Rhamnolipid-based fungicides and effects on soybean germination and initial growth

Abstract: Rhamnolipid is a microbe-produced biosurfactant that has various potential industrial uses, including those relying on its antimicrobial activities. We studied its fungicidal use against the zoosporic Phytophthora sojae, a major soybean pest, to reduce excessive use of chemical fungicides. In solutions, rhamnolipids were found to effectively lyse zoospores, the life form responsible for the spread of P. sojae infection. We then developed several delivery formulations and compared their efficacy in protecting soybeans planted in soil. While some coated formulations negatively affected the soybean germination, others significantly improved the germination under simulated conditions of P. sojae zoospore inoculation and flooding. We further investigated the interactions of rhamnolipid with soybeans during water imbibition and germination. Seed coat was determined as the main barrier that regulates water imbibition, and rhamnolipid increased the imbibition rate. Rhamnolipid could slightly decrease the soybean germination rate, but only at high concentrations, e.g., down by about 15% at 20 g/L rhamnolipid. Rhamnolipid had far more pronounced effects on root development. At proper concentrations, rhamnolipid could stimulate lateral root development while reducing the primary root extension. Detailed results and discussion will be presented.