2.4 Preliminary screening of Tibetan yeasts to control blue mold of pears
Preliminary screening of Tibetan yeasts for an- tagonistic activity was based on their ability to inhibit the infection of pear fruit by P. expansum. The bio- control activities of 20 yeast isolates from Tibetan fermented products were evaluated in vivo in the pear fruits at 20 °C, according to the method described by Wilson et al. (1993). The fruits were wounded with a sterile cork-borer (5 mm diameter×3 mm deep) and six wounds were bored on each pear. An aliquot (50 μl) of the yeast cell suspensions in sterile distilled water at a concentration of 1×109 cells/ml was inoc- ulated into each wound, while the same volume of sterile distilled water was applied as a control. Each wound was dried at room temperature for about 3 h, and then injected with 30 μl of P. expansum spore suspension (1×104 spores/ml). Then the samples were kept in enclosed plastic trays so as to maintain a high relative humidity at 20 °C in dark. Each yeast treat- ment consisted of three replicates, each consisting of 12 fruits. The whole experiment was performed twice.The decay incidence and lesion diameter on the treated pears were observed on the third day. Inhibition rates were calculated from the results using the following formulas: DI=(NC−NT)/NC×100% and LI=(dC−dT)/dC× 100%, where DI is decay inhibition (%), NC is the number of decay wounds for the control, and NT is the number of decay wounds for the treatment; LI is le- sion diameter inhibition (%), dC is lesion diameter of decay wounds for the control, and dT is lesion diam- eter of decay wounds for the treatment. Treatments that showed a decay incidence inhibition rate higher than 35% were selected and re-evaluated in further experiments.