August 30, 2001 http://commodities.caes.uga.edu/fieldcrops/cotton/
IN THIS ISSUE:
Managing Weeds at Defoliation
Aim as a Harvest-Aid
Cotton Disease Update
Richard Davis Joins USDA Research in Tifton
Localized Heavy Rains May Cause Harvest Problem
Click HERE for the 2001 Cotton Defoliation/Harvest Aid Suggestions
Crop Update.(Brown) USDA has projected Georgia producers to harvest 2.11 million bales from 1.49 million acres, which calculates to an average of 680 lb/A. The crop has reached that in-between stage in which rain is still needed in many fields but will hurt others. Particularly in the central and eastern portions of the state, dry, hot weather over the past three weeks has taken a serious toll on late-April and May-planted cotton. A few fields have already been defoliated and picking will begin shortly.
Managing Weeds at Defoliation. (Culpepper and Brown) Preharvest herbicide applications are of questionable value in most cases. Desiccating mature weeds likely will not increase harvesting efficiency nor reduce harvesting losses. The major exception would be fields heavily infested with viny weeds such as morningglory and cowpea.
Research in this general area has been very limited. There are no established guidelines for determining when the level of weed infestation justifies a preharvest herbicide application. Additionally, research is needed to better determine the response of various weed species to the herbicide and defoliant options available. Growers have four options to consider for defoliation or desiccation of annual weeds prior to harvest.
1) Harvade combined with other defoliants such as Prep will defoliate or desiccate certain weed species. Relatively good activity, especially if applied under warm conditions, has been observed on common ragweed, tropic croton, prickly sida, hemp sesbania, sicklepod, and mature morningglory (morningglory with seed pods). Harvade-Prep combinations are not effective on pigweed or lambsquarters and are generally poor on immature morningglory. Good spray coverage is essential for weed desiccation. In the case of morningglory, good defoliation and vine desiccation have been observed in cotton with open row middles, while poor results have been observed in rank cotton where spray coverage on the vines is poor.
2) Glyphosate applied in combination with a defoliant. In non-Roundup Ready cotton, tank-mix 1 to 2 lb AI/Aof glyphosate with the defoliants and apply in 15 to 25 gallons of water when at least 60 percent of the bolls are open. Glyphosate-defoliant combinations generally have been effective on annual grasses, common ragweed, lambsquarters, pigweed, cocklebur, tropic croton, cowpea, and sicklepod. In Roundup Ready cotton, glyphosate can be mixed with the defoliant as discussed above. Alternatively, current labeling allows for separate application of glyphosate after 20 percent of the bolls are open. Glyphosate will not suppress regrowth or juvenile growth of Roundup Ready cotton.
3) Paraquat (Boa or Gramoxone Max) can be applied after cotton defoliation. In this program, the cotton is defoliated as normal. After at least 75 to 80 percent of the bolls are open, the remaining bolls expected to be harvested are mature, and most of the cotton leaves have dropped, apply 0.25 to 0.5 lb AI/A of paraquat. Apply in a minimum of 20 gallons of water per acre, add 1 pint of nonionic surfactant per 100 gallons of water, and be aware of drift concerns. Wait approximately 5 days before picking, and then pick as soon as possible. If spray coverage is good, paraquat will desiccate most annual weeds.
4) Aim recently obtained a defoliation/desiccation label. In early season weed control trials, Aim has shown desiccation of small weeds such as morningglory when applied as a directed spray. However, it is unknown at this time how effective Aim is in the desiccation of larger and more mature weeds at harvest. Studies are being conducted to obtain further knowledge on Aim=s effectiveness for weed desiccation; thus, one may want to try Aim as a weed desiccate on limited acreage until more experience and research are obtained.
Aim7 As a Harvest-Aid. (Jost) FMC has recently received EPA approval for the herbicide Aim to be used as a cotton harvest-aid. Aim was initially marketed as a broadleaf herbicide for use in corn, small grains and rice. The active ingredient in Aim is carfentrazone, and it is formulated as a 40DF.
The herbicidal activity of Aim is based on its ability to block the enzyme protoporphyrinogen oxidate. This enzyme is necessary for the formulation of chlorophyll. When this enzyme is blocked, reactive oxygen species are produced which then destroy cellular membranes. In essence, the mode of action is somewhat similar to paraquat.
