Screening for Susceptibility of Macadamia to Euwallacea fornicatus and its Fungal Symbiont Fusarium euwallaceae.

The polyphagous shothole borer (Euwallacea fornicatus, PSHB), an ambrosia beetle, with its fungal symbiont, Fusarium euwallaceae, is responsible for Fusarium dieback (FD) in a wide range of woody hosts. In 2019, the first suspected case of E. fornicatus was reported in macadamia in South Africa. The aims of this study were to confirm the E. fornicatus report and thereafter to assess the susceptibility of commercially planted macadamia cultivars to FD caused by F. euwallaceae. The identities of the beetle and associated fungal symbionts were confirmed by means of DNA sequence analysis of the 28S ribosomal large subunit gene for beetles and the internal transcribed spacer region for fungi. Isolates identified as Fusarium species were further characterized by phylogenetic analysis of the translation elongation factor 1α and the β-tubulin gene regions. Thereafter, Koch's postulates regarding F. euwallaceae were fulfilled on a mature Macadamia integrifolia tree planted at the experimental farm of the University of Pretoria. In order to determine susceptibility against FD, additional cultivar screening was conducted on nine commercially planted cultivars by means of pathogenicity trials using sterilized or inoculated toothpicks inserted into detached branches. Detached branch inoculations showed no significant lesion development six weeks post inoculation, except for cultivar 816. The restricted growth of F. euwallaceae observed in macadamia tissues therefore suggests that macadamia may not be a suitable host for F. euwallaceae and that the threat of FD in macadamia in the event of E. fornicatus infestation is less than for other E. fornicatus hosts. Future work on beetle attraction to macadamia is recommended for a more comprehensive understanding of the interaction between E. fornicatus and its fungal symbionts and macadamia.