Assessing susceptibility of Metrosideros excelsa (pōhutukawa) to the vascular wilt pathogen, Ceratocystis lukuohia, causing Rapid ‘Ōhiʻa death

Metrosideros excelsa (pōhutukawa) is an important native tree species in Aotearoa-New Zealand. Ceratocystis lukuohia is an aggressive wilt pathogen of the Hawaiian forest tree M. polymorpha (‘ōhi ‘a) and could be a threat to pōhutukawa. A host response study was conducted to evaluate the susceptibility of M. excelsa seedlings to C. lukuohia infection. Forty-eight M. excelsa seedlings from six parent trees were inoculated with C. lukuohia. Disease symptoms and mortality were observed in M. polymorpha positive controls inoculated with C. lukuohia, but not in the M. excelsa. Dissections of the inoculated M. excelsa revealed significantly lower disease severity (mean range: 3.9%-9.0%) comparable to a mean of 1.7% for the M. excelsa negative control treatment. In comparison, the mean disease severity for the M. polymorpha positive control plants was 43.3 ± 6.1%. Aleurioconidia were present in the stem tissue of 86–100% of C. lukuohia-inoculated M. excelsa selections compared to 100% of M. polymorpha and 0% of M. excelsa inoculated with sterile water. These results suggest that C. lukuohia can infect M. excelsa if introduced to a wound, but the pathogen is not aggressive enough to cause wilt and mortality. There is potential for wild M. excelsa to become infected with C. lukuohia if a pathway allowed for the accidental introduction of the pathogen into Aotearoa-New Zealand. Differential susceptibility of other M. excelsa genotypes or Metrosideros species also needs to be considered. Therefore, measures should be taken to avoid the introduction of this pathogen into the country.