Survival of rhizobia on seeds, nodulation and growth of soybean as influenced by synthetic and natural seed-applied fungicides

Soybean seeds (cv. Aldana) were treated with a synthetic seed dressing composed of carboxin (20%) and thiuram (20%) as the active ingredients or with a natural fungicidal preparation containing 23.8% of natural oil extracted from tea tree, and next day untreated (control) and treated with the above mentioned preparations soybean seeds were pelleted with inoculant containing Bradyrhizobium japonicum – the symbiotic bacterium of this plant. Shortly (within 1–2 hours) after seed inoculation and after 24 hours of storing the inoculated seeds at room temperature seed samples were collected to count rhizobial numbers on seeds. At the same sampling times seeds were also sown into pots filled with perlite moistened with water solution of plant nutrients (without N). After 4 weeks of plant growth in a growth chamber numbers of root nodules were counted and fresh and dry mass of shoots were determined. The studied chemical seed dress- ings had no significant effects on numbers of soybean rhizobia on seeds, and on nodulation and growth of soybean plants when seeds were analyzed or planted shortly after their inoculation with B. japonicum. However, the studied preparations significantly reduced numbers of soybean rhizobia on seeds stored for 24 h at room temperature, but only the synthetic one affected adversely nodulation and some growth parameters of soybean plants that developed from the stored seeds