The symptoms that can be expressed by this particular bacterial disease are quite unique. Bolls, bracts, flowers, leaves, petioles, and stems can all exhibit symptoms but they can and will appear different, making disease diagnosis complex at the field level. Characteristic symptoms on leaves are more commonly referred to as “angular leaf spots” that are confined by the veins (see photo at left). The angular nature of the leaf spots will be the main difference between foliar fungal diseases. In addition, the center of lesions caused by bacterial blight will not fall out of the lesion. Leaf lesions will first begin to develop as water-soaked spots and will progress to black (oftentimes referred to as greasy in their wet appearance), sunken spots that will ultimately result in leaf defoliation. Lesions on bolls will appear as if hot oil splashed on the boll. In addition, I have heard people characterize the lesions on bolls as having the appearance of a cigarette burn depending upon the stage of the disease as well as the stage of the bolls.
Bacterial blight is most severe when temperatures are between 86 and 97F and when relative humidity is above 85%. Temperatures throughout the state have been well within this range for the better part of the last 6 weeks. Moreover, the bacterial organism responsible for the disease can be spread to noninfected plant tissue by windblown moisture and in general by water splash. Therefore, overhead irrigation can exacerbate the disease. In addition, equipment can easily spread the bacterium to plant material when driven through the field at periods of the day when dew is present on foliage. Avoid equipment moving through the field when dew is present.
Once cotton has been determined to be infected by the bacterium there are no curative measures that can be taken. In several of the conversations over the past few days fungicides have entered the conversation. A fungicide application WILL NOT be beneficial since that entire group of products only benefits plants infected with a fungus.
Scouting cotton fields and determining whether or not the disease is present should be done at this time. Keep in mind, the disease can be spread to fresh host tissue by the movement of moisture on something as simple as your pant leg. If you are scouting infected fields and moving into fields without infection keep scouting to those periods of the day when dew will be absent. Should you need help with lesion type or a place to send samples for proper identification please feel free to call myself (662-402-9995), Darrin Dodds (662-418-1024), or Clarissa Balbalian (662-325-2146) in the diagnostic laboratory.