Control of root-knot nematode disease in Aucklandia lappa Decne. by endophytic Serratia liquefaciens ML4.

INTRODUCTION: Aucklandia lappa Decne. is a medicinal plant. In recent years, Meloidogyne hapla has been found to cause severe damage to A. lappa. Given the medicinal nature of A. lappa, disease control must prioritize safety concerns such as pesticide residues. Therefore, environmentally friendly and highly safe biological control measures are of considerable significance for managing root-knot nematode disease in this plant, and endophytic bacteria represent a valuable source of biocontrol resources. METHODS: A strain of Serratia liquefaciens ML4 with the ability to control root-knot nematode disease was screened from endophytic bacteria of A. lappa. The nematicidal activity of its fermentation broth was evaluated through in vitro assays, pot experiments, and field micro-plot trials. The composition of the fermentation broth was analyzed by headspace solid-phase microextraction coupled with gas chromatography-mass spectrometry (HS-SPME-GC-MS). RESULTS: The 2-fold dilution of the fermentation broth achieved a corrected mortality rate of 95.82% against J2s of M. hapla after 24 h of treatment. The egg hatching rate was inhibited by 99.51%, the pot control efficacy was 57.14%, and the field micro-plot test control efficacy was 62.79%. At the same time, it promoted the growth of A. lappa. Active volatile organic compounds (VOCs), including 1-octen-3-one, 6-methyl-5-hepten-2-one, dimethyl trisulfide, 2-phenylethanol, dimethyl disulfide, and 2-isobutyl-3-methylpyrazine, were screened from the fermentation broth and demonstrated nematicidal activity against J2s of M. hapla. Among them, 1-octen-3-one exhibited a fumigation LC50 of 5.45 × 10-3 µL/L and an aqueous contact LC50 of 5.79 µL/L against J2s. The pot experiment showed that at a concentration of 1 µL/L, 1-octen-3-one inhibited gall formation by 79.66%. CONCLUSION: S. liquefaciens ML4 can effectively control root-knot nematode disease on A. lappa and promote plant growth. The VOCs in its fermentation broth are the key active ingredients responsible for nematicidal activity and disease control, indicating this strain is promising for biocontrol of root-knot nematodes.