Septoria Brown Spot Vs. Bacterial Blight In Soybean

 

Dr. Kiersten Wise at Purdue University wrote an excellent article to help growers distinguish Septoria brown spot and Bacterial blight in soybeans in the July 16th issue of Purdue’s Pest and Crop Newsletter. Our climate has been favorable for both diseases, and while I would not expect any economic loss from either, it is important to be able to identity these diseases. Consequently, we are reprinting this article in this issue of the Crop and Pest report. If the name Kiersten Wise sounds familiar it is because she graduated with a Ph.D. in plant pathology from NDSU seven years ago.

 

Excerpt from - Kiersten Wise. Septoria brown spot vs. Bacterial blight in soybean. Purdue Pest and Crop Newsletter: Issue 16. July 16, 2015.

 

How do we distinguish between these two diseases? Leaves infected 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 1). 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 2).  Lesions may or may not have the yellow halo of bacterial blight lesions, but leaves with lesions can turn yellow due to senescence (Figure 3).  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.