Cortical responses in Japanese black pine to attack by the pine wood nematode

Abstract

Anatomical and cytochemical changes in the current-year stem cuttings of Japanese black pine (Pinus thunbergii) were investigated in the early stage of infection by a virulent isolate and an avirulent isolate of pine-wood nematode (Bursaphelenchus xylophilus), and an avirulent isolate of another nematode species, Bursaphelenchus mucronatus. Accumulation of lignin- and suberin-like substances around the resin canals in the cortex was shown as a new symptom of the infection by these isolates. Experiments with girdled cuttings demonstrated that more nematodes inhabit and move in the bark than in the xylem and pith at the early stage of infection by the virulent isolate. Death of pine cells occurred first in the epithelial cells of resin canals in the cortex after inoculation with the virulent isolate and then in the cortex and periderm, pith and xylem, and finally the cambium. In branches of 5-year-old seedlings inoculated with avirulent and B. mucronatus isolates, wound periderm was formed surrounding resin canals in the cortex, and cortical cells surrounding the wound periderm were alive. Evidence indicates that nematodes first enter resin canals in the cortex and then invade the surrounding cortical tissue, and that the ability of the virulent nematodes to move into the cortical tissue may be greater than that of the other isolates, accounting for differences in virulence. Key words: Bursaphelenchus xylophilus, Bursaphelenchus mucronatus, virulence, periderm, pine wilt disease, pine-wood nematode.