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Disease and insects hit cotton

Abundant rain and unusually cool temperatures have produced symptoms of disease and insect damage in Southern Plains cotton, according to Terry Pitts, Oklahoma State University Extension integrated pest management specialist.

Pitts, stationed at the OSU Southwest Research and Extension Center south of Altus, says wet cotton may develop some yellow cotton plants, but sunlight and warmer temperatures will fix that development. At the same time, a reddening of browning of cotton leaves could be Alternaria, a condition seen in cotton in the area for the past two seasons.

"The red/brown color is due to low potassium as the plant has pushed off all available potassium into boll production. Studies have shown fungicides don't reduce the incidence and it doesn't reduce yield," Pitts said. "We are starting to see some dying plants where Fusarium has caused severe leaf drop, which results in no viable bolls on each plant. The condition has been seen in several dryland fields east of the irrigation district here at Altus. Resistant varieties are the best defense against this disease."

Although leafhoppers have been seen in cotton fields, populations have not been above 40 percent infested plants, Pitts said. The cotton has not grown to a stage where leafhoppers are not a problem;

Grasshoppers have been seen on the edges of cotton fields as well as corn, grain sorghum, alfalfa and peanut fields, he said. They are easy to control by spraying the edges of fields, ditches and pastures.

Pitts recommended that farmers begin to scout fields for stink bugs. Thirty percent or more of the plants in a field must be infested before chemical control is needed, he said.

Bollworms and armyworms have been seen in alfalfa and other crops. Cotton producers can expect to find their eggs in their cotton, so they should monitor for any damage.

Current Bt technology contained in modern cotton varieties is working to control the worms, Pitts said. "I mention this as a precaution for producers with conventional cotton varieties," he said. He cautions farmers to remember more than 10 to 12 percent of the plants must be infested with bollworms before using any control methods on conventional cotton varieties.

He said bollworm moths are laying eggs low in cotton plant canopies, on blooms, bloom tags and/or bracts with some larvae escaping predation and mortality.

He urged producers to scout the entire plant and to reduce scouting intervals to three to four days during periods of increasing bollworm egg lay, especially during peak bloom. Hatching larvae feed on the cotton plant to receive a toxic dose. Treatment with foliar insecticides for tobacco budworm or bollworm should only be considered on Bt cotton when eight to 10 larvae larger than one quarter inch are present and five to 15 percent of the squares or bolls are worm damaged, Pitts said.

"Increasing spray volume above what would be normally used to control other pests will usually improve fall armyworm control," Pitts said. "But any control over 60 percent to 80 percent should be considered good."

Unlike bollworm and budworm larvae, Pitts said fall armyworms do not have numerous tiny spines on most parts of the skin when viewed with a 10- or 20-power hand lens.

Fall armyworms are likely to feed on both blooms and bolls. Small larvae are difficult to detect because they often feed on boll bracts and on the surface of bolls, hidden behind the bracts. Fall armyworms feed on a relatively small number of bolls, he said, taking more larvae to do as much damage as a smaller number of bollworm or tobacco budworm larvae.

Treatment, he said, is justified when four or more small larvae, less than one quarter inch, are found in 100 bloom and bolls or when 10 to 20 larvae are found per 100 plants. Timing applications to control small larvae is more effective than trying to control large larvae. Small larvae are often found in white blooms, pink bloom tags or behind the bracts of medium or large bolls, he said.

Pitts can be contacted by calling the Southwest Center's telephone, 580-482-8880 or his cell phone 580-318-3121. Pitts' email address is [email protected]. Pitts' original IPM newsletters can be found at the website.

TAGS: Cotton
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