Bacterial Blight (Angular Leaf Spot) Observed in Louisiana Cotton Fields

Originally published July 18, 2014

Bacterial blight was once (prior to 1991) a major disease of cotton causing average annual losses of as much as 3.4%. In severe cases, losses ranged from 50 to 70%. From 1991 to 2000, average losses due to bacterial blight averaged 0.1%. Over the past few years, a resurgence of the disease has been noted in the Mid-South. Since 2009, the disease has been observed on many different varieties in many counties in Texas, Mississippi, Arkansas, and Missouri. This year the disease has been observed in three parishes to date. The causal agent of the disease is a bacterium, Xanthamonas campestris pv. malvacearum. This pathogen may affect all plant parts of cotton causing seedling disease and/or infection of leaves, vascular tissues, stems, petioles, bracts, and bolls. Foliar symptoms begin with small, water-soaked lesions on the underside of leaves that are “angular” because of leaf veins that restrict the movement of the bacteria. The lesions become visible on the upper surface of leaves and become necrotic (Figure 1), and the pathogen also may affect vascular systems in leaves resulting in purplish lesions that follow veins (Figure 2). Severe infections may result in defoliation. Stems and petioles also may be infected causing lodging, loss of branches, and/or leaf drop (Figures 3 and 4). Bolls also may be infected resulting in stained lint and possible transfer of the bacterium to seed.

The pathogen may be seedborne, and in the past was successfully managed by acid-delinting and chemical treatments of seed. Infection occurs through natural openings or wounds to cotton plants. Survival of the pathogen on plant debris in the field may serve as initial inoculum the following cropping year. The bacterium is spread by wind, rain, insects, and equipment. Overhead irrigation or wind-driven rain will spread the bacterium during an active epidemic. Optimal conditions for disease development are high relative humidity (>85%) and temperatures ranging from 86-97°F.

Planting acid-delinted or chemically-treated seed will reduce the chances of infection. Sanitary measures to avoid spreading the bacterium should be used in fields where infection has occurred. Rotation to a non-host or planting resistant varieties also are management options. Turning under plant debris will help by reducing the number of bacteria that serve as primary inoculum at the beginning of the next growing season. Avoiding rank canopy growth will reduce leaf wetness periods and may help to reduce disease severity. Managing insect pests will likely lessen the spread of the pathogen. Do not stop irrigating, but do not over-irrigate. Water stress could be more detrimental than the disease, and cotton plants may compensate for foliage loss. Most importantly, there are varieties available that are resistant to bacterial blight. Below are some external sources with more information. Please contact your local parish agent, specialist, or nearest research station if you suspect bacterial blight or require additional information.

Angular lesions caused by bacterial blight

Figure 1. Angular lesions caused by bacterial blight.

Bacterial blight lesions following leaf veins.

Figure 2. Bacterial blight lesions following leaf veins.

 Angular leaf lesions infection of petiole, and leaf drop.

Figure 3. Angular leaf lesions infection of petiole, and leaf drop.

Infection of stem and petioles causing leaf drop.

Figure 4. Infection of stem and petioles causing leaf drop.

9/4/2019 7:53:52 PM
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