Ceratocystis ficicola 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. interjectus 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.
Pinewood nematode (PWN), Bursaphelenchus xylophilus, is the causative agent of pine wilt disease (PWD) of pine trees and is transmitted by cerambycid beetles belonging to the genus Monochamus. PWN is believed to have been introduced into Japan from North America at the beginning of the 20th century. In this article, we first provide an outline of the PWD system and the range expansion of PWN in Japan and then review the literature, focusing on the virulence of PWN. Virulence is a heritable trait in PWN, with high virulence being closely related to a high rate of reproduction and within-tree dispersal. When two PWN isolates with different virulence levels are inoculated into pine seedlings, the more virulent nematodes always dominate in dead seedlings. In a laboratory setting, many more virulent nematodes board the insect vectors than avirulent ones. The age at which vectors transmit the most abundant PWNs to pine twigs changes during the course of a PWD epidemic. However, the relation between virulence and transmission of PWN remains as yet relatively unknown. Such information would enable ecologists to predict the evolution of the PWD system. In this review we also compare ecological traits between the PWN and the avirulent congener, B. mucronatus.
Bursaphelenchus mucronatus is closely related to Bursaphelenchus xylophilus, the causative agent of pine wilt disease. Both nematodes are transmitted between host pine trees as the fourth-stage dispersal juveniles (JIV) by insect vectors. After the invasion of Japan by B. xylophilus, B. mucronatus, native to Japan, appears to have been replaced in the pine forests during the spread of the disease. To help understand this species replacement, the number of JIV carried by an insect vector (the initial nematode load) was compared between the two nematode species by using the beetle, Monochamus alternatus, in the laboratory. The initial load of B. mucronatus was signi cantly smaller than that of B. xylophilus although the number of third-stage dispersal juveniles (JIII) concentrated at the pupal chambers did not differ. Statistical analysis showed that the proportion of JIII moulting to JIV was the most important among three components explaining the difference in the initial load of B. mucronatus while the number of JIII concentrated at the pupal chamber was the most important for B. xylophilus. The phoretic af nity between the nematode and its vector is discussed in relation to its role in the species replacement.
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