Bacterial blight, a disease that has not been of economic significance in Arkansas for years, has been detected in numerous cotton fields in northeastern Arkansas, as well as in some fields in the southeastern part of the state and some areas of Mississippi over the last two weeks. The disease has been confirmed in Desha and Mississippi counties and pathologists are working to determine the extent of its spread. Bacterial blight was historically a major cotton disease across the U.S. Cotton Belt, but the discovery and use of resistance to the pathogen (Xanthomonas axonopodis pv. Malvacearum) in most cotton cultivars and modern seed processing and handling procedures, particularly the use of acid-delinted seed, lowered incidence of this disease substantially for the last two decades.
The pathogen can infect all parts of the plant. Symptoms of bacterial blight vary according to the plant part that is infected and the stage of the crop when infection occurs. Early symptoms include small water-soaked spots on leaves that rapidly enlarge to take on an angular shape (Fig. 1). Lesions may coalesce rapidly and follow the main leaf veins (Fig. 2). At this point in the season, lesions on leaves and lesions on squares and bracts (Fig. 3) are likely present in fields. In very susceptible varieties, or where the disease is severe, lesions may form on petioles and stems, and leaf defoliation may occur (Fig. 4). As bolls develop, water-soaked lesions may form. These lesions are probably of the most concern from a yield loss standpoint because they will result in stained lint and provide entry for secondary boll rot pathogens.
Bacterial blight severity is greatest at high ambient temperatures (86-97 F) and high relative humidity – conditions we have had throughout the state for several weeks. The pathogen is spread most effectively by splashing water, particularly driving rain – also conditions we have had occasionally this summer in certain parts of the state. Bacteria can enter the plant through natural openings like stomates, nectaries, or through wounds. The extent of the bacterial blight problem in a field will depend on the susceptibility of the cotton cultivar, the extent to which the pathogen has spread through the field, and the duration of favorable weather for disease development.
What Should Growers Do Now?
The most important thing to do right now is to survey all fields to determine if the disease is present. Scout fields for the presence of leaf symptoms, particularly in the lower to mid canopy. Look for excessive leaf defoliation. Dryland fields and fields that have been stressed from other reasons may show considerable leaf drop due to bacterial blight. Because other factors may cause spots on leaves or squares or defoliation, when in doubt, submit a sample to the University of Arkansas Division of Agriculture Plant Health Clinic for an accurate diagnosis. For help in collecting and submitting samples, contact your local county agent. Also remember when scouting fields that this pathogen can be spread by contact so avoid walking (or driving through) fields when foliage is wet. It is vital that growers identify this disease in their fields this season while symptoms are visible. The bacterium can survive in undecomposed plant debris for at least several months, so knowing where the disease is this fall will allow appropriate cultivation to destroy residue before next season.
What Can I Do If I Find Bacterial Blight in a Field?
First, and foremost, do not panic. While this disease can be rather severe under the right conditions, it will not result in a crop failure. The extent of the crop loss in any given field will be determined by the incidence (distribution) of the disease in the field, the susceptibility of the cultivar, and the weather conditions from now until harvest. Here are some suggestions for dealing with fields where bacterial blight is present.
1) Continue to manage the crop for yield. While the disease may hurt, it will not likely hurt nearly as bad as abandoning the field or cutting back on inputs. Abandoning (or severely curtailing inputs) now to save a little money will also eliminate any chance of estimating the actual damage the disease caused.
2) Recognize that the disease can be spread by equipment or people moving through the field when the leaves are wet. Avoid running ground rigs through or scouting fields when the foliage is still wet from rain, dew, or irrigation.
3) Be realistic regarding irrigation. While overhead irrigation may contribute to the spread of the pathogen, lack of irrigation will be of much greater concern. Continue to irrigate as needed to meet crop demands, but do not over-irrigate.
4) There are no chemical control methods for control of bacterial blight in the field. Maintain good insect control to minimize the possibility of infection through wounds created by insect feeding.
5) Do not over fertilize. Lush, rank foliage will contribute to higher humidity in the canopy, a longer period of leaf wetness, and will enhance infection. In fields or cultivars where the crop tends toward rank growth, consider using plant growth regulators to maintain an open canopy.
Remember: bacterial blight has not been a significant disease in Arkansas for over 20 years. Some fields that were infected early and that were in regions with frequent driving rains and summer storms may be damaged considerably. However, in a majority of cases, this disease may hurt but it won’t likely cause crop failure.
Information by Cliff Coker, Tom Barber, Craig Rothrock, and Terry Kirkpatrick. Photos courtesy of Cliff Coker, Extension Plant Pathologist and Amanda Greer, Program Technician, Southeast Research and Extension Center.