Bacterial blight of cotton (also called angular leaf spot, boll rot, and black leg) is a potentially destructive bacterial disease for North Carolina cotton production, but has only been detected sporadically in North Carolina. Cotton yield losses caused by bacterial blight have been previously reported in excess of 10%; however, significant losses to this disease in the United States have not been recently reported.

Bacterial blight is caused by a bacterium, Xanthomonas axonopodis pv. malvacearum (Formerly Xanthomonas campestris pv. malvacearum).

Bacterial blight infections can affect any above-ground portions of the plant and at any point of plant development. Lesions associated with Xanthamonas begin as an angular leaf spot with a red to brown border (Figure 1). Tissues surrounding lesions may also become yellow, giving the appearance of a "halo" surrounding the lesion. The angular appearance of lesions is due to restriction of the lesion by fine veins of the cotton leaf, and bacteria may spread along the leaf veins causing blackened lesions along major veins.

Leaf petioles and stems may also become infected (cotton black leg) (Figure 2), which can lead to premature defoliation of the plant. Black cankers on stems or branches may girdle the plant causing the portions to die above the canker. Additionally, a white waxy crust containing the bacterium may also form on old leaf spots or cankers.

Infected bolls have round, as opposed to angular, lesions that initially may appear water-soaked, and then become sunken and dark brown to black. Bolls infected with Xanthamonas may result in rotted seed and discolored lint.

Figure 1. Bacterial blight (angular leaf spot) caused by Xanthomonas campestris pv. malvacearum.

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Figure 1. Bacterial blight (angular leaf spot) caused by Xanthomonas campestris pv. malvacearum.

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Figure 2. Bacterial blight lesions on leaf and the blackleg symptom on the leaf petiole.

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Figure 2. Bacterial blight lesions on leaf and the blackleg symptom on the leaf petiole.

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Significant rainfall events and high humidity combined with warm temperatures favor disease development. The bacteria enter leaf tissues through the plants natural openings (stomates) and wounds, and disease is most severe following storms that produce heavy rains and/or hail.

X. axonopodis survives in infested crop debris or seeds. Although infections may be seedborne, acid-delinting cotton seed has been instrumental in minimizing the spread of bacterial blight through contaminated seed. Volunteer seedlings may also be a source of primary inoculum for bacterial blight.

There are no remedial management tools available to arrest disease development. Regulating plant growth during the growing season to prevent rank growth may help reduce disease incidence. Additionally, reducing the humidity within the canopy and increasing the rate of drying of the foliage may also limit the progression of this disease. Avoid cultivation or other activities (moving equipment through fields) that may cause damage to plant tissues when foliage is wet to reduce potential infection sites on cotton plants.

Should a field become infested, harvest that field as soon as possible. Additionally, the cotton variety, seed lot, and a disease severity rating should be determined. Destroy remaining stalks at the first opportunity to reduce the survival of bacteria in crop debris. In the year following a bacterial blight infestation, fields should be planted to a blight-resistant variety or rotated to a different crop.

Preventative actions for bacterial blight of cotton:

• Plant high-quality, disease free, acid delinted seed.
• Plant blight-resistant varieties where available.
• Scout fields and identify infected plants and varieties.
• Shred stalks and incorporate cotton debris.
• Do not cultivate or move equipment through fields when foliage is wet.

• The NCSU Plant Disease and Insect Clinic provides diagnostics and control recommendations.
• The NC State Extension Plant Pathology portal has disease information and news updates on disease in North Carolina.
• The Cotton portal has agronomic and pest information related to cotton production.

This factsheet was prepared by the NCSU Field Crops and Tobacco Pathology Lab in 2017.

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