Assessment of two endemic rock phosphate solubilizing Streptomyces spp. on sugar beet (Beta vulgaris L.) growth under field conditions

•Streptomyces bellus and Streptomyces saprophyticus increased the concentration of soluble phosphate and potassium in the soil.•Inoculation of sugar beet seeds by Streptomyces increase shoots and roots length.•At field, the effect of S. saprophyticus on root yield was similar to conventional fertilization. Biocontrol agents and plant growth-promoting rhizobacteria represent promising tools to improve plant growth and health as environmentally friendly biopesticides and fertilizers. Streptomyces bacteria are commonly used for biocontrol, but have been poorly investigated for biofertilizer activity. In this study, we evaluated at field scale the performance of rock phosphate (RP) solubilizing Streptomyces spp. (S. bellus (SB) and S. saprophyticus (SS) in promoting the growth of sugar beet under RP fertilization. Inoculation of seeds with SB stimulated root elongation and inoculation with SS enhanced shoot elongation. At the end of the trial, chlorophyll levels in the leaves were higher with SS. The highest root yield was recorded with SS, and root diameter increased with both bacteria. The levels of soil available phosphorus (P) and potassium (K) increased particularly with SB, but also with SS inoculation. In sum, potassium and phosphate solubilizing and sugar beet growth promoting activities of SB and SS were lower in the field than previously observed in the greenhouse. That the activities were present in the field speaks for improving the formulation and optimizing the application strategy.