Bacterial Blight

Bacterial blight is caused by a bacterial pathogen that is present in most soybean fields.Young leaves are most susceptible to infection. Click on image to view a larger version. Photo credit: Dean Malvick, University of Minnesota

Leaf spots caused by the bacterial blight pathogen, Pseudomonas syringae pv. glycinea can be found in most soybean fields every year in the Midwest. They are most common during rainy, humid periods in July and August. The bacteria can also infect snap bean and lima bean.

Bacterial blight can be confused with brown spot (Septoria leaf spot) and with bacterial pustule. Bacterial blight and brown spot are especially common. Both diseases often occur in the same fields and even the same plant, and symptoms can be difflcult to separate.

Bacterial blight generally occurs at low levels that do not limit yield.

 

Disease cycle

The bacteria that cause bacterial blight overwinter in crop residue and on seed. Initial infections can occur during seedling emergence, especially if infected seed is planted. The bacteria are spread by wind and rain, and outbreaks that occur later in the season often follow wiindy rainstorms.

Bacterial blight is favored in continuous soybean fields, no-till soybean fields, or fields planted with seeds from infected soybeans.

 

New lesions of bacterial blight typically develop after a rainstorm. Photo credit: Carl Bradley, University of Illinois Click on image to view a larger version.

Agronomic impact

Bacterial blight occurs every year in the Midwest without causing significant yield losses except in unusually rainy growing seasons. However, bacterial diseases should be monitored carefully in seed production fields because they are seedborne and can affect seed quality.

In severe cases, bacterial foliar disease may cause some early defoliation, but it will not kill whole plants. Severely affected plants may have smaller seeds. Soybean varieties differ in their susceptibility.

 

Symptoms and scouting

Outbreaks typically develop several days after a rainstorm or hailstorm. Symptoms are most evident on new growth that is expanding at the time of the rain event. New lesions of bacterial blight are small, angular, water-soaked, yellow to brown spots on leaves.

The angular lesions enlarge in cool (70-80° F), rainy weather and merge to produce large, irregular patches of dead tissue. The centers of these patches often drop out, giving infected leaves a ragged appearance.

The bacteria can also infect stems, petioles, and pods.

Each lesion caused by bacterial blight (above) will have a yellow halo, especially in the early stages. Compare to brown spot lesions, caused by the fungus Septoria, below. Photo credit: X.B. Yang, Iowa State University Click on image to view a larger version.

Brown spot lesions caused by Septoria develop a more generalized yellowing where brown spots are present. Photo credit: Carl Bradley, University of Illinois Click on image to view a larger version.

Look-alikes
Bacterial blight is often confused with brown spot, a leaf spot disease caused by the fungus Septoria glycines. Bacterial blight occurs on upper new leaves and brown spot infection begins on older leaves, or leaves on the lower part of the plant. A characteristic yellow halo forms around lesions caused by bacterial blight, whereas leaves infected by brown spot show a more generalized leaf yellowing when numerous brown spots are present.

 

Management

Control measures are generallly not needed. However, if bacterial blight was severe this season, the following agronomic practices will reduce the disease next season:

• rotate soybean with a non-legume
• cover soybean residues after harvest by tillage, if possible, or shred residue for quick decomposition.
• plant pathogen-free seed. Do not use seed from plants infected with bacterial blight.

Foliar fungicides will not affect bacterial blight because it is a caused by a bacterial pathogen, not a fungus.

 

Resources

Bacterial Blight - University of Nebraska

Bacterial blight and brown spot - Iowa (pdf)

Bacterial Blight - Minnesota Crop Diseases

Get to Know the Common Foliar Diseases of Soybean - University of Illinois