Pathogens of soybean in the Guinea Savanna Zone of Nigeria were isolated from plants and seed sources and identified.  Pathogenicity tests using the blotter method were carried out to determine the virulence of the organisms isolated on seeds, seedlings and leaves of soybean. This would help in the effective protection of the crop in the zone and increase yield, seed quality, and storability.  Results showed that eight organisms namely Aspergillus flavus, A. niger, A. tamarii, and Botryodiplodia theobromae ,were predominantly affecting soybean seeds post harvest while  Phomopsis sojae, Fusarium spp, Cercospora kikuchi and Cercospora sojina were predominantly, affecting soybeans plants in the field.  These organisms caused damages like staining of seeds in storage.  However, contrary to previous reports, Aspergillus spp were found to improve seed viability. Seeds treated with A. flavus, A. niger, and A. tamarii germinated more than the control by 35%, 17.5% and 22.5% respectively.  A. flavus was found to have protected the seeds up to the 10th month in storage.  A. flavus therefore has the potential for biological control of other pathogenic organisms of soybean in the zone. Among the organisms isolated, Phomopsis sojae was found to be the most virulent organism affecting both plants and seeds of soybean in the zone.  Farmers in the zone should take measures that will control these organisms especially P. sojae in the field and storage so as to avert losses of soybean in the zone.

Keywords: Soybean, disease, pathogenicity, organism