Cotton Disease Update: August 19, 2016

Target spot of cotton as indicated by lesions containing concentric rings.

Target spot

Over the past decade the occurrence of target spot in the MS cotton production system has been on the rise. Target spot, until this year, had been a disease that tended to follow large amounts of rainfall.  Three years ago target spot was bad in southern MS due to tropical conditions that persisted during the end of the season.  The disease itself, produces sizeable roundish lesions on cotton leaves in the lower to middle canopy.  Lesions tend to be 1/8^th inch to ½ inch in size and can, in some cases, be surrounded by a yellow halo.  More often than not, the lesions have a concentric pattern to them, hence the name target spot.

Over the past several years fungicide trials have been conducted across the southern U.S. to attempt to manage target spot. Trials have been conducted in AL, AR, GA, LA, MS, and TN.  In several of the trials the disease did not develop on what were considered to be susceptible varieties (e.g., Deltapine 1137, Phytogen 499).  In general, and based in large part on reports from the southeastern U.S., a positive response to a fungicide appears to be more likely when cotton is treated around first bloom or at most two weeks after first bloom.  Reports this year have been rampant regarding the necessity for two foliar fungicide applications: one at first bloom followed by one two weeks later.  In 2015, trials were conducted in Stoneville, MS with the two aforementioned varieties.  Target spot was readily observed in all plots and caused substantial defoliation, in some cases greater than 50% of the lower to middle canopy.  The trial consisted of one or two applications of either Headline (6 fl oz/A), Priaxor (4 fl oz/A), Quadris (6 fl oz/A), or Topguard (7 fl oz/A).  For the most part, regardless of cotton variety, fungicides did not result in a positive response.  On average, regardless of product or number of applications, fungicides resulted in 60 lbs less lint yield than the non-treated in Phytogen 499.  In Deltapine 1137, a similar trend was observed; however, the reduction was on the order of 166 lbs less lint yield in the treated versus the nontreated.  The only positive response, on the order of less than break even (based on cost of application and air plane and general selling price of cotton) was a single application of Priaxor at white flower which resulted in a 29 lb increase in lint yield over the nontreated.

Cotton boll rot as observed in the lower canopy.

Boll rot

Numerous calls have been received in the past week regarding the initiation of boll rot. The environmental conditions at present remain conducive for the development of boll rot.  A long list of organisms can be responsible for the initiation of boll rot in the cotton canopy including bacteria (see below), fungi and some yeasts.  In years where conducive conditions persist, including extended periods of cloudy, humid, rainy weather, for extended periods of time boll rot can become severe in the lower canopy.  First and foremost, once boll rot has been initiated probably the best medicine would be for sunshine and dry weather.

Managing boll rot has been a common topic of conversation for decades. Numerous fungicide trial plots have been conducted in almost every cotton growing state to determine the benefits of carefully timed foliar fungicide applications.  However, several general comments can be made about application technology as well as the class of chemistries available at this time.  Aerial application does a good job of applying fungicides over a large number of acres in a relatively short period of time.  However, the aerial application technology oftentimes cannot deliver the fungicide product to the portion of the plant canopy where the disease has occurred and therefore cannot protect bolls in the lower canopy.  Fungicide products in the strobilurin class are the predominant products labeled for application in cotton, with a few products that contain a strobilurin and a triazole.  Strobilurin products are preventive and should be applied prior to disease onset.  Triazoles provide a limited “curative” potential and can be applied after disease onset, but much like strobilurins are better when applied prior to disease.  Neither of the fungicide classes are effective at arresting the development of boll rot once this has started in the lower canopy.  In addition, reduced volume aerial applications likely do not provide the canopy penetration or coverage of the plant part necessary to aid in reducing boll rot.  Aerial application is good at applying products to the plant’s leaves, but bolls are oriented in a different fashion and much more difficult to cover.  Long story short, the jury is still out as to whether or not foliar fungicides effectively reduce boll rot.

Bacterial blight of cotton as determined by the water-soaked lesion on a boll.

Bacterial blight

Once again this season, bacterial blight has been observed in several counties within the MS cotton production system. Angular lesions on leaves, lesions that can surround the petiole (blackarm), lesions on bracts, and water-soaked lesions on bolls have been quite common in counties reporting bacterial blight.  Even though bacterial blight can incite boll rot the cause of boll rot cannot always be attributed to bacterial blight.  A laundry list of organisms, in some cases greater than 175 organisms that includes bacteria as well as fungi can cause boll rot.  Bacterial blight can initiate boll rot; however, all boll rots should not be blamed on bacterial blight.  In some cases, bolls can completely rot from bacterial blight, but a secondary organism will feed on the rotted boll.  Do not stop managing the cotton crop if bacterial blight is observed in the cotton canopy.  To prevent the risk from bacterial blight occurring in 2017 one of several options can be selected.  Firstly, rotate to a non-host.  Corn and soybean are not hosts of the bacterial blight organism.  Secondly, deep tillage can reduce the overwintering potential of the bacterial organism.  Thirdly, choose a bacterial blight resistant variety.  Several resistant varieties are available on the market and will reduce the losses associated with bacterial blight.