Migration of pine wood nematodes in the tissues of Pinus thunbergii

Abstract

The distribution of pine wood nematodes (Bursaphelenchus xylophilus, PWNs) in Japanese black pine (Pinus thunbergii) tissues was investigated by staining with fluorescein isothiocyanate-conjugated wheat germ agglutinin. After PWNs were inoculated to current-year stems of pine seedlings, their distribution at about 5 cm below the inoculation site was confined only to cortical resin canals 1 day after inoculation, and then spread to other tissues, including resin canals of short branches. When PWNs were inoculated onto cross or tangentially cut surfaces of stem segments, maximal PWN migration speed was estimated to be faster through cortical resin canals and xylem axial resin canals vertically (>6.7 and <2.3 mm/h, respectively) than through cortical tissues both vertically and horizontally (<1.2 and <0.2 mm/h). To examine whether PWNs in cortical resin canals could invade surrounding tissues, segments in which PWNs resided only in cortical resin canals were prepared by removing the top portion 6 h after inoculation. Additional incubation of such segments caused extended PWN distribution to xylem axial resin canals and then to other tissues. A similar experiment with top portions of girdled segments removed 12 h after inoculation also showed extended PWN distribution from xylem axial resin canals and pith to cortical resin canals and then to other tissues. These results provided direct evidence that PWNs have the ability to migrate from cortical resin canals and xylem axial resin canals to other tissues.