Originally written by Emmanuel Byamukama, former SDSU Extension Plant Pathologist.
Several soybean fields scouted the week of July 15, 2019 were found with bacterial blight developing. The frequent rains experienced in most soybean growing counties have led to the development of this disease. Bacterial blight affected leaves are most evident on younger leaves in the upper canopy (Figure 1).
Disease Symptoms
The common symptom of bacterial blight is the leaf tattering (tearing) (Figure 1). This happens when expanding bacterial blight lesions coalesce and form large brown patches. High wind and raindrops lead to these lesions tearing apart. Individual bacterial blight lesions start as small angular spots surrounded by a yellow halo with a water-soaking characteristic.
Bacterial blight is caused by a bacterium, Pseudomonas syringae pv. glycinea. This bacterium survives in infested crop debris and in seed, however, the infested soybean residue is the main source of inoculum. The bacteria is splashed onto leaves by windy rainfall. The bacteria enter the plant through natural openings and wounds. The bacteria multiplies under temperatures <81° F. Hot and dry weather halt disease progress.
Management Strategy
Since bacterial blight is caused by a residue-borne pathogen, residue management through crop rotation or tillage (where practical) can help reduce the inoculum. Resistance to this disease is available, therefore for future soybean seasons, varieties with resistance or tolerance to bacterial blight should be selected for planting.
