Bacterial blight is the most common bacterial disease in soybeans, rarely causing significant yield losses. It is very similar in appearance to bacterial pustule, but can be differentiated through its water-soaked lesions which are contrary to the lesions of bacterial pustule (not water-soaked).
Development of bacterial blight is favored by temperatures between 70 and 80 degrees F. In hot, dry weather the bacterial blight will not progress. It survives in infected crop residue and is spread through water splashing from infected tissue to the live plants. In can also be established through the planting of infected seeds.
Symptoms begin as small, water-soaked, and light green lesions on the surface of the leaves. Surrounding them are yellowish-green halos. As the lesions mature, they darken in color and cause large areas of the leaf surrounding them to die. As the dead areas begin to die, they are ripped away by the wind and rain, leaving the foliage with a tattered appearance. Infected pods begin with small, water-soaked lesions that enlarge to encompass most of the pod. Lesions will turn dark brown or black in color. Symptoms of bacterial leaf blight resemble those of bacterial pustule, often occuring together on the same plant. Other similar diseases are soybean rust and Septoria leaf spot. Unlike soybean rust, bacterial leaf blight will at times have green leaves below it in the canopy. Septoria leaf spot also tends to occur lower in the canopy than bacterial leaf blight.
As the bacterial leaf blight advances, more parts of the leaves will die, and eventually the plant may be defoliated. Plants that are infected at a young age may be stunted and usually killed by the bacteria. Overall damage is usually insignificant, but in sesceptible varieties, yield losses have been as high as 40 percent. Excessively rainy growing seasons warrant close monitoring of crops.
Tilling crop residue under is important for promoting decomposition of diseased tissue. It's also important to use disease-free seed. Sometimes bacteria can be spread through the use of wet machinery from an infected field, so avoid using machinery during wet conditions.
Please contact your local plant pathologist to learn what is most effective in your area and how to use it best. Always read and follow all label instructions.
Planting disease-free and resistant varieties to avoid infection is a common method used by producers. Rotating in nonhost crops can also be a very effective method of controlling the growth of bacterial leaf blight.