Assessing susceptibility of Metrosideros excelsa (pohutukawa) to the vascular wilt pathogen, Ceratocystis lukuohia, causing rapid ‘Ohi‘a death

Authors: LUIZ, BLAINE - Akaka Foundation For Tropical Forests; MCNEILL, MARK - Agresearch; BODLEY, EMMA - Auckland Botanic Gardens; Keith, Lisa

Submitted to: Australasian Plant Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/23/2022
Publication Date: 3/8/2022
Citation: Luiz, B., McNeill, M., Bodley, E., Keith, L.M. 2022. Assessing susceptibility of Metrosideros excelsa (pohutukawa) to the vascular wilt pathogen, Ceratocystis lukuohia, causing rapid ‘Ohi‘a death. Australasian Plant Pathology. 51:327-331. https://doi.org/10.1007/s13313-022-00858-9.
DOI: https://doi.org/10.1007/s13313-022-00858-9

Interpretive Summary: The rapid and widespread mortality of M. polymorpha is called Rapid 'Ohi'a Death and is a significant threat to the ecological, cultural, and economic systems supported by the species. While the effects of C. lukuohia on M. polymorpha biology and ecology are being studied, little is known about its effects on other Metrosideros species in Hawai'i and throughout the Pacific. Pohutukawa (Metrosideros excelsa) has been described as one of the best-known and best-loved native trees in Aotearoa-New Zealand. To determine if M. excelsa is susceptible to ROD, seeds were collected in and around Auckland, NZ and sent to USDA ARS in Hilo, HI to be propagated and tested. Pathogenicity testing results suggested that C. lukuohia, the main vascular wilt pathogen causing ROD in Hawai'i, can infect M. excelsa if the fungus is introduced to a wound, though the pathogen may not be aggressive enough on this host to cause wilt and mortality. However, measures should be taken to avoid the introduction of this pathogen into New Zealand.

Technical Abstract: Metrosideros excelsa (pohutukawa) is a culturally, ecologically, and economically important tree species in New Zealand. Ceratocystis lukuohia is an aggressive wilt pathogen of the Hawaiian forest tree M. polymorpha and could potentially be a threat to pohutukawa, though little is known about the host range of the pathogen. In this study, pathogenicity experiments were undertaken to evaluate the susceptibility of M. excelsa seedlings to C. lukuohia infection. In total, forty-eight M. excelsa seedlings from six seed families were inoculated with C. lukuohia. Disease symptoms and mortality were observed in M. polymorpha positive controls inoculated with C. lukuohia, but not in any of the M. excelsa. Dissections of the inoculated M. excelsa revealed significantly lower disease severity (3.88-8.98) compared to the M. polymorpha positive control plants (43.25) and were comparable to the M. excelsa plants inoculated with the sterile water negative control treatment (1.67). Aleurioconidia were present in the tissues of 86-100% of C. lukuohia-inoculated M. excelsa compared to 100% of M. polymorpha and 0% of M. excelsa inoculated with the negative control. These results suggest that C. lukuohia can infect M. excelsa if propagules are introduced to a wound, though the pathogen is not aggressive enough on this host to cause wilt and mortality. There is potential for wild M. excelsa to become infected with C. lukuohia if it is introduced to New Zealand, and lineages of the pathogen that are more aggressive or that can infect new hosts present in the country could arise over time. Therefore, measures should be taken to avoid the introduction of this pathogen into New Zealand.