Identification of a Biocontrol Agent Bacillus velezensis XJ20 and Assessment of the Antifungal Efficacy Against Stagonosporopsis caricae Causing Pumpkin Fruit Rot

During the late stages of pumpkin (Cucurbita moschata) production, storage, and transportation, pumpkins are highly prone to infection by pathogenic fungi, which leads to fruit decay and subsequently causes severe losses. In this study, a fungal isolate obtained from rotten pumpkin fruits was identified as Stagonosporopsis caricae. Ten biocontrol bacteria with antagonistic properties against S. caricae were isolated from the pumpkin surface, and one strain of biocontrol bacterium, XJ20, which exhibited the most remarkable biocontrol performance among them, was isolated and identified as Bacillus velezensis. XJ20 effectively controlled the decay of pumpkin fruits, with a biocontrol efficiency of over 73%. Moreover, it could effectively improve the activity of pumpkin defense enzymes, contributing to the maintenance of resistance against pumpkin fruit rot. The XJ20 lipopeptide extract demonstrated significant inhibitory activity against S. caricae. At a concentration of 10 μl/ml, it achieved an inhibition rate of 82.7% for mycelial growth. Additionally, it induced mycelial deformation. The lipopeptide was relatively insensitive to temperature and UV irradiation to a certain extent, but extremely alkaline conditions would result in the loss of antifungal activity. The lipopeptide extract was purified using a protein purifier and analyzed by MALDI-TOF mass spectrometry, suggesting that the substances demonstrating antifungal activity were surfactin, fengycin, and iturin, which could lay the groundwork for the development and application of B. velezensis XJ20 in the subsequent stage.