Bacterial blight was found in several soybean fields scouted in Brookings, Deuel and Moody counties. Recent heavy storms and/or hail injury may have increase the prevalence of bacterial blight. As the weather heats back up, typically over the optimum 75 degrees, the growth of this bacterium will be stalled.
Bacterial Blight vs. Brown Spot
Symptoms of bacterial blight are often confused with soybean brown spot, which is a fungal disease.
Bacterial blight is caused by Pseudomonas syringae pv. glycinea. Bacterial blight can occur on all above ground plant parts, but is most evident on leaves in the mid to upper canopy. Bacterial blight lesions start as small angular spots surrounded by a yellow halo. The lesions expand and coalesce to form large brown patches. The brown patch centers may fall out leading to leaf tearing/tattering (Figure 1).
Figure 1. Bacterial blight symptoms on top soybean leaves. Notice the halo around the lesions and the tearing of the leaf.
In contrast, soybean brown spot lesions are also dark brown but the yellow halo around the lesion is not obvious except in advanced heavy spots (Figure 2). Brown spot, also known as Septoria leaf spot is a fungal disease caused by Septoria glycines. Usually brown spot remains in lower leaves and does not cause significant yield loss. However, under heavy infection, brown spot can result in premature leaf fall leading to significant yield loss.
Figure 2. Brown spot symptoms on soybean leaves.
Management & Diagnosis
Both diseases are residue-borne and can be seed-borne; however, the main source of inoculum is soybean residue. Residue management through crop rotation or tillage (where practical) are effective cultural practices for both of these diseases. If brown spot is found in mid-canopy at the beginning of pod set (R3), a foliar fungicide may be advised.
Soybean disease samples from South Dakota will be diagnosed free of charge this year, thanks to a grant from the South Dakota Soybean Research and Promotion Council. For help diagnosing problems with your soybeans, mail or bring soybean samples to the SDSU Plant Diagnostic Clinic.
|South Dakota State University
153 Plant Diagnostic Clinic (SPSB), Box 2108
Brookings, SD 57007