A new semi-selective agar medium for recovery and enumeration of antagonistic yeast, Pichia guilliermondii strain Z1 from orange fruit surface

The aim of this work was to develop a semi-selective medium for recovery and enumeration of Pichia guilliermondii strain Z1, a reliable biocontrol agent against postharvest pathogens of citrus fruit, and to assess the population dynamic of this antagonistic yeast on orange fruit in relation to incubation temperature and time of incubation. PDA is the basal medium used in this study which allows the antagonistic strain Z1 source of carbon and nutrients. Different chemicals (thiophanate-methyl, thiabendazole, thiram and imazalil) and antibiotics (hygromycin, tetracyclin, ampicillin and chloramphenicol) have been tested individually and based on the plating efficiency up to 90%, 40 combinations have been assayed between antibiotics and fungicides. Ten combinations proved to be highly selective against citrus pathogens and laboratory microflora, but only one consisting of tetracyclin (1 g/L) and thiabendazole (60 mg/L) was retained based on plating efficiency up to 99%, total selectivity against laboratory microflora, epiphytic microflora from washed orange fruit and lower cost.
The semi-selective medium TET-TBZ-PDA has been used to assess the impact of temperature and incubation time on the survival of strain Z1. It appears that population density was significantly influenced by both factors and the highest population size was recorded at 25°C followed by 5 and 35°C, respectively. This strain required a time of adaptation before entering the exponential growth phase with a maximum growth observed at 25°C relative to others. The semi-selective medium TETTBZ- PDA could be an efficient and valuable way to track the population density of this strain on the surface of orange when applied pre-or postharvest. This semi-selective medium may also aid in reaching a population density allowing a better efficiency in relation to environmental conditions.