Isolation, Screening And Security Study Of New Antagonists Against The Postharvest Diseases Of Pears

A huge loss has been made by spoilage in the course of fruit picking, distribution and storage, which is also globally concerned. In developed countries,10-30% fresh fruits perished caused by postharvest disease, while this number was up to 40-50% in developing countries, where the freezing storage and transportation facilities are limited.At present, effective control methods of low temperature and synthetic fungicide treatment are most commonly used. Although refrigeration technology was widely developed in our country recent years, there was still a considerable number of pathogens could grow on fruits and result in decay under low temperature storage. Synthetic fungicide treatment was significantly effective, but it killed both of the pathogens and the beneficial microorganisms. Moreover, fungicide residues do great harm to human health and the environment. Environmental pollution caused by chemical pesticides and the resulting food safety problem has become the focus of attention around the world. To control postharvest diseases of the fruits and vegetables, antagonistic bacteria and yeasts has became an effective way to substitute or reduce the using of synthetic agents and has been proved to be the new developing trend.In this research, the antagonistic bacteria and yeasts were isolated, screened and identified with various methods. The research has discovered strains with excellent biocontrol effect. The biological safety of the antagonistic strains was investigated and the production processes of their commercial preparation as well as the preservation method were preliminary discussed. Main conclusions of the research are as follows:(1) The antagonistic yeast Y20 and Y21 were sieved by in virto and in vivo experiments which were tested to show preferable antibacterial effect to pathagens of Botrytis cinerea in pears and Penicillium expansum in peachs. Experiments on morphological characteristics, nutritional characteristics and physiological characteristics as well as the DNA molecule sequencing analysis of Y20 and Y21 were conducted. The results identified that Y20 is Rhodotorula mucilaginosa and Y21 is Zygoascus hellenicus. There is no report on these two kinds of yeasts (Y20, Y21) used in postharvest diseases control currently.(2) The two strains of Y20 and Y21 grow up fast without apparent lagging period. The gray mold and blue mold in pome fruit were significantly inhibited by antagonistic yeasts Y20 and Y21. The fruits'disease incidence could be prominently reduced at the concentration of 1×107CFU/mL, 1×108CFU/mL and 1×109CFU/mL, and the higher the yeast concentration was the better the effect was. Antagonistic yeast B19 at the concentration of 1×108CFU/mL had a stable biocontrol effect on gray mold and blue mold in pome fruit. Treated by antagonistic yeasts Y20 and Y21(1×108CFU/mL), then stored at room temperature(20℃) for 45 days, the desease incidence of pome fruit was remarkablely lower than that of the control fruit, and had no adverse effect was found on storage quality of the fruits.(3) Acute oral toxicity test indicated that the acute peroral toxicity MTD of yeastsY20 and Y21>21.5g/kg. Based on acute toxicity dose classification of the GB15193.3-2003 appendix, Y20 and Y21 both belonged to non-toxic level. Ames test, mouse bone marrow cells, micronucleus test, mouse sperm abnormality test and other genetic toxicity tests showed that Y20 and Y21 yeasts had no teratogenic and mutagenic effect to mice. In the 30-day feeding trial, no harmful effects of antagonistic yeas were found on mice's growth, normal blood index, physicochemical index, physiological functions and major organs and tissues (liver, spleen, kidney, etc.).(4) Optimizeing Y20 medium using cheap molasses, uniform experimental result showed that using the optimal medium combination:16.58% of molasses, 0.05% of magnesium sulfate,0.05% of potassium dihydrogen phosphate and 0.3% of urea to conduct yeast culture could get the maximum biomass 1.402g/100mL.(5) Research on freeze-dried agent of Y20 and Y21 indicated that the activity of Y20 and Y21 could be maintained over 75.6% within 6 months. Stored at 4℃, the biological activity of the two yeasts'freeze-dried powder could be well kept within 6 months. Research on Y20 and Y21's liquid agent showed that under the condition of 20℃and 4℃, the best storage group of Y20 was 10% of trehalose,0.0176% of Vc and PBS mixing liquid, and the best storage group of Y21 was 5% of trehalose, 0.0176% of Vc and PBS mixing liquid.(6) During the storage period of Y20 and Y21, research on the rotten incidence of pome fruit indicated that both of the freeze-dried and liquid storage bacterial agents had a good effect on the diseases of pome fruit.In summary, as postharvest disease biocontrol antagonists of pome fruit, antagonistic yeast Y20 and Y21 have significant and safe control effect. Research on biocontrol effect, safety, and commercialization technology indicated that the two antagonistic yests of Y20 and Y21 were effective, security and has the potential of commercial application. This research lay a good foundation for studies of postharvest biocontrol preservation, security of strains and commercialization of antagonistic yests of Y20 and Y21.