Even in the arid air of West Texas, cotton diseases, both soil-borne and leaf infection, do occur and can cause significant yield and quality losses.
Jason Woodward, who has a joint appointment as a plant pathologist with Texas A&M and Texas Tech in Lubbock, said Verticillium wilt “is currently the most economically important disease in West Texas.”
It has staged a bit of a comeback. “It was a major problem in the 1970s and 1980s,” Woodward said during a pest management update at the Beltwide Cotton Conferences in San Antonio. “But widespread use of HS-26 limited the disease. Now, Verticillium wilt also affects peanuts and chili pepper.”
Effect on cotton production may be severe, resulting in yield reduction, low micronaire and poor uniformity.
“We are evaluating management options,” he said. “Currently, control choices are limited. Terry Wheeler (Texas A&M research plant pathologist at Lubbock) has an extensive cultivar screening program.”
Woodward said infections typically occur early in the season and symptoms develop as the plant’s water requirement increases. “The fungus is long-lived in the soil.”
Fusarium wilt may cause significant damage, too, but distribution is limited to counties southwest of Lubbock. The disease is more common in sandier soils and shows an interaction with root-knot nematodes.
“Fusarium wilt develops from about 30 to 45 days after planting and historically shows symptoms later in the season.”
As with Verticillium wilt, management options are limited. But Woodward said ongoing research with nematicide efficacy trials, characterization of pathogen populations, a cultivar screening program initiated in 2007 and a study of the pathogen population dynamics may help identify control options. “We are studying the influence of cultivar selection on populations of the fungus."
He said Alternaria stem blight was identified in 1999. It shows up as circular areas in the field and resembles lightning strike damage. Plants display a purplish discoloration and infections occur at leaf margins and progresses down the leaf petiole and main stem. “Sporulation is prolific,” Woodward said.
The disease is associated “with premature senescence and is common in fields exhibiting drought stress and in early maturing cultivars. Necrotic zones appear on older leaves.”
He said infected plants have a desiccated appearance and the disease may cause defoliation. Alternaria diseases are not widespread, but producers are concerned.
“Stem blight is currently a minor problem, but is increasing every year. It is found in Upland cultivars and cuts across genetics. Stem blight may result in significant yield loss and quality and loan value reductions.”
Woodward said leaf spot epidemics have occurred in two of the last three years. Defoliation has occurred, but effects on crop yields are uncertain.
Nematodes also damage Texas cotton. Reniform nematodes cause more trouble in South Texas, Woodward said, but they have infested several fields in the High Plains. “We’re identifying more fields each year (with Reniform nematode infestations).”
The nematode may cause stunting, chlorosis and wilting. Reduced root growth may resemble root-knot nematode damage.
Root-knot nematode is the most important nematode species on the High plains.
Woodward said best control options for either nematode infestations or disease outbreaks begin with proper identification. Cultivar selection is “the cornerstone” for management.