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August Cotton Newsletter

Cotton Management Considerations for the Remainder of 2018 (Mark Freeman)

 

The 2018 Georgia cotton crop is extremely variable in maturity. Much of the state has had cotton blooming and setting fruit for weeks where as other parts of the state still has a large portion of the crop yet to bloom. There is no one size fits all approach to management of this crop and decisions should be made accordingly.

 

With much of our crop planted late due to the rainfall in late May and June, we must remember that the window for blooms to become harvestable bolls is compressed compared to a full season crop. It is important during that early bloom period to minimize stress to the crop in order to retain as much lower node fruit as possible as the cotton does not have time to compensate for losses early on. Remember that the best way to manage vegetative growth is by having high early season fruit retention. This limits the amount of available carbohydrates that the plant can use towards vegetative growth later in the season. PGRs should also be used to limit vegetative growth and enhance lower node retention.

 

When should we terminate our PGR applications? Research conducted at UGA and other parts of the country suggest that PGR applications should cease when cotton reaches 5 nodes above white flower. In theory a late season mepiquat application could stop further vegetative growth and divert carbohydrates and resources to boll production, but research has shown that there is no benefit to cotton yields, plant heights, fiber quality, or regrowth potential after defoliation.

 

One more note on PGRs, with the widespread rain across the state, the issue of rainfastness has been a concern when applying mepiquat. The rain free period after mepiquat chloride application is 8 hours. Although tank mixtures with an adjuvant are not necessary, their use may aid in plant uptake and help shorten that rain free window. Another option is Pentia (mepiquat pentaborate). Pentia requires a rain free period of two hours after application and that can be reduced to one hour when Pentia is tank mixed with an adjuvant.

 

If irrigation water is available, all steps should be taken to ensure that the crop does not encounter drought stress when adequate rainfall has not occurred. This is especially true for the first six weeks following first bloom as this is the period of peak water demand. Irrigation scheduling has been a topic of conversation and research for the past few years and several quality cotton irrigation scheduling methods exist which use soil moisture sensor technology. However, if a grower does not intend to utilize one of these methods they are encouraged to follow the UGA Checkbook irrigation schedule which can be found on page 132 of the 2018 UGA Cotton Production Guide. One thing to note when using this method is that rainfall or irrigation amounts do not “carry over” to the next week. For example, if water demand for a particular week was 2’’/week and you receive 3”, that extra inch does not affect the amount of water required for the following week.

 

 

Insects (Phillip Roberts)

 

Silverleaf whitefly (SLWF) adults have been observed in low numbers in cotton in some areas during the past week to ten days.  To date very few immature whiteflies have been observed in cotton.  We are not aware of any field which has exceeded threshold for SLWF.  Most reports include observations of individuals or a few adults when searching plants for corn earworm.  However, the presence of SLWF in a field is worth noting and management of all insect pests must consider the presence of SLWF.  All efforts should be made to minimize the need to treat SLWF with insecticide.

Management Considerations:

  1. Scout for the presence of SLWF adults.  It is important to know if SLWF is present!
  2. Conserve beneficial insects, do not apply insecticides for any pests unless thresholds are exceeded (beneficial insects will also suppress corn earworm).
  3. If SLWF is present in a field, avoid use of insecticides for other pests which are prone to flare SLWF.
  4. Scout fields frequently for adults and immatures once fields are infested with SLWF.
  5. Be timely with SLWF insecticides when thresholds are exceeded (many learned in 2017 that it is difficult to play catchup with SLWF).
  6. Be very aware of SLWF infestations in hairy leaf varieties and late planted cotton, these are high risk fields.

There is no question that agents, scouts, consultants, and growers are looking more closely for SLWF this year based on the problems we had in 2017.  Historically if we see SLWF in cotton during the month of July we should anticipate problems with SLWF, especially on late planted fields, and manage appropriately.  Infestations do not come close to where we were a year ago.  In 2017 treatable populations first occurred during the last week of June and many acres were treated in July; so we are in a much better situation this year compared to last.  It will be important that all fields are monitored closely for SLWF and hopefully proper proactive management can minimize damage and the need for SLWF insecticides.  Your County Agent has additional information on management and scouting of SLWF in cotton.

Scout All Cotton For Corn Earworm

Bt cotton technologies have allowed cotton growers to significantly reduce insecticide inputs for caterpillar pests.  However Bt cotton is not and has never been immune to corn earworm (CEW).  Since commercialization of Bt cotton, a percentage of Bt cotton grown in Georgia has required supplemental treatment of CEW with foliar insecticide in most years.  Introduction of 2-gene Bt cottons significantly reduced the need for CEW sprays compared with the original single gene Bt cotton.  The industry is beginning to transition to 3-gene Bt cottons which will improve efficacy against CEW compared with 2-gene Bt cottons.

In recent years susceptibility of CEW to Bt cotton has significantly declined in parts of the US (especially in the Midsouth and North Carolina).  Only a small percentage of 2-gene Bt cotton has required supplemental treatment for CEW in Georgia during recent years so we have not observed this decline in efficacy in the field (or have we?); this is due in part to low corn earworm infestations in cotton.  However we did see a slight increase in CEW insecticide applications during 2017.

