Bacterial pustule on soybean
Compiled by: Buyung Hadi, from the materials by: Loren Giesler and Dean Malvick
Bacterial pustule is caused by the bacterium Xanthomonas axonopodis pv. glycines.
Symptoms and Signs
Early symptoms consist of small, pale green spots with raised centers on leaves in the mid- to upper canopy. As the disease progresses, small brown-colored pustules form in the middle of the spots and the spots turn yellow. The spots may merge, forming large irregularly yellowing lesions. Bacterial pustule symptoms are easily confused with soybean rust pustules. Soybean rustpustules have circular openings at the top as gateways for spore spread. Bacterial pustules do not have this feature although under microscope bacterial pustules may appear to have crack openings on their surfaces. Bacterial pustule lesions are sometimes confused with the lesions caused by bacterial leaf blight. Bacterial leaf blight lesions appear water-soaked while the lesions of bacterial pustule do not. As in other bacterial diseases, if a soybean leaf with lesions due to bacterial pustule is cut and submerged in water, bacteria will stream out of the infected tissue.
Bacterial pustule rarely cause defoliation and yield loss.
Life Cycle and Epidemiology
Similar to bacterial leaf blight pathogen, X. axonopodis survives the winter on crop residue in the field. Plant infection occurs when infected crop residue on soil surface is dislodged by the wind or rain splash onto healthy plants. Leaf wetness is required for infection to occur. Thus, field cultivation while the leaves are wet (e.g. from previous rain) may also blow infected crop residue onto healthy soybean tissues and promote new infection. Unlike bacterial leaf blight pathogen, X. axonopodis development is not hampered by high temperatures. In fact, the optimum temperature for bacterial pustule pathogen ranges between 86-92˚F. In the field, this ability to thrive in high temperatures translates into continuous disease development in wet summers. Bacterial pustule pathogen may infect soybean seeds .
Scouting is recommended between R1 through R6 of soybean developmental stages. Resistant varieties and pathogen-free seeds should be used to manage bacterial pustule. Crop rotation and plowing the field before planting also help to reduce field inoculum level of the pathogenic bacterium. Try to avoid cultivation when the leaves are wet.