Antagonistic Efficacy and Chemical Basis of Endophytic Serratia plymuthica WF63 Against Colletotrichum Species in Nutgall Tree (Rhus chinensis)

Nutgall tree anthracnose, caused primarily by Colletotrichum species, acts as a primary bottleneck restricting the sustainable development of the Rhus chinensis industry. Developing green biocontrol strategies by screening molecular targets for novel fungicides is highly imperative. A strain designated as Serratia plymuthica WF63 was isolated from healthy R. chinensis tissues. The strain exhibited broad-spectrum antifungal activity and multiple plant growth-promoting (PGP) traits, including the production of protease, cellulase, and indole-3-acetic acid (IAA). In vivo experiments revealed that S. plymuthica strain WF63 achieved a biocontrol efficacy of over 50% against anthracnose pathogens (Colletotrichum nymphaeae and C. fioriniae) and demonstrated significant plant growth-promoting effects. Gas chromatography–mass spectrometry (GC-MS) analysis, combined with in vitro toxicity validation of pure compounds, identified hexahydro-2H-pyrido [1,2-a]pyrazin-3(4H)-one as a core antifungal component in the fermentation broth, with a half maximal effective concentration (EC50) of 133.88 mg·L−1 against the target pathogen. These findings not only highlight S. plymuthica strain WF63 as a promising antifungal biological agent but also suggest that the specific nitrogen-containing heterocyclic compound may serve as a candidate scaffold for further fungicide optimization, pending comprehensive ecotoxicological evaluation.