Identification and Difference Between Soybean Bacterial Pustule and Bacterial Blight

The correct identification of any disease is critical to the management of that crop during the growing season. Additionally, being familiar with the environmental conditions, management practices, and symptomology associated with diseases are keys to help manage a disease. There are quite a few diseases that can look and act very similar, which makes diagnosing the disease difficult. Two of those diseases are bacterial pustule and bacterial blight in soybean.

This disease favors wet and warm weather, which can lead to infection occurring at any point in the growing season. When trying to identify this disease, the initial symptomology can look very similar to bacterial blight. The key difference between the two is, bacterial pustule does not lead to leaf tattering. Bacterial pustule also has smaller chlorotic halos around the lesions and on the underside of the leaf there are very small bumps. The small, tan, bumps are unique to bacterial pustule and is one of the disease identifiers. One method of managing this disease, is to plant disease free seed. The pathogen for bacterial pustule can survive on seed that will be planted the next year, so it is important to plant pathogen free seed. Crop rotation with non-host crops such as corn and small grain crops are beneficial for bacterial suppression. There are no in season rescue applications that can be made to fight this disease. Due to the pathogen being a bacteria, fungicides and other chemical applications do not have efficacy against bacterial pustule.

Bacterial blight is a disease of opportunity. It is very common for plants to be infected after storms produce mechanical damage to a plant because the bacteria can enter through those wounds. Unlike bacterial pustule, blight favors cool and wet weather conditions. This can also be an identifying factor of which disease is present. The symptomology associated in the early stages of infection can include brown spots on the cotyledons. If young plants are infected, this can lead to premature death. When plants are infected later in the growing season, there can be yellowish-brown lesions on the leaves. These lesions can turn a darker color and the leaves can dry out and appear tattered and torn. Finding cultivars that are not susceptible to bacterial blight, is one of the recommended management practices. Also, rotating with non-host crops and practicing cultivation to bury the residue is recommended. Typically, there is very little yield loss associated with bacterial blight. 

¹Malvick, D. 2018. Bacterial pustule on soybean. University of Minnesota Extension. https://extension.umn.edu/pest-management/bacterial-pustule-soybean/.

²Giesler, L.J. 2019. Bacterial blight. CROPWATCH. University of Nebraska-Lincoln. https://cropwatch.unl.edu/plantdisease/soybean/bacterial-blight/.

Websites verified 2/10/23

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