Biotechnological Potential of Metschnikowia pulcherrima Yeasts for Biomass Production in Agricultural Biocontrol

This study aimed to select Metschnikowia pulcherrima strains with antimicrobial potential and high biomass content, optimize their cultivation conditions, evaluate growth characteristics at different scales, and assess antimicrobial activity on apple plants (Malus domestica cv. Golden Delicious) infected with phytopathogens. Of the nine tested strains, M. pulcherrima D2 was selected for its strong inhibitory activity against all tested phytopathogenic molds: Venturia inaequalis, Botrytis cinerea, Phoma exigua, Colletotrichum coccodes, Monilia laxa, Alternaria alternata, Alternaria tenuissima, Fusarium sambucinum, and Fusarium oxysporum, both in vitro on laboratory media (inhibition zones from 13.5 to 35.0 mm) and in vivo on stems, leaves, flowers, and fruits of apple. Morphological observations of treated plants showed the complete absence or significant delays of disease symptoms for up to 10 days. Disease symptoms for several pathogens (V. inaequalis, A. alternata, A. tenuissima, B. cinerea, F. sambucinum) remained reduced by ≥50% for up to 31 days post-treatment compared to the untreated control. Optimal cultivation conditions for M. pulcherrima D2 were established: a complex medium containing yeast extract (5.0 g/L), soy peptone (5.0 g/L), and glucose (2.6 g/L), at pH 5 and 25 °C, with shaking at 180 rpm, resulted in high biomass contents (107–108 CFU/mL). Scale-up in 5 L bioreactors confirmed efficient biomass production (108 CFU/mL and from 3.1 to 3.9 g/L of dry biomass). These findings highlight the strong biotechnological potential of M. pulcherrima D2 for the development of a biocontrol agent to protect apple fruits and trees against fungal phytopathogens.