Pseudomonas fluorescens RB5 as a Biocontrol Strain for Controlling Wheat Sheath Blight Caused by Rhizoctonia cerealis

Wheat sheath blight is a soil-borne fungal disease caused by Rhizoctonia cerealis and is a serious threat to wheat worldwide. A microbial fungicide is a promising alternative to a chemical fungicide for wheat disease control. In this study, strain RB5 against R. cerealis was isolated from wheat rhizosphere soil, which was identified as Pseudomonas fluorescens according to physiological, biochemical, and 16S rRNA gene sequence analyses. For improving the antifungal activity of RB5, the response surface methodology (RSM) was used to optimize the culture conditions for strain RB5, and the optimal culture conditions are 8.7 g/L of cassava, 5.2 g/L of soybean meal, pH 6.8, a 218 r/min speed, a 31.5 °C temperature, and 54 h of culture time. The inhibition rate of the culture filtrate obtained under this culture condition was up to 79.06%. The investigation of action mechanism showed strain RB5 could produce protease, chitinase, and siderophore, and its culture filtrate disrupted the mycelial morphology and inhibited the activities of three cell-wall-degrading enzymes of R. cerealis. Furthermore, the pot experiment exhibited that RB5 significantly controlled the wheat sheath blight with an efficacy of 71.22%. The evaluation of toxicological safety on an animal indicated that the culture filtrate was safe on mice. Overall, the culture filtrate of RB5 is a very promising microbial fungicide for the control of wheat sheath blight.