Southwestern Oklahoma cotton is showing signs of premature senescence syndrome, according to Terry Pitts, Oklahoma State University Extension integrated pest management specialist.
"This condition," Pitts says, "is generally thought to be caused by insufficient potassium in plant leaf tissue which predisposes the foliage to secondary pathogen infection. This occurs when extreme wet conditions are present with the roots being soaked for an extended period."
Pitts said during this time the plant becomes starved for potassium. This condition can be brought about by dry weather during the boll-filling period followed by extended periods of wet, cloudy weather. This happens after cutout, when the cotton plant stops producing bolls, and the plants have a decent load of bolls, Pitts said.
"Symptoms are seen in the upper third of the plant canopy, beginning with yellowing between the plant veins, followed by a rapid change to a red/orange/bronze color," Pitts said. "The affected leaves continue to deteriorate, eventually showing brown, necrotic lesions and leaf margins."
Generally, secondary foliar pathogens such as alternaria, cercospora and stemphyllium can be isolated from affected leaf tissue, Pitts said. These are not considered primary pathogens, but they attack these debilitated plants and contribute to premature senescence and defoliation, he said.
"Sixty percent of the total plant potassium is in the cotton boll", Pitts said. "Adequate potassium is necessary for fiber and seed development. Potassium is important for enzyme activation, pH balance, stomatal control and translocation of photosynthates."
Both extended dry periods and late season rains with waterlogged soils contribute to reduced root function, Pitts said. Relatively non-functioning root systems can't uptake enough potassium, and perhaps other nutrients, to meet boll demand, causing the deficiency, Pitts said.
"Barren plants and those with very little boll load will generally appear unaffected because their demand for potassium and other nutrients is much less," Pitts said. "In addition, the plant hormone cytokinin is important in regulating senescence and roots are a major site of cytokinin production. As root function decreases, so does the production of cytokinin, which leads to senescence."
For nutrients to be absorbed by plant roots, Pitts said, they must come in contact with the root surface. There are generally three ways this occurs: root interception in very small amounts, movement of ions by mass flow with the soil solution, and diffusion of ions through the soil solution.
Most of the potassium moves to roots by diffusion, he said. Diffusion occurs when an ion moves from an area of high concentration to one of low concentration. As plant roots absorb nutrients from the surrounding soil solution, a diffusion gradient is established. Under low soil moisture conditions water films around soil particles, becoming thin and discontinuous, and slowing the movement of potassium to the roots, thereby reducing uptake.
"Under these conditions, plants cannot absorb enough of the nutrient to meet boll demand," Pitts said. "Research has indicated this condition can occur even in fields that contain ample soil potassium. In 2002 and 2003, soil samples were collected from about 20 fields showing signs of premature senescence. Results indicated soil test potassium levels in 18 of these fields were in the moderate to high category and presumably sufficient for optimum plant growth.
"However, it is important to test soil annually to determine seasonal crop needs. In addition, annual soil testing will provide a good history for tracking levels over time. Moderate soil potassium levels in fields with a history of premature senescence may signal the need for supplemental fertilizer."
Soil applications of potassium fertilizer may be justified in some situations, Pitts said. In-season foliar potassium applications have been evaluated, but generally have been effective only about 20 percent of the time.
"In addition to alternaria on the leaves we are also seeing some bacterial blight on bolls. This is occurring in full growth bolls and is a result of the cool, wet, cloudy weather that just passed. There are no remedial actions that can be taken except to let the field dry out before applying more water in irrigated fields. Probably a week after the last week of August would be time to start applying water again, if you need to mature more bolls."
Pitts can be reached at 1-580-482-8880. His email address is firstname.lastname@example.org.
Secondary pests such as the Southern green stink bug and brown stink bug have also become a problem in Oklahoma cotton fields, Pitts said.
"We are seeing some damaged bolls in several fields in southwestern Oklahoma," he said. "Research has shown that stink bugs feeding on bolls less than 12 days old can result in abscission. When stink bugs attack older bolls, they remain on the plant, but can cause damage to seed, lint staining and yield reductions."
Stink bugs can damage bolls that are less than 22 to 24 days old or have not reached 450 heat units of age.
"Because stink bugs are very mobile, scouting them can be difficult," Pitts said. "One scouting method is to examine 6 row feet at several locations in the field. When there is an average of one or more stink bugs per 6 feet of row, you have reached the threshold and both nymphs and adults can inflict damage. The best method for determining stink bug infestations is to examine bolls for external and internal damage."
Stink bugs, Pitts said, cause a wart-like structure to form on the internal carpel wall. When 20 percent of quarter-sized bolls show internal damage, insecticide applications are justified.
Research, he said, has shown that Bidrin, Vydate and Orthene are effective on both brown and Southern green stink bugs, while pyrethroids are effective on the Southern green stinkbug only. Although stink bugs can be somewhat of a "phantom pest," samplings bolls to determine internal damage takes much of the guesswork out of the process, he said.
This report is made available by NTOK Cotton, a cotton industry partnership which supports and encourages cotton production in north Texas, Oklahoma and Kansas. For more information on the cotton scene, see okiecotton.org and ntokcotton.org.