The Study Of Biocontrol And Mechanism Of Action Of Postharvest Disease Of Citrus Fruits By Two Antagonist Yeasts

Orange is a nutritious fruit and is welcomed by consumers. In the world its yield is the number one in fruits. But green mold is its major postharvest disease and could cause big losses. Fruit diseases are usually controlled by application of synthetic fungicides. However, due to the development of fungicides-resistant of pathogens, the environment contamination and human health risks and so on, alternative methods are needed. Biocontrol is an alternative one to synthetic fungicides for the control of postharvest decay of fruits. As yeasts have shown many merits in biocontrol of postharvest diseases, now antagonist yeasts have been given more and more attention to apply to postharvest biocontrol.In this research 70 antagonist yeasts were isolated from the surface of fruits and the healthy leaves of citrus fruits. And two antagonist yeasts were screened that had very good suppression effect on the green mold of citrus fruits. The two antagonist yeasts were identified as Candida sp.and Saccharomycodes sp. separately. The biology of the strains was further tested. The main results were as followings:1. The optimized conditions for the growth of the strain C. sp. were: culture medium was NYDB, growth temperature was 24℃, culture time was 48h, pH value was 6.0 and installing liquid volume was 100mL/250mL.The optimized conditions for the growth of the strain S.sp. were: culture medium was NYDB, growth temperature was 28℃, culture time was 72h, pH value was 4.0 and installing liquid volume was 50mL/250mL.2. The strain C. sp.could use many diffrent kinds of nitrogen and carbon resources. But its growth could be influenced by them. When using yeast extract as nitrogen resource and glucose as carbon resource, the growth was the best. The influence of nitrogen resource for the growth was far bigger than the one of carbon resources.3. The concentrations of the yeast cells obviously influenced disease incidence and lesion development in citrus fruits. The bigger the concentration was, the better the result was. When the concentration of yeast cells was 1 × 10~8CFU/mL, it could control the infectivity of the pathogens.4. The result of biocontrol was influenced by inloculating time of antagonist yeast and Penicillium digitatum. The good result was got when the yeasts were inloculated 12 or 24 hours before pathogens, but when inloculated simultaneously with pathogens, the biocontrol result was reduced siginificantly.5. The strain C. sp. not only can inhibit spore germination of P. digitatum on PDA plate, but also influence the hyphae growth of the fungus. When the concentration of yeast cells was 1×10~8CFU/mL, the strain could completely inhibit the spore germination.6. The both of the yeast strains growed well on the peels of citrus fruits. In 1st day two antagonist yeasts were inoculated, the yeast's population increased more than 10 folds comparing to the start point. In 3rd day, the population of C. sp. increased 26.2 folds comparing to the start point and came to the largest value. In 4th...