Abnormal ascent of xylem-sap was investigated after the inoculation of Pinus thunbergii with Bursaphelenchus xylophilus, using acid-fuchsin solution injected into the bottom of trunks of standing trees at one week intervals prior to harvest. Results were considered in relation to decrease in xylem water content, anatomical changes, and increase of nematode number in the trees. When sap-flow was disturbed two weeks after nematode inoculation, the water content began to decrease, after which whitish streaks became conspicuous in the cross section of trunks very clearly bounded by the area stained with the dye. Tracheids in those areas showed “cavitation”, that is, the phenomenon that tracheids come to contain gas instead of water. Xylem-sap flow, which ascended in spiral direction in healthy trees, was disturbed by the blockage of flow at cavitated tracheids. Areas of cavitation gradually spread to cover the entire cross sectional surface. Anatomical changes in pine tissue and increase in nematode number were delayed until the fourth week when water contents of xylem decreased to about 30% of healthy trees. Necrosis of cambium and phloem seemed to be caused by water shortage at the adjacent part to cavitated tracheids. Water deficit induced by cavitation was judged to be a major factor of inducing lethality in pine trees. Because resin leakage from epithelium was restricted to very small areas, mechanical plugging of water flow by leaked resin did not appear to be a major factor in disease induction. Clear boundaries between the cavitation and dye-stained areas suggested that hydrophobic substances were incorporated in cavitation areas. Investigations on the causal factors of cavitation may be required to understand the mechanism of pathogenesis associated with nematode infection.
The Phytopathological Society of Japan