Bacterial blight is a cotton disease that, over decades, has accounted for an average loss of only about 1 percent across the U.S. cotton belt, and many younger growers and consultants may never have seen it in the field, says Don Blasingame, Mississippi State University plant pathologist emeritus.
“It isn’t what I’d term a limiting factor in cotton for most producers — unless they happen to have one of the ‘hot spots’ where it occurs, and in those cases losses can be as high as 25 percent to 40 percent, or more,” he said at the annual meeting of the Mississippi Agricultural Consultants Association.
“To those growers, it’s not a minor problem — it’s a severe problem.”
The organism for the disease, which has been known since late 1800s and occurs in all cotton producing areas of the world, is most often the result of infected or contaminated seed, Blasingame says.
“When I was in graduate school at Texas A&M, we saw bacterial blight quite often. Back then, producers would ‘brown bag’ seed from the previous year’s crop. If the field had bacterial blight that year and saved seed was planted the following year, it would occur again.
“The first outbreak I saw in Mississippi was in 1972, and it was pretty severe. Fields were affected from Rolling Fork up to Clarksdale, with thousands of acres involved. At that time, growers were primarily using mechanical/flame delinted seed, which did a good job of removing lint, but it didn’t control some of the disease organisms that were on or in the seed.
“Then the move was made to acid delinting, which did a good job of removing some of those pathogens from the seed, and for a number of years we saw very little bacterial blight.”
In recent years, however, he says the system of acid delinting has changed from the old type of turning seed over in an acid bath to a mist-type application, “which sometimes can’t adequately penetrate the cracks and grooves of the seed to destroy the bacteria.” The bacteria can be carried internally as well as externally, he says.
Three conditions are necessary for a severe disease outbreak, Blasingame says: the virulent pathogen, a susceptible host, and the proper environmental conditions.
“For most Mid-South producers, the environmental conditions favorable to bacterial blight — rain and blowing wind — occur at some point in the season almost every year. In 2011, we had plenty of inoculum, and a few cotton varieties were fairly susceptible to the pathogen.
“Under certain conditions, the disease can carry over in the field from one year to the next, if there is a mild winter and field debris isn’t destroyed or turned under — but I consider that a fairly minor means of getting the organism into the field. It is most commonly seedborne.”
When an infected seed is planted, Blasingame says, the bacteria “grow right along with the cotyledon and the plant until about mid-season, when the canopy overlaps and produces a greenhouse effect in the bottom of the canopy. The bacteria then grow and multiply, and when a storm comes through with rain and blowing wind they can be dispersed throughout the field, causing seedling blight, damping off, or boll rot.”
The organism for bacterial blight is fairly easy to isolate and identify in the lab, he says, and in the field it “isn’t difficult” to spot the resulting defoliation and loss of bolls that occur in the lower part of the canopy.
“Last year, we had outbreaks in the Clarksdale, Miss., area and adjoining counties, and also in the Wilson, Ark., area,” Blasingame says. “Of the seed samples sent in last year, about a third had the organism. It doesn’t take very many infected seed in a bag to cause a potential problem.
“For example, you have a bag of seed with only 2 percent or 3 percent of infected seed, and you plant it on 40 or 50 acres. When the canopy closes and the bacteria begin to multiply, if a storm comes through, all of a sudden instead of a few plants infected, you can have a spot as big as a tennis court. If there are subsequent storms, you may have a spot as big as a football field — which started from just a very small number of infected seed.
“Once you see bacterial blight in the field, you really can’t confuse it with anything else, because it doesn’t look like any other cotton disease. The angular leaf spot will begin as a small, water-soaked spot. It won’t be round like cercospera or other fungal-type diseases.
“Many times it’s restricted by some of the leaf veins; sometimes it will carry down the leaf and become almost systemic. It can go to almost any part of the plant.
“It normally enters through natural openings, the stomata, and once inside the plant begins to multiply rapidly, producing a chemical that breaks down cell walls as it moves through plant tissue. It can get on any plant part that is soft enough and has a natural opening. Many times, we’ll see bolls that may not have a lot of bacterial blight on the boll itself, but will have infection on the petiole, which will then drop.
“It doesn’t take a lot of bacteria to cause defoliation, and you can lose the bottom canopy very quickly under the right environmental conditions.”
Given the outbreaks in 2011, Blasingame says, “I think there is plenty of inoculum present for recurrences this year.”
He suggests looking at screening results for seed varieties and “stay away from those that are highly susceptible.” Also, where possible, debris in the field should be destroyed and turned under in order to limit carryover of the disease.
If there are areas where the disease is suspected after the canopy covers, he says, “Get out in the field and look into the lower part of the canopy for signs of the disease.
“Unfortunately, once infection occurs there’s not really much you can do except take good notes on the variety involved and seed lot so you can take precautions the following year.”
Cotton can tolerate a certain amount of defoliation, Blasingame says, “but once the disease gets on bolls, that’s when the losses start. Even if the boll isn’t lost, it may be hardlocked.”
Plant breeders have done a good job of identifying genes for resistance to the bacterial blight pathogen, he notes.
“There are at least 22 strong genes of resistance, but for most plant breeders, the first thing they look at is yield. And if they’re dealing with an organism that has a history of only 1 percent to 2 percent loss over 50 years or more, they’re likely not going to spend a lot of time on incorporating genes for resistance.
“So, when choosing your cotton seed, look at tests where varieties have been screened for this particular disease, and factor those results into your seed choices.”