XYLEM DYSFUNCTION IN FICUS CARICA INFECTED WITH WILT FUNGUS CERATOCYSTIS FICICOLA AND THE ROLE OF THE VECTOR BEETLE EUWALLACEA INTERJECTUS

摘要

Ceratocystisflcicola causes serious wilt disease in many fig orchards in Japan. The transmission of this pathogen is thought to occur via soil to host roots, and an ambrosia beetle, Euwallacea interjectus, has been reported as a vector of the pathogen. Anatomical investigations were made on the disease development process with a particular focus on the responses of host tissue to the activities of the vector beetle and the pathogen. Living 26- and 8-year-old Ficus carica trees that were naturally infected with C.ficicola and had holes excavated by E. inter jectus were used for analysis. Dark brown discoloration was observed in the sapwood of specimens with poor shoot elongation and slight leaf wilt at harvest. Discolored sapwood coincided with the distribution of hyphae of the pathogen, which was verified by the presence of conidiophores. Most of the beetle's gallery was distributed inside the discolored area. In the non-discolored sapwood adjacent to the border of the discolored area, some galleries were elongated and contained living new generation adults and larvae of E. interjectus. Hyphae of the pathogen and colored substances were identified also around those new galleries.The present study showed that elongation of galleries by E. interjectus in the functional sapwood induces the wide distribution of the pathogen and contributes to the expansion of the discolored area in which vessels were dysfunctional. This process causes a shortage of water supply and wilting in the infected trees. Euwallacea interjectus must be contributing to the symptom development of this wilt disease.