Alabama Cotton Shorts
August 10, 2020
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August 10, 2020
Situation. The crop has advanced rapidly in the last weeks. Many April and early May plantings are showing signs of maturity as the crop blooms in the upper canopy, leaves decline with age and leaf spots, and large bolls begin to speckle and harden. As of August 10, over 75 percent of the crop is rated good to excellent. This week’s forecast has good probabilities for rain in the entire state, so we’re likely to see some much needed relief across Alabama. Had we had a broad inch or so each of the last couple of weeks, we might not could gin all the cotton we’d make. We have a good crop. In places, it’s exceptional; in others, heat and drought have diminished it somewhat. (Brown)
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Stressed cotton near Prattville on July 20.
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Implications of Leaf Productivity in Cotton. The following is a quote from the May 1990 entry from Cotton Physiology Today, a multi-year research-based educational series on cotton management produced by the National Cotton Council.
Cotton leaves have a limited productive life, just like an athlete, but leaf productivity is measured in days instead of years. For the first 16 days after a leaf begins to unfurl, carbohydrates produced by the photosynthetic machinery are directed towards growth of the new leaf. But as soon as that leaf is near full size (80% expanded), this 16 to 18 day old leaf hits its prime carbohydrate exporting capability. And by the time a leaf is 25 days old, it starts a downhill slide until age 60 to 65 when it no longer exports carbohydrates to fill bolls. During mid-season, the most active main-stem leaf is approximately 5 nodes from the top, while the leaf 13 nodes from the top has already aged to the point where it is non-functional.
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The research on which these comments are based indicates that the production of a leaf declines after a few weeks, and eventually lower leaves contribute little to boll fill in the upper canopy. This is further compounded by photosynthesis-limiting shade in the lower portions of the crop canopy. The article goes on to state, “One of the most important management factors to derive from leaf aging studies is that maturation of late set bolls requires late season leaves.”
Two thoughts from this: (1) A foliar disease such as Corynespera (target spot), if it is not already established in the crop, poses little threat in mid- to late-bloom cotton. Also, since target spot thrives down in the high humidity of the crop canopy, air movement in the upper, exposed terminal area limits its effects in upper vegetation and fruit. (2) Good fertility – especially K and N – matters. It matters because proper plant nutrition sustains leaf health late into the season. A healthy, strong upper canopy is needed to finish a crop. The N part of this is easy. Soil, sidedress, and foliar applications readily deliver sufficient N for strong yields. Potassium is more challenging because pre-bloom stresses can limit K uptake despite proper fertilization, despite the fact that adequate K is in the soil.
The dominant, hard-to-escapet challenge is moisture/heat stress. While many parts of the state have enjoyed favorable rainfall during much of June and July, the last couple of weeks have stressed crops in many places. Some good rains would help us toward a strong finish. (Brown)
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K Deficiency and Related Foliar Diseases. Over the past few weeks we’ve observed several cotton fields with Stemphylium leaf spot. The ultimate cause is a depletion of potassium (K) in leaf tissues. Potassium is rapidly exported from leaves and stems to fill developing bolls, and as K becomes scarce in leaves, plants become vulnerable to fungi such as Stemphylium and, less commonly, other secondary pathogens such Alternaria and Cercospera. Potassium adds strength to leaf cells and the lack of potassium in leaf tissues makes them weak and susceptible to these fungal infections. These secondary invaders initially appear as small brown lesions, and as they enlarge, they can lead to massive premature defoliation.
Incidence of Stemphylium is most common in dry land fields under drought stress. Lack of soil moisture reduces uptake of K during vegetative growth and early flowering, and thus the reserves in stem and leaves are insufficient to sustain the leaves during boll fill. Adding to the challenge is that root activity tends to decline as plant resources are directed towards reproduction. Short season varieties can sometimes be more susceptible to Stemphylium since they often have an intense demand for K in a short time. These secondary pathogens can also be a problem under irrigation, particularly in the latter stages of flowering (4th week or later) in the presence of a heavy boll load and if heavy rainfall or watering has promoted K leaching. In recent years there have been a few cases of Stemphylium attributed to high soil magnesium levels competing with K for uptake in plant.
Note, foliar fungicides do not help with these secondary invaders. Potassium deficiency is the issue. Other foliar diseases such as target spot and areolate mildew may respond to timely applications of fungicides, but they are ineffective for Stemphylium and related pathogens. (McGriff and Brown)
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Cotton Scout School. The 61st annual consecutive Cotton Scout School was held on July 24 and recorded on YouTube. It can be referenced at: https://www.youtube.com/watch?v=thTs78VahqE&t=2s
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North Alabama’s Turn for Bollworms. Thankfully bollworm pressure in south and central Alabama was relatively light this year, and we did not receive any calls on escaped worms in those regions. However, we have received reports from the Tennessee Valley of bollworm egg lays from 5 to 20 percent over the past 7 to 10 days. Currently, we are still recommending allowing the Bt technologies in two-gene cottons the opportunity to control these populations. In fields that have been treated for other pests, such as plant bugs or stink bugs in the past 2 weeks with hard chemicals (e.g., pyrethroids, organophosphates, or neonicotinoids), we recommend treating when 5 small larvae are found per 100 plants. In fields that have not been sprayed in the last 2 weeks, we recommend treatment when 10 small larvae are found per 100 plants. The reason for the difference is the presence of beneficial insects that should be in fields that haven’t been treated recently. You can watch Eddie McGriff’s presentation on beneficial insects on YouTube. here.
