Spatial distribution of Raffaelea quercivora in xylem of naturally infested and inoculated oak trees.

Mass mortality of Japanese oak caused by Raffaelea quercivora due to Japanese oak wilt (JOW) has been tremendous since the late 1980s in Japan. We investigated detailed distribution of R. quercivora hyphae in a naturally infected Quercus serrata tree using fluorescein-conjugated wheat germ agglutinin and also examined spatial relationships between hyphal distribution, water conduction loss, and host reactions in xylem in inoculated Q. crispula saplings. Hyphae of R. quercivora elongated longitudinally in vessels and transversely in ray tissues in both naturally infected and inoculated Quercus trees. Hyphae were confined within a relatively small area near the inoculation site. Vessel dysfunction was also confined and overlapped with hyphal distribution. The reaction zone consisting of two types of fluorescent substance was formed surrounding the outside of the hyphal area in xylem and was always found in non-water-conductive zone. These results suggest that wilting of Quercus trees caused by JOW may not be induced by dysfunction of a small number of vessels, but by that of many vessels, and it requires that R. quercivora hyphae spread from many galleries bored by beetles during mass attacks.