The effect of some compounds in enhancing biological control on citrus fruit

This study concerns the antagonistic activity of Pichia guilliermondii isolate 5A against Penicillium digitatum on mandarin type fruits cv Fairchild. In order to enhance the inhibitory activity of the yeast 5A, we tested the effect of some salts, Na sub(2)CO sub(3) and CaCl sub(2), at concentrations of 1-2-4%, and of fractions of essential oils of orange (citral and carvacrol) and Thymus capitatus (thymol) at concentrations of 1-2-4%. Inoculation was done by pipetting into each wound 10 ml of a conidial suspension of the pathogen at 108 cells ml super(-1) on artificially wounded fruits. The high inhibitory property of isolate 5A was confirmed in three consecutive trials (90.2, 98.2, 92.2% inhibition). When 5A was combined with the salt Na sub(2)CO sub(3), even at a dosage of 1%, values increased (6-7%), and the same occurred at the other two dosages. The use of Na sub(2)CO sub(3) alone gave a high inhibition at the three dosages used (78, 84, 92%). The salt CaCl sub(2) had lower inhibitory effect (74-82-87%) compared to Na sub(2)CO sub(3) but when combined with 5A increased inhibition of P. digitatum by 8-9%. When the isolate 5A was used together with fractions of essential oils, the increase in the efficacy of the yeast was not significant, probably due to the high volatility of the tested compounds. Each of the fractions alone at the three tested dosages showed highly variable effectiveness. Furthermore, dosages of 4% of citral and carvacrol damaged the fruit epicarp. In vitro trials demonstrated low toxicity against the cells of the antagonist.