August 30, 2001
http://commodities.caes.uga.edu/fieldcrops/cotton/
IN THIS ISSUE:
Crop Update
Managing Weeds at
Defoliation
Aim as a Harvest-Aid
Too Late
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.
Prepared by:
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
Putting knowledge to work
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