Pilot-scale production and liquid formulation of Rhodotorula minuta, a potential biocontrol agent of mango anthracnose

Aims:  To develop a pilot-plant fermentation process for the production of the yeast Rhodotorula minuta, to be used as a biocontrol agent of mango anthracnose, using a low-cost culture medium. To develop a stable liquid formulation that preserve high viability of the yeast stored at 4°C. Methods and Results:  Keeping constant the volumetric power input, a fermentation process was scaled-up from shake flasks to a 100 l bioreactor. Preharvest applications of the yeast resulted in postharvest anthracnose severity equal or lower than that observed with a chemical fungicide. Glycerol was added to the formulation as water activity reducer and xanthan gum as a viscosity-enhancing agent. Yeast initial concentration of 1010 CFU ml−1 resulted in 4–5 orders of magnitude decrease after 1 month of storage at 4°C, whereas when it was formulated at 109 CFU ml−1, the decrease was of two orders of magnitude in 6 months. Conclusions:  The fermentation process was successfully scaled-up using a low-cost culture medium. Postharvest anthracnose severity could be considerably reduced using this yeast. Formulating the yeast at 109 CFU ml−1 and adding glycerol (20%) and xanthan (5 g l−1) avoided both contamination and yeast sedimentation and it was able to preserve up to 107 CFU ml−1 after 6 months at 4°C. Significance and Impact of the Study:  The yeast R. minuta is reported as a novel antagonistic micro-organism against the pathogen Colletotrichum gloeosporioides. Pilot plant production of this yeast allowed us to conduct field tests in commercial orchards during three harvest seasons. Yeast suspensions applied to mango trees reduced the fruit anthracnose severity in levels similar or better than chemical fungicides.