Bacterial blight is a widespread soybean disease that is most common during cool, wet weather. This disease usually occurs at low levels that don't result in yield loss. Bacterial blight can be mistaken for Septoria brown spot. The two diseases can be distinguished by the presence of a halo around bacterial blight lesions. Both diseases can occur together on the same plants, but bacterial blight is most common on young leaves whereas brown spot is usually seen on older, lower leaves in the plant.
Bacterial blight can occur on all above ground plant parts, but is most evident on leaves in the mid to upper canopy. Infections begin as small, angular, water-soaked spots that turn yellow and then brown as the tissue dies. The spots darken and are surrounded by yellowish-green halos. Spots often merge to form large, dead patches on the leaves. The dead tissue may fall out giving the leaves a ragged appearance. These infected leaves usually remain on the plant. Infection can also occur on stems, petioles, pods, and seeds in infected pods. Infected seedlings may be stunted or killed in severe cases.
Conditions and timing that favor disease
Plants can be infected at any time during the growing season. Cool, wet weather and rain storms favor disease development. Disease progress stops in dry, hot conditions. Bacterial blight is spread by wind and rain and by cultivation when foliage is wet.
Bacterial blight is caused by Pseudomonas syringae pv. glycinea, which can also infect snap bean and lima bean. The pathogen overwinters in crop residue and can be seed transmitted.
Plant resistant soybean varieties and rotate with non-host crops. Always use pathogen-free seed. Field cultivation should be avoided when the foliage is wet. Deep tillage may help in the rare fields where this disease is frequently severe.
Reviewed in 2018