Acta Hortic. 1325. ISHS 2021. DOI 10.17660/ActaHortic.2021.1325.33 Proc. V Int. Symp. on Postharvest Pathology: From Consumer to Laboratory - Sustainable Approaches to Managing Postharvest Pathogens Eds.: M.H. Jijakli et al. 233 In vitro and in vivo screening of yeast isolates on Penicillium digitatum and Galactomyces citri-aurantii of citrus N.C. Mbili a and S.S. Buthelezi Discipline of Plant Pathology, University of KwaZulu-Natal, Private Bag X01, Scottsville 3201, South Africa. Abstract 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. Keywords: Penicillium digitatum, Galactomyces citri-aurantii, yeast, biocontrol, citrus INTRODUCTION South Africa (SA) is one of the top producers and exporters of citrus, with the industry contributing about R19 million in exports in 2018 alone. Economic losses due to postharvest pathogens present a huge challenge to SA’s citrus industry. Penicillium digitatum and Galactomyces citri-aurantii are two of the most prevalent pathogens associated with sweet oranges in South Africa (Christie, 2016). Sweet orange cultivars such as ‘Navel’ and ‘Valencia’ are susceptible to sour rot and green mould infections. The most common method of control is chemical control, which involves the use of fungicides such as thiabendazole, imazalil and guazatine (Ait Ben Aoumar et al., 2014). However, the emergence of fungicide resistant strains and safety concerns have made it difficult to control these pathogens (Perez et al., 2016). This predicament has opened a gap for alternative control strategies with minimal health and environmental hazards, such as biological control agents. The use of microorganisms as biological control agents against postharvest pathogens has since yielded positive results in both in vitro and in vivo studies (Perez et al., 2016; Aloui et al., 2015; Klein and Kupper, 2018). Abraham et al. (2010) observed that yeast and bacillus species prevented green mould decay on ‘Navel’ oranges and reduced the disease incident to less than 5% on ‘Valencia’, when applied preventively. Furthermore, fruits treated with the yeast Aureobasidium pullulans ACBL-77 cultured in YMM without other nutrients showed 65 and 85% curative and preventive control of sour rot, respectively. These results were reported in a study conducted by Klien and Kupper (2017). Biological control agents are a promising solution to pest control. Furthermore, these microorganisms can compete for nutrients, secrete enzymes that inhibit the pathogen and induce resistance with low risk of resistance (Talibi et al., 2012). The aim of this research was to investigate the in vitro and in vivo antagonistic activity of yeast species in controlling P. digitatum and G. citri-aurantii of sweet oranges postharvest. a E-mail: mbilin@ukzn.ac.za