Cotton Boll Rot: Potential Implications of Bacterial Blight

Boll rot caused by bacterial blight at the base of a boll.

Bacterial blight (or angular leaf spot) continues to be the hot topic in cotton production even at this late stage in the season.  With continued temperatures between 86 and 97°F as well as scattered rainfall throughout much of the state it is likely that bacterial blight will continue to defoliate plants and infect bolls.  A few consultants have called and suggest that bacterial blight has almost shut down in some fields.  However, I’m still able to observe fresh infection on bolls and active lesions present on leaves throughout the plant canopy.  As a brief update, 11 counties have confirmed bacterial blight infections (Bolivar, Calhoun, Coahoma, Grenada, Leflore, Monroe, Quitman, Sunflower, Tallahatchie, Washington, Yalobusha).  In addition, 4 varieties in MS have been observed with bacterial blight symptoms.  Conservatively, 50,000+ acres of cotton have bacterial blight present on the plants and infection ranges from mild (light leaf spotting present) to severe (excessive leaf spotting, with undercanopy defoliation, boll rot, and little to no bolls present). 

At present, fields are starting to be observed with what could be considered to be mild symptoms of boll rot.  In one particular situation, on Tuesday, in a field south of Highway 82 in Sunflower County with light bacterial blight symptoms on leaves, symptoms of boll rot could clearly be attributed to bacterial blight.  Dark, circular lesions were observed at the base of the boll where the peduncle attaches to the plant.  In some cases, secondary boll rot was prevalent on those bolls that had a primary infection site caused by bacterial blight.  I scouted several other fields on Wednesday and found similar symptoms in the north Delta.  Keep in mind, there are at least 170 organisms (bacteria, fungi) that can cause boll rot.  One of the single worst boll rotters is the bacterium that causes bacterial blight, Xanthomonas axonopodis pv. malvacearum (= Xanothomas campestris pv. malvacearum).  The particular bacterium can infect bolls, stain cotton yellow, cause hard lock, and increase the likelihood of the transmission of the disease through infected seed.  In addition, if conducive environmental conditions are not present for an extended period of time, bacterial blight opened a wound in the developing boll and it becomes much easier for a secondary organism to infect the boll at that site.  The release of nutrients from the boll makes for an easy entry point for another organism.          

Boll rot caused by a fungus, likely a secondary infection on a boll that was initially infected by the bacterium that causes bacterial blight.

Once boll rot has begun, a management attempt to arrest the development of the organism(s) involved is mostly ineffective.  I’m fairly sure that in the next few weeks there will be numerous questions and concerns posed about the efficacy of a fungicide or put most simply “would a fungicide have benefitted my crop?”  Three main issues make the expectation of a fungicide as a preventative tool in managing boll rot a difficult one: fungicide timing, canopy penetration, and specific organism attributed to the boll rot.  Fungicide timing is likely the main reason for this issue since there are currently products available for application to cotton that are labeled for boll rot management but it is often difficult to determine what specific organism was the root cause of the boll rot.  With that in mind, a fungicide would not be expected to provide “control” of boll rot caused by bacterial blight.  A fungicide product should be chosen based on the type of organisms present in the past and whether or not boll rot is present prior to application.  Applying a fungicide to cotton with boll rot already detected in the lower canopy will not stop the boll rot on bolls with infection the products labeled are preventive but would only be expected to provide some level of “control” if application were done prior to infection.  But, again, if bacterial blight started the infection and a fungus causes a secondary infection a fungicide would still not have prevented this from happening.  The issue of timing is one that continues to be researched.  One of the other main issues would appear to be application method used.  In almost all cases, boll rots begin in the lower canopy, and depending upon the specific environment encountered (high relative humidity) boll rot can move up the plant.  Expecting a fungicide to arrest development of boll rot in the bottom of the plant canopy with aerial application methods is simply expecting too much.  Ground application would be the best method to apply a fungicide but then we’re right back to a timing issue.  Moreover, many of the organisms involved in boll rot would likely not be properly managed with a fungicide application especially in a situation where bacterial blight was the initial causal agent of the boll rot.