In vitro and in vivo screening of yeast isolates on Penicillium digitatum and Galactomyces citri-aurantii of citrus

Penicillium digitatum and Galactomyces citri-aurantii are two of the most important fungal pathogens of citrus causing green mould and sour rot, respectively. These pathogens are responsible for about 90% of the postharvest losses in the citrus industry worldwide. P. digitatum and G. citri-aurantii were isolated from untreated diseased citrus fruit in KwaZulu-Natal, South Africa. A total of 200 yeasts were isolated from orchard soils, weeds, grasses, citrus peels and leaves; and screened against green mould and sour rot. The highest antagonistic activity was shown by 20 yeast species isolated from lemon and sweet orange fruits and leaves, with biocontrol efficacies ranging between 20 and 86%. These isolates were further applied as preventative treatments of 'Navel' and 'Valencia' cultivars against both pathogens. Some yeast species reduced green mould incidence on fruits by 94 and by 77% on sour rot when applied preventively. Scanning electron micrographs showed shrinkage and the collapse of hyphal structures of P. digitatum and G. citri-aurantii after in vitro interaction with yeast isolates. These results suggest that yeast antagonists could be an alternative control method for green mould and sour rot in the citrus industry.