The 2021 Alabama cotton crop is all over the board. According to Alabama Cooperative Extension System Cotton Agronomist, Steve Brown, this year’s crop is great in places and woeful in others, with several variations everywhere in between.
“Even though it has been wet all summer, the dry weather in early August has dried out many fields, so much that a rain would be welcomed,” Brown said.
This time of year, rain often comes on the backs of a tropical storm or system. While rain is a need, producers are certainly not looking to receive the complications that a tropical storm would bring.
Rain is not the only issue. While some fields have good cotton, there are others with pale yellow plants that have poor root development and/or leached fertilizer.
“Some foliar nitrogen may help, but soil applications this late in the season are ill-advised because of the potential for even further maturity delays, rank growth and defoliation challenges,” Brown said.
Amanda Scherer, an Alabama Extension plant pathologist, said warm, wet weather will continue to drive disease pressure in August.
Target Spot and Areolate Mildew
Target spot is present in several fields in south and central Alabama, as of Aug. 11. She said although areolate mildew has not been reported, it won’t be far behind.
“Symptoms of target spot and areolate mildew will begin in the lower canopy and infection can result in premature defoliation,” Scherer said. “Target spot pressure is normally heaviest in southwest Alabama, and symptoms include irregularly-sized leaf spots with concentric rings. In contrast, areolate mildew is easily identified by a white mildew covering parts of entire leaves.”
Scherer suggests a continued, proactive crop protection approach through scouting and making timely fungicide applications. She said fungicides will not only protect the cotton crop from target spot and areolate mildew, but will also offer yield protection.
“Producers should consider fungicide applications if their crop has good yield potential, is between the first and sixth week of bloom and if disease is present or conditions remain conducive for disease development,” Scherer said.
Cotton Leafroll Dwarf Virus
Researchers have identified cotton leafroll dwarf virus (CLRDV) in sentinel plots in Brewton, Fairhope and Prattville, Alabama.
Scherer said the most common symptom in the plots is leaf bronzing or reddening.
“In Alabama, yield losses can vary from zero percent to 30 percent,” Scherer said. “Later planted cotton seems to be more at risk for yield impacts. Unfortunately, we do not have any management recommendations to offer at this point in the season.”
Stemphylium Leaf Spot
Regional Extension Agent Eddie McGriff said Stemphylium leaf spot is present in north Alabama cotton.
“Stemphylium leaf spot starts as small brown lesions, and as they enlarge they can lead to premature defoliation,” McGriff said. “There are several cotton leaf viruses with similar visual symptoms, so it is important to diagnose the disease correctly.”
A potassium deficiency brings on Stemphylium leaf spot. For this reason, fungicides will not provide effective control. The disease often occurs in dryland fields under drought stress or in fields where roots are restricted because of a hard pan. He said the disease can also be a problem under irrigation or fields with a high yield potential.
“Stemphylium leaf spot starts to appear around the fourth week of bloom with a heavy fruit/boll load which corresponds to a heavy demand for potassium,” McGriff said. “The roots of the plant also start to decline then due to competition for carbohydrates by developing bolls. This adds to the challenge of taking up enough soil potassium.”
Producers should conduct soil tests in fields with Stemphylium leaf spot to be sure there is adequate potassium. Petiole testing can help detect potassium deficiencies up to two weeks in advance, especially as the cotton crop moves toward peak bloom.
Alabama Cotton Entomologist, Scott Graham, said the cotton insect situation ramped up significantly in August.
“Plant bugs are still heavy in spots statewide, but stink bugs are becoming more of an issue as the crop continues to bloom,” Graham said. “Heavy bollworm egg lays have also been reported in parts of the Tennessee Valley. Spider mites are also in fields and have reached treatment levels in the Wiregrass.”
Graham said the overarching theme appears to be the drawn-out planting of corn because of the wet weather this spring.
Plant bugs—both tarnished and clouded—still require treatments in central and south Alabama fields. Graham said some cotton in the Tennessee Valley is under substantial plant bug pressure.
“The preferred method to scout for plant bugs is the black drop cloth,” Graham said. “The threshold is three tarnished plant bugs per five row feet. If clouded plant bugs are found, count them as 1.5 tarnished plant bugs.”
He said clouded plant bugs tend to cause more damage to small bolls than tarnished plant bugs, thus the threshold change.
“Thus far, stink bug pressure seems higher than normal,” Graham said. “This is especially true in the Tennessee Valley. Stink bugs are a consistent threat to cotton in central and south Alabama, so treatments with activity on both plant bugs and stink bugs are common.”
The most reliable scouting method for stink bugs is boll sampling. Sample bolls that are one inch in diameter for internal signs of injury.
“The threshold is 10 percent internal injury from weeks three through six of bloom,” Graham said. “Regardless of location, if a ‘hard’ chemistry has not been used by the third week of bloom, I would highly recommend finding a reason to use one.”
Graham said there are reports of heavy bollworm egg lays in the Tennessee Valley. Producers have not reported escaped worms, but he said if there is a high percentage of egg-infested plants there is a possibility of escaped worms.
“Focus egg-scouting efforts on the terminals and the bloom zone, particularly on bloom tags,” he said. “Currently our recommended thresholds are based on the number of escaped worms found per 100 plants.”
The threshold is five worms (0.25 inches or bigger) per 100 plants.
Spider mites have been active in the Wiregrass in the second week of August.
“Mites are in nearly every field statewide at some level all year, just waiting for a hot, dry stretch to ‘blow up,’” Graham said. “Treatment decisions require a bit of professional judgement. The threshold is when spider mite injury is widespread across the field.”Source : aces.edu