Biocontrol potential of Streptomyces sp. N2 against green and blue mold disease in postharvest navel orange and the action mechanism

The objective of this study was to provide a promising alternative to chemical fungicides for management of postharvest citrus decay, thereby promoting sustainable citrus fruit production. The postharvest decay of citrus fruit caused by Penicillium digitatum and Penicillium italicum results in substantial economic losses in citrus industry worldwide. With growing fungal resistance issues in P. digitatum and P. italicum, there is an urgent need for searching new methods to address above problems in a safe and environmentally friendly way. Streptomyces sp. N2, a new species from Streptomyces genus, exhibits significant antagonistic activity against Rhizoctonia solani. However, its biocontrol efficacy against postharvest decay caused by P. digitatum and P. italicum and its action mechanism remain unknown. In this study, Streptomyces sp. N2 was found to have significant potential in controlling green and blue mold diseases in postharvest navel oranges. Moreover, the action mechanism of Streptomyces sp. N2 against both P. italicum and P. digitatum was elucidated. On the one hand, Streptomyces sp. N2 stimulated fruit resistance to fight against invading fungal pathogens. It significantly reduced ROS content in navel orange upon the infection of mold disease, increased the production of defense-related enzymes including peroxidase (POD), polyphenol oxidase (PPO), and phenylalanine ammonia-lyase (PAL) and pathogenesis-related proteins of chitinase and β-1,3-glucanase. On the other hand, Streptomyces sp. N2 secreted bioactive substances to inhibit the growth of P. italicum and P. digitatum so as to prevent the development of postharvest decay. The bioactive substances secreted by Streptomyces sp. N2 significantly inhibited the spore germination and mycelial growth and led to microstructural damages to the cell wall and membrane, ROS burst, and mitochondrial dysfunction in both P. italicum and P. digitatum. This study provides a theoretical reference and application potential for the biological control of Streptomyces sp. N2 on green and blue mold diseases.