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Bacterial Blight Appearing in Soybeans (8/03/17)

I have been receiving some questions about foliar lesions in soybeans.

Bacterial Blight Appearing in Soybeans

I have been receiving some questions about foliar lesions in soybeans. What I have seen has primarily been bacterial blight. Symptoms often begin as small angular lesions often with a yellow halo (Figure 1), will eventually enlarge, turn black and tatter the leaves (Figure 2). The disease not usually of economic concern and fungicides do not manage the disease.

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To help distinguish bacterial blight from Septoria brown spot I am reprinting a newsletter article from an NDSU graduate who became a faculty member at Purdue and is now the University of Kentucky (Dr. Kiersten Wise). Kiersten provides excellent diagnostic photos of both diseases (Figures 3-5) and a very nice 3 minute video comparing the diseases

https://www.youtube.com/watch?v=elVSfq_FFyI&index=6&list=PLnQq3T4QUKpZQyI3lcokex_DoIzqUR35N

Excerpt from - Kiersten Wise. Septoria brown spot vs. Bacterial blight in soybean. Purdue Pest and Crop Newsletter: Issue 16. July 16, 2015. Available at http://extension.entm.purdue.edu/pestcrop/2015/Issue16/

How do we distinguish between these two diseases? Leaves inmarkell.3fected by Pseudomonas sp. bacteria have brown angular lesions that are surrounded by a yellow ring or halo, and may have a water-soaked appearance. As lesions age, they turn dark brown and fall out of the leaf tissue, giving leaves a tattered appearance (Figure 3). Bacteria survive on soybean residue and in seed, and enter plants through stomates and wounds caused by equipment or other mechanical

damage, or from weather events such as heavy rains, wind, and hail. Long periods of leaf wetness and cool weather favor infection. Hot, dry weather will limit disease development. Yield loss may occur if disease is severe and plants defoliate. However, most fields in Indiana exhibiting symptoms of bacterial blight are only lightly to moderately affected by the disease and we would not expect to see yield loss due to this disease in these fields.

Symptoms of brown spot are typically observed in the lower canopy first, and are characterized by brown to black spots on upper and lower leaf surfaces (Figure 4). Lesions may or may not have the yellow halo of bacterial blight lesions, but leaves with lesions can turn yellow due to senescence (Figure 5). Research indicates yield reduction from this disease will be minimal if it stays confined to the lower 2/3 of the canopy.

Preventative management options for both diseases include crop rotation, tillage, and planting less susceptible varieties. These methods can lower the risk of disease developing in the subsequent soybean crop. Fungicides will not manage bacterial blight. Fungicide applications for brown spot are rarely warranted and may not be consistently profitable.

A past video comparing symptoms of bacterial blight and brown spot is available here: https://www.youtube.com/watch?v=elVSfq_FFyI&index=6&list=PLnQq3T4QUKpZQyI3lcokex_DoIzqUR35N

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Sam Markell

Extension Plant Pathologist, Broad-leaf Crops

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