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.

SummaryFicus erecta, a wild relative of the common fig (F. carica), is a donor of Ceratocystis canker resistance in fig breeding programmes. Interspecific hybridization followed by recurrent backcrossing is an effective method to transfer the resistance trait from wild to cultivated fig. However, this process is time consuming and labour intensive for trees, especially for gynodioecious plants such as fig. In this study, genome resources were developed for F. erecta to facilitate fig breeding programmes. The genome sequence of F. erecta was determined using single‐molecule real‐time sequencing technology. The resultant assembly spanned 331.6 Mb with 538 contigs and an N50 length of 1.9 Mb, from which 51 806 high‐confidence genes were predicted. Pseudomolecule sequences corresponding to the chromosomes of F. erecta were established with a genetic map based on single nucleotide polymorphisms from double‐digest restriction‐site‐associated DNA sequencing. Subsequent linkage analysis and whole‐genome resequencing identified a candidate gene for the Ceratocystis canker resistance trait. Genome‐wide genotyping analysis enabled the selection of female lines that possessed resistance and effective elimination of the donor genome from the progeny. The genome resources provided in this study will accelerate and enhance disease‐resistance breeding programmes in fig.

Ceratocystis canker, which is caused by the fungus Ceratocystis fimbriata Ellis et Halsted, is one of the most severe diseases of the common fig (Ficus carica L.). In contrast, the wild fig species F. erecta Thunb. is resistant to this fungus. We performed interspecific hybridization between the common fig (seed parent) and F. erecta (pollen parent) through artificial pollination. Even though hybrid seeds showed high germination rates, the seedling survival rates were low. All of the seedlings contained the expected simple sequence repeat (SSR) alleles from both common fig and F. erecta at each of the three loci tested, thus confirming the parentoffspring relationships of the interspecific hybrids. The leaf morphological characters of hybrid seedlings were intermediate between those of the parents. Cuttings of cultivars of common fig, F. erecta, and hybrid seedlings were inoculated with C. fimbriata by direct wounding of the shoot. All of the common fig cultivars tested withered and died within 10 weeks. Leaves and shoots of the hybrids and F. erecta were healthy 100 days after inoculation. Our results suggest that interspecific hybridization between the common fig and the wild species F. erecta is a breakthrough in the breeding of a new fig rootstock source with resistance to Ceratocystis canker.

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