Relationship between the distribution of the pinewood nematode (Bursaphelenchus xylophilus) and the development of xylem embolism in the stems of Japanese black pine (Pinus thunbergii) seedlings monitored by magnetic resonance imaging

The development of xylem embolism in 1-year-old stems of Japanese black pine (Pinus thunbergii) seedlings was monitored by compact magnetic resonance imaging (MRI) after inoculation with the pinewood nematode (Bursaphelenchus xylophilus). In parallel, the nematode distribution and population structure in the stems were examined by isolating the nematodes using the Baermann funnel technique. The vertical length and volume of massive embolisms in each seedling were strongly correlated with the maximum relative embolized area (REA) in stem cross-sections. Embolism development and nematode reproduction were not restricted to the inoculation site, as any portion of the stem could be the initial point of a population burst. The nematode population in the stem xylem was strongly correlated with the REA and with the circumferential proportion of cambial death in cross-sections monitored by MRI. The proportion of second-stage juveniles was also correlated with the REA in the xylem. In contrast, the nematode population in bark tissue was not correlated with either the REA or cambial death. These results suggested that nematode reproduction in the cambial zone is the key step in pine wilt disease, and second-stage juveniles were suggested to induce massive embolisms in the advanced stage of the disease.