Screening Antagonistic Bacterial Strains To Control Citrus Green/Blue Mold Diseases

Citrus(Citrus reticulata)is the most popular fruit in the world,but during storage and transportation,decays are often encountered.Most of these decays are postharvest diseases caused by fungi.The most common diseases are green and blue mold caused by Penicillium digitatum and P.italicum,respectively.Chemical fungicides are widely used to control postharvest diseases.However,the use of fungicides is being restricted due to the stringent regulation,carcinogenicity,high and acute residual toxicity,long degradation period,environmental pollution,and growing public concern about the chemical residues in fruits.In recent years,biological control has emerged as one of the most promising alternatives to chemical fungicides.In this study,we aimed to screen bacterial strains from rhizosphere soils and citrus peel to the biocontrol green/blue mold of citrus.The main results are as follows:Initially,from 148 strains isolated,strains SZS-6,Zj1-3,Zj1-9,Zj2-3,Zj2-9,and Zj2-11 were proved to show inhibitory effects on the growth of P.digitatum strain Pd-1 and P.italicum strain Pi-1 on potato dextrose agar(PDA)plates.Among them,strain Zj2-3,Zj2-9,and Zj2-11 explored strong effects on P.digitatum by over 80%.Next,the antagonistic ability of the substances produced by the strains after 3-day shaking cultivation were tested.The results showed that 20%of the culture filtrate produced by strain Zj2-3,Zj2-9,and Zj2-11 could profoundly inhibit the growth by more than 75%and spore germination by40%of strain Pd-1 and Pi-1.Whereas,the volatile substances of the strains were less effective in inhibiting the growth of the pathogens Pd-1 and Pi-1.The possible effects to other posrharvest pathogens were examined.Strain Zj2-3,Zj2-9,and Zj2-11 could inhibit the growth of Colletotrichum gloeosporioides strain ATN-1and Alternaria alternata strain BR-1.None of the bacterial strains showed distinct effects on Geotichum citri-aurantii strain HYS1-1.The strains could form stable population in wounds of citrus.In the inoculated wounds of citrus fruit,after being incubated at 25°C for 28 days,the populations of strains SZS-6,Zj2-3,Zj2-9,and Zj2-11 increased from 2×10~5 colony-forming units(CFU)/wound to7.07×10~7,1.15×10~6,1.77×10~6 and 1×10~6 CFU/wound,respectively.These bacterial strains were able to significantly reduce the lesion diameter of green mold and blue mold diseases.The biocontrol effect of the antagonistic bacterial strains was related to the concentration of the bacterial cells.Higher efficacy was achieved when the fruits were treated with a high concentration of antagonistic bacteria(1×10~9 CFU/m L)at25°C.Moreover,strain Zj2-9 and SZS-6 were the most effective in reducing the size of the lesion by more than 90%.The application of strains SZS-6,Zj2-3,Zj2-9,and Zj2-11 by dipping the fruits with the bacterial suspension(1×10~8 CFU/m L)effectively reduced the natural decay incidence from 46.0%among the untreated control fruits to 28.0-23.0%after30 days of storage at 15°C.Moreover,the control efficiency of strain Zj2-9 and SZS-6 was50.0%and 47.8%,respectively.Based on the 16s rRNA and recA gene sequence,strain Zj1-3 and Zj1-9 were identified as Bacillus pumilus,strain Zj2-3,Zj2-9,and Zj2-11 were Bacillus velezensis,and strain SZS-6 were Burkholderia cepacia.In summary,six antagonistic bacterial strains were obtained to be potential in controlling the green and blue mold diseases.