it is pulled from the tube, discarded, and date of removal logged. We continue this procedure until all plants have been removed or senescence begins,” Wheeler said. “Then we statistically analyze the data to identify superior Verticillium-wilt-tolerant plants.
“The following season we plant stored seeds from the parent of each of the superiors in the Verticillium wilt nursery, and repeat the entire selective process.
“Using field and greenhouse plantings enables us to screen two generations of material each year. This minimizes the time required to develop superior breeding lines for subsequent production of commercial varieties with higher levels of Verticillium wilt tolerance.”
Financial support for the 2005 research project was provided by Plains Cotton Growers Inc. Subsequent projects have been funded by Cotton State Support Committee.