Development and validation of a screening method for actinomycetes from Protaetia brevitarsis larval frass with biocontrol potential

• A novel QC-PPT system was developed for screening biocontrol strains against soil-borne diseases. • Seven potent antagonistic strains were isolated from WSFC larval frass. • Strain X15 boosts root growth and controls blight effectively. • X15 has broad-spectrum anti-pathogen and biosynthetic genes. This study aimed to establish an efficient screening method for identifying biocontrol strains effective against soil-borne pathogens of plants. To achieve this objective, we developed a q uantitatively- c ontrolled p otted p lant t esting (QC-PPT) system by optimizing cultivation devices, growth substrates, pathogen inoculation methods, and quantitative evaluation of root infection. This design effectively confines the roots and facilitates uniform pathogen infection. The optimal inoculation timing was determined to be between the 10th and 16th day of growth. Furthermore, a substrate composed of vermiculite with 4% organic compost was identified as ideal, supporting vigorous peanut growth while allowing sufficient pathogen infection required for reliable biocontrol evaluation. Using this system, seven strains with strong antagonistic effects against Sclerotium rolfsii were isolated from White-spotted Flower Chafer (WSFC, Protaetia brevitarsis ) larval frass. In vitro assays showed that strain X13 inhibited Sclerotium rolfsii growth by 44.56%, while strain X15 performed more excellently in the QC-PPT system: it increased peanut root dry weight by 32.8% and reduced root lesion area by 51.2% compared to the control group. Genomic sequencing data revealed that the superior strain X15 possesses the most diverse set of secondary metabolite biosynthetic gene clusters. Collectively, strain X15 is a promising candidate for biopesticide development, and the QC-PPT system we established can be extended to other crop-soil-borne pathogen systems. This study not only provides an effective biocontrol resource for peanut southern blight but also facilitates the development of sustainable disease management strategies in agriculture.