Kemerait: Protecting Georgia Cotton Against Leaf Diseases: It’s not what it used to be

There was a time in the not so distant past when cotton growers did not need to worry about protecting their crop from leaf diseases.  That time has passed.  Three foliar diseases are expected to affect cotton production in Georgia this year and all of them are likely to become more apparent after the crop begins to bloom, especially as we move into July and the “dog days” of August. One of them, Stemphylium leaf spot, is likely to be more problematic when hot and dry conditions prevail; however early leaching of potassium because of heavy rains early in the season could also bring this disease on.  Target spot is likely to be problematic in fields with good growth and good yield potential and where conditions are warm and humid and chances for rainfall are favorable.  Areolate mildew typically appears during the second half of the growing season and, like target spot, is favored by rainfall and humidity.  Areolate mildew has become the leaf disease of cotton that is of greatest importance across much of Georgia and for cotton growers throughout the southeastern United States.

It is important to be able to differentiate these diseases of cotton because effective management options are different and timeliness if critical where a fungicide could be effective.  Stemphylium leaf spot is associated with a deficiency in potassium in the cotton plant.  This deficiency may occur because the soil is deficient in potassium or because the roots have not been able to translocate sufficient potassium from the soil.  This could be the result of drought where water was not available to move the potassium into the plant, or it may occur in areas where plant-parasitic nematodes have damaged the roots and inhibited nutrient uptake.  It could also occur where potassium has been leached from the soil because of heavy rainfall.  Severe symptoms of Stemphylium leaf spot often occur in the sandier areas of a field, whether because nutrients are more easily leached from those areas, because drought is felt more intensely there, and because sting and root-knot nematodes are more commonly found in sandier areas of a field.  Plants affected by Stemphylium leaf spot show poor growth and significant reddening and yellowing in the foliage.  Spots are found on leaves throughout the canopy, and especially at the top of the plant.  Centers of the spots are papery and bleached-gray; often having a “shot-hole” appearance.  As Stemphylium leaf spot is the result of a potassium deficiency, fungicides are not effective in managing this disease.

Target spot, caused by the fungal pathogen Corynespora cassiicola, is a disease of cotton that is most likely to appear when the crop has a dense canopy of leaves and high yield potential.  Target spot is especially problematic where there are extended periods of leaf wetness, either because of frequent rain showers or where a dense leaf canopy and high humidity reduce airflow and keep leaves in the lower part of the plant from drying.  Target spot can also be a problem in fields with excess nitrogen.  Target spot is rarely found on poor-growth cotton or where Stemphylium leaf spot occurs.  Target spot begins deep in the lower leaf canopy and can move quickly up the plants causing significant premature defoliation when conditions are favorable.  The development of target spot is quite variable as it is largely driven by a disease-favorable environment.  The first week of bloom is an excellent time to begin scouting for target spot, as first bloom often corresponds with a time when the canopy is closing.

Judicious use of fungicides can help to protect yield and maximize profit for growers.  As target spot may not appear in every field every year and, therefore, not all fields will need to be treated.  However, where conditions are favorable and target spot appears early enough in the season, growers can increase their yield by 200 lb lint/A, or more, with use of fungicides.  Growers should consider use of a fungicide for management of target spot as early as the first week of bloom.  However, if the disease is not present, growers can safely delay application if they are willing to scout for onset of the disease.  The single most important timing for application is the 3rd week of bloom; multiple fungicide applications may be necessary to protect the crop and yield.  The most effective fungicides to date include Priaxor and Miravis Top.  Headline is effective as well; Quadris may be somewhat effective.  Target spot is nearly impossible to manage once significant defoliation occurs.  Growers should not need to protect their crop after the 6th week of bloom.

Prior to the 2017 growing season, areolate mildew (Ramularia/Ramulariopsis) was uncommon except later in the season in southeastern Georgia.  Since 2017 this disease has been observed in cotton across the Coastal Plain and has resulted in yield loss.  The determination that areolate mildew is often associated with yield loss is based on results of multiple field trials conducted in cooperation with UGA Extension agents Stephanie Hollifield (Brooks County), Jeremy Kichler (Colquitt County), Bill Tyson (Bulloch County, and Seth McAllister, Braxton Crews, Bill Starr, and Taylor McDaniel (Terrell County).  Basically, if areolate mildew appears within 4 weeks of anticipated defoliation, or if the crop is already 20% or more defoliated, I would not recommend use of a fungicide.  It is too late.  If areolate mildew shows up earlier, and the cotton crop looks to have reasonable yield potential, I would recommend use of a fungicide to protect yield.  In our UGA Extension trials, judicious use of fungicides has protected 100-250 pounds of lint/acre.  Again, if the disease is not detected until significant defoliation has occurred, then there is little chance of protecting yield with a too-late-applied fungicide.

Areolate mildew is easier to control than is target spot.  This is because the fungus is much more exposed (it looks a lot like powdery mildew) to the fungicide in the mid-to-upper canopy and because it is less confined to the lower canopy.  Use of Priaxor and Miravis Top will protect the crop against target spot and areolate mildew, as will Headline and Quadris.  Areolate mildew alone can be managed with timely applications of any of these fungicides; however azoxystrobin (e.g., Quadris) is more effective for management of areolate mildew than it is target spot.

Because of crop development, it is important to consider protecting a cotton crop with fungicides, especially after blooming has begun.  I recommend 5 basic steps in considering the need for a fungicide for management of foliar disease in cotton.  First, scout the field and determine what diseases are present.  Second, decide on management options.  If the disease is Stemphylium leaf spot, then a fungicide will not control the disease.  If the disease is target spot or areolate mildew, then a fungicide could be beneficial.  Third, consider the crop before applying the fungicide.  Does the field have a reasonable chance for good yields?  How advanced is the disease?  Fourth, decide on a fungicide.  Priaxor and Miravis Top are the best fungicide for management of target spot, though other fungicides, to include Headline and Quadris are also good.  These fungicides will control areolate mildew as well, though areolate mildew is easier to control than is target spot.  Fifth, timing of fungicide application is critical.  If applied too late, there will be little hope for controlling the disease and protecting yield.  Applying a fungicide when it is too late or is not needed is a waste of time and money.  Failure to apply a fungicide when use of a fungicide is justified could result in loss of up to 200 to 300 pounds of lint per acre.


Robert C. Kemerait, Jr.
Professor and Extension Specialist
Department of Plant Pathology
University of Georgia

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