Biological Control Of Postharvest Soft Rot Of Strawberry By Debaryomyce Hansenii And Its Formulation

Strawberry is one of the most widely cultivated berries in the world,which is rich of many essential nutrients and biologically active substances,which are very effective in improving the immunity of humanbeings and preventing diseases.Strawberry is also a fruit of great economic value.However,strawberry is delicate and susceptible to mechanical damage during storage,transportation and marketing,which provides conditions for pathogen to infest strawberry fruits,resulting in postharvest diseases,especially soft rot caused by Rhizopus stolonifer.The occurrence of this disease seriously affects the quality and economic value of strawberry,so it is necessary to take measures to effectively control the postharvest soft rot of strawberry.This can be achieved by using biological control methods through antagonism between antagonistic yeasts and pathogen in the fruit.Antagonistic yeasts have been widely used in the field of biological control of postharvest diseases of fruits and vegetables because of their safety,efficiency and environmental friendliness,but the application of Debaryomyces hansenii to the biological control of postharvest soft rot of strawberry has not been reported yet.In this thesis,an antagonistic yeast strain of D.hansenii was isolated and purified from healthy strawberry fruit,which was effective in the biological control of post-harvest soft rot of strawberry,and various physiological mechanisms of D.hansenii for the biological control of post-harvest soft rot of strawberry were investigated.The effect of D.hansenii on the species composition,species diversity and colony structure of strawberry surface microorganisms was then investigated by microbiome technology based on 16S r DNA and ITS sequencing,and finally D.hansenii was formulated with a view to providing a theoretical basis for its future commercial application.The main findings of this thesis are as follows.1.R.stolonifer,the causal pathogen of postharvest soft rot of strawberry,was isolated and purified from rotting strawberry fruit.Four strains of suspected antagonistic yeast were also isolated and purified from healthy strawberry fruit.and D.hansenii Y3 was found to be the most effective in controlling postharvest soft rot of strawberry.The acute toxicity test determined the toxicity class of D.hansenii to be practically non-toxic.2.By investigating the biological control of soft rot of strawberries after harvest at different concentrations of D.hansenii,1×10~8 cells/m L was obtained as the optimum concentration of D.hansenii for use.After 4 d of storage,D.hansenii significantly reduced the natural post-harvest decay rate of strawberry fruit,while not significantly affecting the storage quality and delaying its deterioration to a certain extent.3.The physiological mechanisms by which D.hansenii controls postharvest soft rot include:(1)D.hansenii is able to colonise the surface and wounds of strawberry at low(4°C)and ambient temperatures(20°C)to control postharvest soft rot by competing with R.stolonifer for nutrients and space;(2)D.hansenii significantly inhibited spore germination and germ tube length of R.stolonifer;(3)The activities of strawberry antioxidant-related enzymes including superoxide dismutase(SOD),peroxidase(POD),catalase(CAT)and ascorbate peroxidase(APX),and resistance-related enzymes including polyphenol oxidase(PPO)and phenylalanine aminolase(PAL)were all induced and enhanced by D.hansenii.Resistance-related substances including total phenols,flavonoids and anthocyanins also increased significantly.D.hansenii treatment could improve disease resistance in strawberry.4.High-throughput sequencing of 16S r DNA and ITS revealed that the main dominant phylum of the bacterial microbiome after D.hansenii treatment was Proteobacteria,while the dominant genera of Pseudomonas,Pantoea and Brevundimonas can be used as a biocontrol agent against postharvest bacterial diseases caused by Erwinia.The main dominant phylum in the fungal microbiome is Ascomycota,with Debaryomyces being the dominant genus after treatment with D.hansenii,which plays an important role in regulating fungal species composition,species diversity and colony structure on the surface of strawberries.In addition,although the effect of D.hansenii on species diversity and colony structure of the bacterial group was not as significant as that of the fungal microorganisms,it was also able to modulate them to some extent.5.The optimal protectant regimen for freeze-drying D.hansenii was analysed using the response surface methodology as 2.22%alginate,17.83%skimmed milk powder,0.06%L-ascorbic acid and 4.06%sodium L-glutamate.The freeze-dried formulation of D.hansenii obtained by the optimal compounding solution maintains a high survival rate over a long storage period and provides good control of post-harvest soft rot of strawberries.