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Figure 1. Cotton boll with bacterial blight lesion.
In cotton, bacterial blight symptoms are first observed as small, water-soaked lesions that are noticeable on the upper and lower leaf surfaces. The lesions appear to be angular due to the restrictions by veins within the cotton leaf.1 Other parts of the cotton plant may become infected following the production of secondary inoculum. The systemic spread of infection to leaf petioles, stems, and branches may result in black cankers (“Blackarm” lesions) that can cause portions of the plant to die above the canker. Boll lesions first appear to be “water-soaked” and round, and later sunken and dark brown or black.1 Low levels of bacterial blight are commonly observed and reported in various cotton-producing regions; however, symptoms of bacterial blight will be most severe and spread when humidity is high and air temperatures average 86 to 95 °F.2
Prior to the widespread commercial use of acid delinting of planting seed, seed-borne inoculum of Xanthomonas citri pv. malvacearum was reported to occur. The bacterium is capable of overwintering on plant debris. Wind, water, and thunderstorm fronts can disseminate the bacterium beyond the initial point of infection.3 The pathogen enters host plants through open stomata or wounds. Blowing dust and sand events have been linked to bacterial blight epidemics.
Figure 2. Cotton leaf infected with Xanthomonas citri pv. malvacearum, commonly known as bacterial blight of cotton. Wright, R., Texas Tech University.
A field survey was conducted to help determine if certain weed species could serve as plant hosts for overwintering inoculum in fields with a history of bacterial blight. The four-site study sampled seven different weed species. Results concluded that three of the four sites had weed samples that tested positive for bacterial blight. This indicates that asymptomatic winter weeds remaining in a field could potentially serve as a source for bacterial blight inoculum the following season.4
While no fungicides are recommended for bacterial blight, growers may choose to apply growth regulators to prevent further rank growth of their crop. Dense, rank growth can disrupt air flow within the canopy creating a conducive environment for disease development. There are currently no products labeled for use against the disease. Because blight can disseminate rapidly in warm, wet conditions, any management practice to promote drying of cotton leaves may help reduce the development of blight.
Limit the movement of equipment through wet fields to reduce the spread of blight in season.
Plan to harvest fields with bacterial blight first and shred cotton stalks as early as possible. The pathogen can survive on plant debris under relatively dry conditions for months, but turning the crop residue into the soil can promote decomposition.3
Control post-harvest weeds to help reduce potential plant host populations.
As a preventative measure, select cotton varieties with bacterial blight resistance the following year or plant to a different crop.
Sources: 1 Thiessen, L. and Koenning, S. 2017. Bacterial blight of cotton. North Carolina State University Extension. http://www.content.ces.ncsu.edu/. 2 Cotton pests: scouting and management. 2004. University of Missouri. IPM1025. http://extension.missouri.edu/. 3 Thaxton, P.M. and El-Zik, K.M. 2001. Bacterial blight. Compendium of Cotton Diseases. 2nd edition. 34-35. 4 Koczan, J., Albers, D., and Gholston, K. 2017. Identification of an alternate source of inoculum causing bacterial blight. 2017 Beltwide Cotton Conference. Dallas, TX. Web sources verified 05/10/18.150727135907