The mechanism of disease development in pine wilt

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Substances associated with tissue browning of Japanese red(Pinus densiflora)and black pine(P. -thunbergii)inoculated with pinewood nematode(Bursaphelenchus xylophilus)were identified to be some kind of polyphenolics. A catechin-like substance was the most abundant polyphenolic associated with large numbers of B. xylophilus. Catechin, a tannin monomer was detrimental to the reproduction of B. xylophilus and its relative non-pathogenic species, B. mucronatus. Tannin increased conspicuously in field-planted seedlings when they were inoculated with B. xylophilus, thereby they seem to resist pinewood nematode invasion.Water content, amount of tannin, transpiration rate and nematode distribution were determined for pinewood nematode-inoculated, potted seedlings of Japanese black pine which were watered daily or every-fourth day. Multiple regression analysis revealed negative correlations. between water content of wood tissue and nematode numbers in specific stem segment, suggesting an important causal relation between these two factors. In the following experiment, a significant positive correlation was observed between nematode numbers and amounts of(i)oxidized lipids, (ii)tannin and(iii)leachates at 7, 7 and 14 days after inoculation, respectively. There was also a significant positive correlation between the amounts of leachates and tannin 14 days after inoculation. The results bring us the following hypothesis that generation of super oxide anion or hyperaccumulation of tannin, or both may cause leakage of abnormal substances from parenchyma cells, and the substances plug tracheids, thereby causes host wilting.