Bacterial blight was officially confirmed in Appling County Thursday September 1 by Dr. Kemerait from samples collected by a consultant. Prior samples I sent in to the lab from the same field came back positive for target spot and Cercospora leaf spot. Cercospora leaf spot, like Stemphylium leaf spot, typically occurs in fields where fertility is an issue and neither of these diseases can be controlled using a fungicide. Finding bacterial blight, target spot and Cercospora leaf spot is proof that a grower could have multiple foliar diseases of cotton in a field at the same time. Bacterial blight is very likely in many fields in Appling County this season, but may be at levels where it is not obvious and will have no impact on yield. Until recently cotton farmers in Georgia have had little need for concern over this disease; however it has made its presence known in the 2015 and 2016 seasons.
Bacterial blight of cotton is caused by the pathogen Xanthomonas citri pv. malvacearum. It can cause very diagnostic spots on the leaves (angular/geometric in shape) as the lesions, at least initially, are delimited/bound by the leaf veins. At times, the pathogen can become systemic in the leaf veins causing a dark, “lightning bolt” appearance as the bacterial pathogen moves through the veins. It can also cause a very diagnostic boll rot that starts out as a distinct water soaked lesion on the bolls. This can then progress to a general boll rot, often with the introduction of additional fungal pathogens.
I have been working for extension for over 10 years and Dr. Kemerait for 16 years. For the first 14 years Dr. Kemerait has been at UGA, he rarely saw bacterial blight. The disease was severe in 2015, but largely confined to a single variety, Delta Pine 1454, which was retired in our area because of that disease. We also saw it on Delta Pine 1558 (a very important root-knot nematode resistant variety), but felt confident that the amount of disease was not affecting yield and was certainly not to a level that would overshadow the importance of the strong nematode resistance and high yield potential. See the below trials from last year in Appling county.
Here are some important points about bacterial blight:
-Bacterial blight has returned in 2016 and is very wide-spread. It is perhaps most obvious on DPL 1558, but it is found on many other varieties as well, and on varieties from different companies. This is a cotton issue, not simply one company.
-I believe it is possible to find bacterial blight in nearly every field in the state, but often at levels so low that I do not anticipate any yield loss. In other fields it is more severe and may very well result in yield loss. However, there is nothing that can be done now about the disease– other than possibly managing crop growth, development and irrigation to minimize extended leaf wetness periods.
-How did bacterial blight get here? Many are quick to point out that the bacterial pathogen can be seedborne, and indeed it can be. One of the most important reasons we “acid delint” cotton seed is that this practice greatly reduces the transmission of the bacteria through the seed. However, we cannot be absolutely sure that there is no seed transmission, even with acid-delinted seed. Dr. Kemerait cautions that we should be extremely careful in judging the origin of the recent outbreaks of bacterial blight before research data is available. While it is easy to say the disease “came with the seed” such is not obvious and, in his opinion, poorly explains the outbreak we are experiencing now. There is also the possibility that the pathogen is already in our “agricultural landscape” and that the cotton we plant now is simply susceptible to the disease. There is also the idea that we are witnessing a “race shift” in the bacterial pathogen and our current varieties are susceptible. For many years, we in the cotton belt have been battling X. citri pv. malvacearum Race 18. If for some reason we have a new race, it could be an important factor in what we are seeing. Again, the OVERALL impact of the disease in many fields may be negligible.
– Bacterial blight, though one of many many issues we face, is important and we will continue to address it. This is a disease that cannot be solved overnight no matter what we try to do.
-Understand that simply having bacterial blight in a field does not constitute yield loss. But bacterial blight is an important consideration. Again, below are the results from one of my research trials from last season. Two of the varieties that seem to be more prone to bacterial blight yielded over 1,200 lbs.
|DP 1558 Telone||1200.435||37.39%|
|DP 1454 Telone||1165.551||36.95%|
|PHY 487 Telone||996.2934||35.69%|
|PHY 427 Telone||951.0536||34.73%|
|PHY 333 Telone||949.5486||35.88%|
-More research needs to be conducted to make a judgement on the origin of the disease.
-We will be rating ALL county on farm variety trials (over 20 trials with 14 varieties), and hopefully the information we collect will help growers make informed decisions on the cotton variety selection next season.