The defoliation response of cotton to Aim is due to this destruction of cellular membranes. When these membranes are destroyed, this causes a stress on the plant which stimulates the production of ethylene. Ethylene is the primary hormone responsible for the development of an abscission layer on the petiole of leaves, leading to leaf drop. Due to the fact that Aim is a broadleaf-herbicide, it may be particularly useful in fields with morningglories present at harvest time.
Currently, the rate of Aim being evaluated in other states is 0.6 oz (dry)/A. Aim may be tank-mixed with other harvest aids such as Prep, Finish, and Dropp. In these tests COC is also being used at a rate of 1%v/v.
Currently no defoliation data exists for Aim in Georgia cotton. However, in the Brazos Bottoms of Texas the following data was obtained.
Too Late? (Brown) When is it too late to irrigate? The general rule of thumb is to stop watering once the crop reaches 10 to 20 percent open. In years in which drought persists through late August and mid-September, many growers wish they would=ve wet the crop one more time.
When is it too late to apply foliar urea? Research on this issue is not well defined. Some scientists believe that foliar N applications influence yields only up through the 4th week of bloom. With our potential for making a respectable top crop, that period is probably well past that timeBIF additional N is needed by the plant. In the absence of petiole monitoring, such decisions are very subjective. Adding urea with late season insecticide applications makes for economical delivery of N, but trips-over-the-field solely for the purpose of applying urea are somewhat questionable this late in the year without strong justifying evidence.
When are insecticide treatments for worms or stinkbugs no longer needed? Generally, bolls that are fully sized (about 3 weeks old) are past the point of significant damage from either pest.
Cotton Disease Update. (Kemerait) As of the end of August, a number of cotton samples with foliar symptoms or leaf spots have been submitted from around the state. Most of these leaf spots have occurred on mature foliage and are likely the result of fungal pathogens infecting the older, declining leaves. Such symptoms should be of little concern to the grower now. Stemphylium leaf spot has also been observed. Stemphylium and Alternaria are secondary fungal pathogens that occasionally attack leaf tissues with inadequate levels of potassium.
Boll rot has been reported to be especially severe in some areas along the Florida line. Samples have been collected from one field that has been particularly hard-hit and the rot appears to be the result of a fungal pathogen. Similar fungal structures occur on both rotted bolls and necrotic foliage in the field; however, it will take a little time to determine the exact identity of the organism.
Although Fusarium wilt was identified in at least three counties very early in the season, the disease has not been detected in large acreage as of yet. The problem may become more apparent towards the end of this season if plants that are infected undergo some stress, such as moderate drought. Please contact us if you suspect Fusarium wilt in your area.
Richard Davis Joins USDA Research Team in Tifton. (Brown) Richard Davis has accepted a USDA Nematology position in Tifton and will assume his new duties early this fall. Over the past several years, he provided overall leadership for the UGA Extension nematology effort, and we look forward to his future contributions in nematode biology and management at the Tifton campus.
Bob Kemerait will assume extension responsibilities for cotton nematode management in the state, providing leadership for educational programs, field trials, troubleshooting, and grower training. Cliff Brewer will continue to manage the day-to-day operations of the nematode laboratory in Athens.
Localized Heavy Rains May Cause Harvest Problems. (Bader) Several locations around the state received afternoon thunderstorms. Many of these storms contained isolated areas of very heavy rainfall occurring in a short period of time. This intense rainfall caused excessive soil erosion and small gullies in fields. Some of these gullies may be large enough to hamper cotton harvesting. Even if your cotton is being picked by a custom operator, it may be necessary to address these gullies prior to picking. Chances are that more cotton will be lost by not eliminating the gullies so pickers can cross than making pickers go around them. There is less chance of doing major damage to picker heads, which costs money and down time. Having a field properly prepared for harvesting will speed up harvesting and help maintain cotton quality.
Steven M. Brown, Extension Agronomist-Cotton
Philip H. Jost, Extension Agronomist-Cotton & Ag Crops
Michael J. Bader, Extension Engineer-Cotton
Bob Kemerait, Extension Plant Pathologist
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