Objective To address the significant decline in papaya fruit quality and yield caused by postharvest anthracnose, this study aimed to isolate and screen high-quality antagonistic bacteria, evaluate their antifungal activity against Colletotrichum siamense, and preliminarily investigate their mechanisms of action against anthracnose.

Method Rhizosphere soil samples were collected from diseased papaya plants in Zhanjiang City, Guangdong Province, to isolate and screen bacterial strains exhibiting strong antagonistic activity against C. siamense. Taxonomic identification of the target strain was performed through morphological observation, physiological and biochemical characterization, and phylogenetic analysis. The antagonistic activity of bacteria against C. siamense T3 and seven pathogens was assayed using the dual-culture plate method. Furthermore, the biocontrol efficacy of the antagonistic bacteria against anthracnose was validated through in vivo inoculation of papaya fruits and leaves.

Result A biocontrol strain J28 with antagonistic activity against C. siamense T3 was successfully isolated. Morphological characteristics, physiological and biochemical properties, and phylogenetic analysis identified the strain J28 as Bacillus velezensis. The strain J28 was capable of hydrolyzing starch and litmus milk, liquefying gelatin and reducing nitrate. It could also utilize citrate, glucose, sucrose, and maltose, and exhibited catalase activity, as well as the ability to participate in the Methyl Red and Voges-Proskauer reactions. Strain J28 exhibited high salt tolerance, maintaining robust growth capability on LB plates containing 125 g/L NaCl. B. velezensis J28 significantly inhibited the mycelial growth of C. siamense T3 isolated from papaya, with an inhibition rate of 61.81%. Concurrently, strain J28 exhibited broad-spectrum antifungal activity, inhibiting the growth of C. gloeosporioides, Botryosphaeria dothidea, and five other pathogenic fungal species, with inhibition rates ranging from 60.28% to 70.94%. In addition, the fermentation broth of strain J28 inhibited the germination of C. siamense T3 by 65.89% at a concentration of 100%. In vivo inoculation experiments confirmed that B. velezensis J28 effectively inhibited C. siamense in both papaya fruits and leaves, achieving biocontrol efficacies of 72.35% and 93.33%, respectively.

Conclusion In conclusion, strain J28 possesses broad-spectrum antifungal activity and exhibits significant biocontrol potential against papaya anthracnose. This research provides a theoretical basis and technical support for future exploration of salt-tolerant microbial resources and the development of broad-spectrum biological antifungal agents for plants.