UGA researchers have seen reduced field efficacy in Bt corn in Georgia.  We have also seen reduced efficacy of the original Bt toxin in bioassays for CEW collected in Georgia.  Basically what is happening is CEW is/has developed resistance to the Cry1Ac gene.  If the original Bt gene is no longer providing control of CEW, our 2-gene Bt cottons (Bollgard 2, WideStrike, and TwinLink) will be relying on a single gene and overall efficacy will be reduced.  There is variability in performance of Bt cotton technologies.  The chart to the right illustrates relative efficacy of commercially available Bt technologies.  Note that all Bt technologies continue to provide excellent control of tobacco budworm!

 

Bottom line is that all Bt cotton must be scouted on a regular basis and growers must be prepared to act accordingly if thresholds are exceeded.  When scouting pay close attention to blooms, bolls with stuck bloom tags, and small bolls.  When corn earworm escapes occur in Bt cotton they are usually observed near or just below the uppermost white bloom.  The threshold for CEW in Bt cotton is when 8 larvae ¼ inch or greater are found per 100 plants.  We recommend scouts search the top 12 inches of plants and one bloom, one bloom tagged boll, and one small boll per plant.  Remember that larvae must hatch and feed on the plant to ingest the Bt.  However if larvae reach ¼ inch in length survival is likely.  React in a timely manner with supplemental foliar sprays if thresholds are exceeded.  Coverage and penetration of the canopy with insecticides will be important as escaped CEW will be down in the canopy. Only spray other pests (i.e. stink bugs) based on thorough scouting and appropriate thresholds.  Conservation of beneficial insects such as bigeyed bugs will reduce the risk of CEW issues.

 

Diseases (Bob Kemerait)

 

August is a Critical Month for This Year’s Target Spot Control and Next Year’s Nematode, Fusarium Wilt and Bacterial Blight Control

One of the most important components of disease and nematode control in the cotton crop is “timeliness”.  “Timeliness” means deploying the best management tactic to fight diseases and nematodes before these foes become well-established or before the opportunity to use a tactic has passed.  August is a very important time in the cotton season to both be timely for possible fungicide applications this season and for preparation for next season.

Foliar Diseases.  As of the fist of August, there are four foliar diseases active in some, but certainly not all, cotton fields in Georgia.  These include bacterial blight, caused by Xanthomonas citri pv. malvacearum, Stemphylium leaf spot, caused by the fungus Stemphylium solani, target spot, caused by the fungus Corynespora cassiicola, and areolate mildew, caused by the fungus Ramularia spp.

There is little that can be done to manage bacterial blight at this time in the season, other than to note if it has occurred in your fields and which varieties have been affected.  Because of weather during the summer of 2018, even varieties which have shown some level of resistance to bacterial blight in the past have been affected this year.  However, now is the time that growers should note the level of bacterial blight in their crop and begin to make decisions as to variety selection for 2019.  Growers have an increasing number of “bacterial blight resistant” varieties from which to choose.

 

Photo Greg Slaughter

Stemphylium leaf spot is characterized by numerous small spots with dark purple/brown margins and often times gray, papery centers.  This disease occurs when the cotton plant is deficient in potassium; potassium deficiencies may exist because of poor soil fertility, perhaps from leaching, or during periods of drought where potassium is not taken up into the plant.  Stempylium leaf spot is managed by insuring proper levels of potassium in the plant; fungicides are not an effective management tool.

Photo Jason Brock

 

Target spot became evident in southwestern Georgia in the latter part of July and is likely present in many fields across the Coastal Plain of the state.  Target spot can develop quickly and is most common in good-growing cotton with high yield potential.  Extended periods of leaf wetness, where the foliage in the interior of the canopy remains wet well into the later morning hours, create perfect conditions for rapid development of target spot and premature defoliation from it.  Fungicides are an important management tool for target spot, though use does not always result in increased yields.  From our research, effective use of fungicides should be considered between the first and sixth week of bloom where the third week of bloom is typically the most critical time of management.  Scouting before use of fungicides to determine if the disease is present help to ensure that an application is warranted.  Priaxor is currently the most effective fungicide for control of target spot, though Headline, Quadris and others are also effective.

Areolate mildew was especially severe in 2017 and there are reports that the disease is back in 2018 if a few fields.  Areolate mildew has historically been confined to southeastern Georgia east of I-75; however it can be found elsewhere as well.  Typically arriving too late in the season to cause any damage (in fact, late-season defoliation may be a benefit), use of fungicides had often not been warranted.  However, in severe cases, the same fungicides used to control target spot are also effective in the management of areolate mildew.  Growers within three weeks of defoliating their cotton need not worry about managing areolate mildew.  Where areolate mildew occurs in a crop with anticipated defoliation a month or more away, and weather is favorable for continued development and spread of the disease, then use of a fungicide may be beneficial to protect yield.  Unfortunately, we have very little data at this point with which to refine our recommendations for management of areolate mildew.

 

It is too late to protect our 2018 cotton crop from plant-parasitic nematodes of Fusarium wilt; however now is the time that symptoms become very evident in the field.  Where stunting, poor growth and even dying plants are found in areas of a field, growers should take measures to determine 1) is it caused by nematodes?, 2) if so, what kind of nematodes, and 3) is Fusarium wilt also involved.  Detection and identification now will help growers to make best variety selection and possible use of nematicides in 2019.

 

Important Dates:

 

Field Days:

 

Midville – Southeast Research and Education Center – August 15th

Tifton – Cotton and Peanut Field Day – September 5th

 

For more information on any of the discussed topics please contact your local UGA Extension Agent.