Abstract: Tobacco brown spot,caused by Alternaria alternata,is a prominent leaf disease in tobacco cultivation,often resulting in notable economic losses.Identifying the strain YZ01 with significant antagonistic activity against Alternaria alternata,exploring its active substances,and optimizing its fermentation conditions can provide bacterial resources to control tobacco brown spot disease and guide its practical production and application.Strain YZ01 was identified as Bacillus velezensis through a comprehensive analysis of morphological,physiological,biochemical characteristics,and sequencing of 16S rRNA and housekeeping gene gyrA.Culture filtrate of YZ01 could significantly inhibit various fungi,with the inhibition rate of 86.56% on Alternaria alternata and 100.00% on Valsa mali,Phytophthora capsici,Botrytis cinerea,and Colletotrichum fructicola.Lipopeptides and proteins were identified as the main antimicrobial active components in the culture filtrate.Subsequent investigation revealed that the fermentation broth of YZ01 disrupted the hyphae cell structure of A.alternata through the action of these active components,leading to protoplast exosmosis and inhibition of mycelial growth.Through single factor and orthogonal design experiments,the optimal fermentation conditions for producing antimicrobial active components were determined.The optimal fermentation medium formula for strain YZ01 consisted of 1.50% dipotassium hydrogen phosphate,1.50% tryptone,and 0.50% glucose.The stability test of the fermentation broth showed that its activity was minimally influenced by temperature,pH,or UV treatment.These findings offer important insights for the biological control of tobacco brown spot disease and establish a basis for future field application.

Key words: tobacco brown spot, Bacillus velezensis, antagonism, antimicrobial active component