by Bob Kemerait, UGA Extension
Bacterial blight has quietly found its way back into cotton fields for the second year in a row. There is little we can do about it now. However, by knowing more about this disease, cotton growers can make decisions to minimize impact next year and beyond.
Now in my 16th year as an Extension specialist at the University of Georgia, I have typically seen only light outbreaks of bacterial blight. This changed in 2015, when seemingly out of nowhere, bacterial blight caused significant premature defoliation and boll rot in a number of fields. The disease was found in more than one variety, but DPL 1454 B2RR, a promising variety with resistance to the southern root-knot nematode, was most severely affected. I was hopeful that the disease would go away if our growers planted varieties other than 1454 this season. Unfortunately, it is back.
Bacterial blight is caused by Xanthomonas citri pv malvacearum. The pathogen can survive in infected crop debris and can be seed-borne. However, acid-delinting cotton seed has been an important measure to minimize spread of the disease through contaminated seed. The bacterium enters the leaf tissue through natural openings, such as stomates and through wounds and injuries. The disease can be most severe when storms produce blowing rain and in irrigated fields. Our summer storms are often preceded by wind-blown sand that creates wounds more easily infected by the bacteria moved in the rain and rain-splash from infected leaves.
Symptoms of disease
Because bacterial blight has been uncommon in the southeastern US, growers and consultants may be unfamiliar with the symptoms. Leaf spots are first observed as small, “water soaked” lesions best viewed from the underside of the leaf. With time, these water-soaked lesions become small necrotic spots characteristically delimited, or bound, by the veins in the leaf. This gives the spots an “angular” appearance; presence of water-soaked, angular spots makes diagnosis of bacterial blight relatively easy.
As the disease continues to develop, smaller spots coalesce into larger lesions, sometimes encircled by a yellow halo. The bacteria may enter the leaf veins and spread causing dark, “lightning bolt” streaks to appear. The bacteria can infect the bolls causing symptomatic water-soaked, crater-like depressions.
Bacterial blight can cause significant premature defoliation and in the worst cases significant boll rot. Where damage is limited to premature defoliation, yields may be reduced by 10 percent or so; when boll rot occurs, losses may exceed 20 percent.