Early Invasion Dynamics of the Pine Wood Nematode in <i>Pinus massoniana</i> : Effects of Stem Type and Wound Type

Understanding the early invasion and migration mechanisms of the pine wood nematode (PWN, <i>Bursaphelenchus xylophilus</i>) is crucial for disease control, but the dynamics immediately following inoculation remain unclear. Using histopathological observation (via paraffin sectioning and microscopy) in conjunction with a modified Baermann funnel technique, we investigated the initial stages of PWN invasion and migration across different stem types and wound types to delineate the precise migratory pathways. Results showed that cortical resin canals served as the sole pathway for initial PWN invasion; even low inoculation levels (10 to 100 nematodes) enabled PWN invasion through these canals within 1 h post-inoculation (hpi). Optimal migration occurred in the bark of 2-year-old branches, reaching 15 cm (upward)/18 cm (downward) with approximately 11.7% of the inoculated nematodes by 12 hpi. Migration did not differ between natural beetle-feeding wounds and artificial wounds within 12 hpi. Migration through xylem was severely limited (only 3 to 15 nematodes migrating 1-6 cm within 12 h), yet was notably accelerated when passing through branch cross-sections (17 cm in bark and 16 cm in xylem within 6 hpi) compared to the maximum distance (8 cm) observed in beetle-feeding wounds or artificial wounds. Furthermore, a critical invasion window was identified: wound exposure for ≤ 12 h permitted PWN invasion, while exposures of 16 h or 24 h significantly reduced or completely prevented invasion. These results define the first 12 h post-wounding as the key vulnerability period for PWN infection, providing a critical timeframe for targeted prevention of pine wilt disease.