Discoloration of sapwood and blockage of xylem sap ascent in the trunks of wilting <I>Quercus</I> spp. following attack by <I>Platypus quercivorus</I>

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  • ナラ類の集団枯損にみられる辺材の変色と通水機能の低下
  • ナラルイ ノ シュウダン コソン ニ ミラレル ヘンザイ ノ ヘンショク ト

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Abstract

Large mortality rates of oak trees (Quercus serrata THUNB. and Q. crispula BL.) during summer months has become a serious problem in districts along the Japan Sea. A specific fungus (being identified) has been detected in wilted oaks and mycangia of the ambrosia beetles (Platypus quercivorus (MURAYAMA)) which make deep pinholes into the sapwood from June onward. To explain the mechanism of wilting, we observed the reaction of tree tissues following beetle attack and fungal infection. The xylem had become widely discolored surrounding beetle's galleries. Vessels in such discolored areas are non-conductive. Conductivity of the vascular system, which was estimated by a dye injection at the base of the trunk, was markedly less in trees attacked by the beetles. Anatomical investigation of these tissue samples revealed that fungal hyphae were distributed in the vessels in the discolored areas especially in those open to the galleries. Tylosis formation was active in large vessels adjacent to the discolored areas. By the start of leaf curling or discoloration, the area of uncolored xylem decreased less than 20% of cross-sections near the base, and large vessels were plugged with tylosis even in the outermost rings. Above that area of maximum discoloration, cambial necrosis and xylem desiccation began for the complete blockage of sap-ascent.

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