Current recommended thresholds, insecticides and rates can be found in the Alabama Cotton IPM Guide. (Graham and Smith)
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Spider Mites. Spider mites are still building in areas of Alabama. Spider mite infestations are rarely evenly distributed throughout the field and are almost always in clumps either near field edges or scattered randomly throughout the field. Our threshold in the Alabama Cotton IPM Guide says to treat fields when mites are widely distributed and mottling of leaves is common. Determining when to implement controls can be difficult when trying to decide how many ‘hotspots’ suggest mites are “widely distributed” and justify a spray. Spider mites prefer hot, dry conditions and typically can be suppressed by a rainfall event. Fields with spider mite hotspots should continue to be monitored after rain, because populations can build back after several days of hot, dry conditions. Spider mites also tend to build following broad-spectrum insecticide applications for other pests.
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Typical spider mite damage on the underside of a leaf. Mites are present but barely visible to the naked eye.
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To scout for spider mites, look for leaf stippling or reddening on the top of leaves. If these symptoms are observed, look on the underside of leaves for spider mites, which will be a yellowish color with two black “spots” on each side of its back. Exposing the underside of leaves to the sun may agitate mites, making them easier to see. Also look for mites one or two nodes above the most symptomatic leaves as they may have moved up to fresh leaves. There are a few miticides labeled for use in cotton that can be found in the Cotton IPM Guide. Abamectin (e.g., Agri-Mek 0.15EC) at 8 to16 ounces per acre is the most economical option but rotating chemistries is necessary if multiple applications are needed. Historically, lower rates of abamectin (8 to 10 oz) have provided adequate control in younger cotton, while higher rates (12 to 16oz) are needed later in the season when plants are larger. (Smith and Graham)
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While whole cottonseed is primarily used as premium feed for dairy animals, it can be further processed in an oil mill to obtain additional products. Processes at a cottonseed oil mill render three materials, LINTERS, HULLS, and KERNELS, which find significant downstream use.
LINTERS are the short, fuzzy fibers still present on ginned seed. They are a fine source for cellulose and are used to produce plastic, rocket propellants, rayon, pharmaceutical emulsions, cosmetics, photography and x-ray film, upholstery, fine writing paper, and even paper currency.
HULLS are the tough, protective outer covering of the interior seed kernel and are used mostly in the feed industry as roughage for livestock.
KERNELS are the protein and oil-rich internal portion of the seed. They are flaked and crushed to produce cottonseed oil and meal. One ton of crushed cottonseed can yield approximately 320 pounds of oil. The meal and hulls are used in animal feeds as high protein and roughage supplements. (adapted from georgiacottoncommission.org)
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On Cotton Incorporated’s Cotton Cultivated page there is July 28, 2020, post “Preparing for Defoliation.” The link is:
http://www.plantmanagementnetwork.org/edcenter/seminars/Cotton/Defoliation/
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In the “Old Days”... Ethrel, a 2.0 lb per gallon ethephon product, was first registered with EPA in 1973 as a ripening PGR for fruit, nut, and vegetable crops. In the early 1980s, it was evaluated and approved in cotton as a harvest aid and boll opener. Uultimately, it changed how we grow and harvest cotton. The primary cotton product became Prep, a 6.0 lb per gallon ethephon. Today most ethephon products are similar in concentration and some include other PGRs as additives. Its primary use is for boll opening which makes once-over harvest possible. Ethephon is also a good defoliant and effectively removes immature fruit.
In years prior to the introduction of ethepon, upland cotton was often harvested twice. There was a 1st picking and a 2nd picking, which provided a ready measure of maturity and relative earliness. As commonly observed today, aggression of spindles on the plant often facilitated the opening of green bolls, which helped with the economics of a 2nd harvest. (Brown)
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This issue contributors:
Dr. Scott Graham, Extension Entomologist
Eddie McGriff, Regional Agronomic Crops Agent
Dr. Ron Smith, Professor Emeritus, Extension Entomologist
Dr. Steve M. Brown, Extension Agronomist (Editor) typos and other mistakes are mine
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About the Alabama Cotton Shorts Newsletter
Alabama Cotton Shorts is a newsletter designed to keep cotton producers in the know. From planting dates to crop inputs—there are many factors to consider. The Alabama Cooperative Extension System is dedicated to providing science- and research-based information, derived from field experience and observations. A team of Extension specialists are working to provide Alabama farmers with timely information throughout the growing and harvest seasons.
By subscribing to the newsletter you will receive pest updates, weed management suggestions, market updates, industry news, and other information. Specialists are making field observations and reporting their findings in hopes of helping producers make more informed choices in the field.
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