NPIPM:Bacterial blight on soybean
Compiled by: Buyung Hadi, from the materials by: Loren Giesler, Dean Malvick and Lawrence Osborne
Causal Organism
Bacterial blight is caused by the bacterium Pseudomonas syringae pv. glycinea.
Symptoms and Signs
Brown spots on cotyledon margins constitute the early infection symptoms of bacterial blight. Rarely, plants infected when very young (any growth stage info?) may be stunted and if the infection reaches the plants’ growing points, they may die. When plants of later stages are infected, small, angular, water-soaked lesions surrounded with yellow halo appear on the leaves. The yellow halo surrounding the lesions is a distinctive symptom to distinguish bacterial blight from brown spot, a soybean disease caused by the fungus Septoria glycines. The lesions are usually first observed during rainy weather on the top leaves. If the cool rainy weather prevails, the lesions enlarge and merge, forming larger necrotic area. Dead tissues eventually drop, and leaves appear ragged. Bacterial blight also infects stems, leaf stalks and soybean pods. Infected seeds may appear shriveled and discolored, but may not exhibit any external symptoms. Normal-looking, infected seeds contain the bacteria and will serve as an inoculum source in the next crop season.
Disease Impacts
Bacterial blight is a common secondary disease on soybean in Northern Plains. Typically, soybean plants compensate for lost leaf area and no significant yield losses are associated with this disease.
Life Cycle and Epidemiology
Pseudomonas syringae pv. glycinea survives the winter on crop debris in the field. In the spring, splashes of rain water splash debris with bacteria from the soil surface onto soybean leaves and stems. In the presence of free water on the leaf surface, bacterial infection occurs. Bacteria enter the leaves through the stomata. Inside the host, the pathogenic bacteria produce toxins that interrupt chlorophyll production, causing the distinct yellow halo around the infected tissues. The bacteria can also move among leaves or plants through leaf rubbing during cultivation or heavy wind. Cool and wet weather (70-80˚F) is favorable for bacterial blight development. The disease development stops in hot and dry conditions. Thus, bacterial blight is typically an early- to mid-season disease on soybean.
Management Approaches
Avoid using susceptible varieties. Seeds produced in heavily infected fields are likely carrying the pathogenic bacteria, do not save infected seed and replant it. Crop rotation and tillage reduce the amount of inoculum. Copper fungicides are labeled for bacterial blight control on soybean, but application needs to be conducted early in the disease cycle to be effective.
