Production of Rhamnolipids by a Halotolerant Bacterial Strain with Seawater and Its Application in the Treatment of Powdery Mildew of Strawberry (Fragaria ananassa)

Abstract

Biosurfactants are amphipathic compounds, a kind of natural agricultural chemical, excreted by microorganisms that exhibit surface activity. Biosurfactants have advantages over their chemical counterparts in biodegradability, low toxicity, and ecological acceptability and effectiveness at extreme temperature and pH. Fermentation water and mineral salt costs are among the production costs. The seawater is usually rich in various mineral salts. Biosurfactant production with seawater and waste vegetable oil with simple facilities will lower the cost greatly. In our study, strain screening, shaking flask fermentation, and 5 L liquid fermentor fermentation were carried out consequently, and the fermentation yield reached above 10 g·L⁻¹, confirming this kind of low-cost production is feasible and practicable. And the application of rhamnolipids in plant protection in saline soils was also conducted. The fermentation broth was diluted to 1 g·L⁻¹ and 0.5 g·L⁻¹ separately and was directly used to treat the downy mildew of strawberry. The results showed that the fermentation broth could be used as an efficient kind of fungicide. The pot experiments showed that control efficiency could be reached over 90.8% and 87.6%, respectively, compared to blank group, performing better than the tested dominant chemical fungicides. Furthermore, the rhamnolipids fermentation broth could also enhance the development of root and shoot of strawberry.