Abstract Description

South Africa is the second-largest citrus exporter in the world, contributing significantly to the economy. However, despite its established and growing success, the South African citrus industry faces challenges threatening its long-term sustainability. Among these, postharvest diseases caused by fungal pathogens continue to be a significant and persistent threat. Postharvest diseases cause substantial economic losses, reducing fruit quality, shortening the shelf life of citrus fruits, and affecting their marketability. Among the various postharvest diseases, this study primarily focuses on green mould and sour rot, caused by the fungal pathogens Penicillium digitatum and Geotrichum candidum, respectively. The current control measure of these fungal pathogens relies on the use of synthetic fungicides such as Imazalil or Thiabendazole; however, due to the rise in fungicide resistance and stricter chemical residue regulations, their continued use is becoming increasingly limited. There has been a search for safer and more sustainable control measures, and Killer yeasts have emerged as promising biocontrol agents due to their antifungal activity. In this study, Ceratitis capitata (Mediterranean fruit fly) was explored as a source of potential Killer yeasts for use as biocontrol agents of post-harvest fungal diseases. Yeasts were isolated from lab-reared C. capitata, while additional isolates were obtained from glycerol stocks prepared from field-collected C. capitata. All yeast isolates were identified through PCR amplification and sequencing of the ITS region. Yeast isolates from the genera Wickerhamomyces, Diutina, Meyerozyma, Candida, and Rhodotorula were identified. Subsequently, antagonism against P. digitatum and G. candidum was tested in vitro. Yeasts belonging to the genera Rhodotorula, Debaryomyces, Meyerozyma, and Wickerhamomyces were selected from the yeast bank, set up within our research group, as they have been previously reported to exhibit antifungal activity against other fungal pathogens. A dual culture assay was conducted to investigate the potential antifungal activity of the selected yeast isolates. Fungal plugs of P. digitatum and D. hansenii were placed in the centre of PDA plates, and the yeast isolates were streaked on opposite sides. Following incubation at 25 °C for 7 days, the radial fungal growth was measured. The results revealed that among the tested yeasts, one of the isolates exhibited the strongest antifungal activity against G. candidum, with other yeast isolates showing a less prominent inhibitory effect. However, none of the yeast isolates showed any inhibitory effect against P. digitatum. In vitro suspension assays are currently underway to further investigate potential inhibitory effects of selected yeasts on the growth of P. digitatum and G. candidum.