Histopathological Observations of Bursaphelenchus xylophilus in Symptomatic Tissues of Pinewood

Five-month old seedlings and three-year old Pinus densiflora were used to examine histopathological damage of wood tissues and symptomatic progress of seedlings at established intervals after the inoculation with the pinewood nematode. At the inoculation site, nematodes entered most axial resin canals of the cortex and xylem resulting in destruction of parenchyma cells. Just after inoculation, only a few nematodes were found in the resin canals in the cortex, and beyond the inoculation site. Cell death recognized by granulation of the cytoplasm and brown cell contents were sporadically observed among axial and radial xylem parenchyma cells through the stem as early as 3 days after inoculation. At the sixth day after inoculation, death of axial and ray parenchyma cells of seedlings which showed the typical disease symptom – marked reduction of oleoresin exudation – was widely distributed in the stem. At this stage, no population growth of nematodes was observed throughout the seedling and no destruction of parenchyma cells necessarily occurred in wood tissues. After complete stop of oleoresin exudation in the seedling, destruction of wood tissues, such as parenchyma cells of axial and radial resin canals, ray, cambium and phloem became more advanced as nematode populations grew rapidly in wood of the seedling. Cell death occurring at the initial stage of pathogenesis was indicated as one of the most remarkable pathological progresses of the nematode inoculated seedling. Destruction of wood tissues resulted from nematode feeding on parenchyma cells following disease progress.