Cotton Foliar Disease and Fungicide Application

Tom Allen, Extension Plant Pathologist
By Tom Allen, Extension Plant Pathologist July 1, 2011 13:34 Updated

Cotton Disease Situation:

Foliar leaf spot of cotton.

Over the past few years we’ve received an increasing number of telephone calls regarding foliar fungal diseases in cotton.  Much of the time the caller blames a generic glyphosate application or an insecticide application plus adjuvant for the appearance of the foliar spots in the cotton crop.  Typically what is encountered, rather than being caused by an herbicide or adjuvant, are minor foliar fungal diseases.  In general, the fungi that cause the spots aren’t considered to be overly aggressive and overall could be considered to be weakly pathogenic.  However, many of the fungi responsible for the foliar lesions generally observed following first white flower occur as a result of a potassium (K; potash) deficiency in the leaf.  In fact, in most cases where we’ve encountered foliar disease (or leaf spots) in cotton the leaves on the plants had a nutrient deficient appearance (see attached photos).  In some rare cases the disease appeared to be so severe that tremendous leaf shedding occurred as a result.  Essentially, once a potassium deficiency occurs in the leaf the stressed plant becomes an easy target for what would typically be considered weakly pathogenic fungi. 

Severe foliar leaf spot.

Several foliar diseases of cotton have been identified at an increasing level throughout the southern U.S. over the past 5 years.  Determining the particular causal agent at the field level is incredibly difficult since the majority of the leaf spots appear similar to one another and the only true way to tell them apart is to isolate spores from the infected plant tissue.  In addition, taxonomically speaking determining the name of the fungus that causes the leaf spot in the laboratory is a frustrating endeavor as the names of some of the fungi continue to change.  At the field level, Alternaria leaf spot, Cercospora leaf spot, Corynespora leaf spot, and Stemphyllium leaf spot all appear similar to one another since the spots produced by the particular fungi almost all have a purple margin and a grey center that in some cases will fall out of the lesion.  One exception to that rule is Alternaria leaf spot that typically forms in the lower canopy and has a target-like appearance (think target spot of soybean).  More specifically the diseases are at best a secondary issue following a reduction in leaf potassium as a result of boll set.  Nutrients shifting from the leaves (course) to the bolls (sink) reduce the concentration of nutrients in the leaves and make them easy targets for weak pathogens.  Boll rot incidence is a completely different issue and will likely be the focus of a blog update later in the season.  Moreover, spots can develop on leaves, the stem, and bolls as a result of any of the foliar diseases listed above. 

Fungicide Application?:

Severe defoliation as a result of foliar leaf spot fungi.

Since 2007 strobilurin fungicides (or products that contain a strobilurin active ingredient) have been suggested as a “plant health” application in Mississippi cotton production.  The suggestion is similar to the “plant health” scenarios in corn, soybean, and wheat.  Essentially, advertising suggests that a fungicide application, made around the time of first white flower, and in some cases followed by a second application 14 days later, can be expected to result in an automatic yield increase (pounds of lint/A).  Pyraclostrobin (Headline) was the first foliar fungicide labeled in cotton in 2007.  In 2008, azoxystrobin (Quadris) was labeled for foliar disease prevention and boll rot management.

As a part of a larger fungicide project (including several states, AR, GA, TN, TX, and MS) in 2008 and 2009 fungicide trials were conducted in Starkville and Stoneville to determine if there was a “plant health” benefit to the application of a foliar fungicide made around first white flower.  Trials were conducted with several different fungicides, including products labeled on cotton and those products that are not labeled on cotton, and generally including the strobilurin-based fungicides since those are the products that have been touted to have “plant health” benefits.  Applications were made by ground equipment (either back pack of hi-boy).  Additional data, including number of bolls above white flower, plant height, disease assessments (including boll rot) were collected.  However, only yield on a pound of lint/Acre are included in this update.  As a whole, the greater data set suggested that a yield increase following a foliar fungicide application could not be expected or predicted.  Additionally, there has been tremendous talk about the use of TwinLine on cotton this year in particular.  Few if any trials have been conducted by academic institutions and all the people I called (at least 4 people in neighboring states) suggested they had not looked at TwinLine in cotton.  One trial was conducted in LA and MS with results similar to those contained within the tables.  Other than that, limited data exists within the academic extension or research community regarding the specific use of this product in cotton production.

Alternaria leaf spot of cotton.

Two tables are included with this update.  The tables utilize the same data set where fungicides were applied by ground.  The table presenting cost of aerial application (Table 2) is included to provide the cost difference between aerial and ground application and the expected return on investment based on three different cotton selling prices.  Products were not applied by air to provide this information.  Similar to the MSU corn fungicide trial data sets, fungicides did not significantly increase yield compared to the untreated plots.  However, two applications of pyraclostrobin resulted in a mathematical increase in yield in addition to a potential economic increase but keep in mind that the tables don’t include the cost of many other variables that should be included at the farm level.

In addition, one of the more interesting scenarios between statistics and a cost benefit type of analysis, specifically considering return on investment, has to do with whether or not a treatment that is deemed statistically significant from another is actually economically beneficial.  As presented in the two figures, even though there were significant differences between fungicide treated and the untreated plots (some fungicides significantly reduced yield) as well as significant differences between fungicide treatments this doesn’t always translate into an economic response.       

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Tom Allen, Extension Plant Pathologist
By Tom Allen, Extension Plant Pathologist July 1, 2011 13:34 Updated
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