First report of Ceratocystis huliohia causing mortality of Metrosideros polymorpha trees on the Island of Kauaʻi,Hawaiʻi USA

The fungus Ceratocystis huliohia was detected for the first time in dead and dying Metrosideros polymorpha trees on the Island of Kaua?i, Hawai?i, U.S.A. Fungal isolates were recovered from diseased trees into culture, and identities were confirmed by diagnostic qPCR assay and DNA sequencing. Growth chamber inoculations confirmed C. huliohia pathogenicity on M. polymorpha seedlings and subsequently the same fungus was recovered, completing Koch's postulates. DNA sequence analysis revealed a unique ITS haplotype for Kaua?i isolates of C. huliohia that has not been found on Hawai?i Island.     

Screening of Metrosideros polymorpha (‘ōhi‘a) varieties for resistance to Ceratocystis lukuohia

Stands of the landscape‐dominant tree, Metrosideros polymorpha (‘ōhi‘a) on the Island of Hawai‘i, HI., USA, are dying from a phenomenon known as rapid ‘ōhi‘a death (ROD). Approximately 180,000 acres of forest have already been impacted by the disease, the majority of which is attributed to C. lukuohia, the more aggressive of the two Ceratocystis species responsible for ROD. Three isolates of C. lukuohia were compared for differences in aggressiveness. Mean disease severity and mean number of days to death did not differ among the three isolates of C. lukuohia. A single isolate was selected for use in a resistance screening of 128 plants across four varieties of M. polymorpha occurring on Hawai‘i Island: incana, glaberrima, polymorpha and newellii. Disease severity was lower in early‐successional var. incana (38.84%) and riparian var. newellii (36.11%) compared to late‐successional var. glaberrima (61.27%) and high‐elevation var. polymorpha (70.27%). Var. incana and var. newellii also had the lowest mortality (63% and 77%, respectively) while var. glaberrima (86%) and var. polymorpha (100%) had the highest mortality. Eighty‐five per cent of the plants that died did so within 49 days post‐inoculation and mean number of days to death was significantly higher for var. glaberrima (46) compared to var. polymorpha (31). The results of this preliminary study suggest that some form of resistance might be present in natural populations of at least one variety of M. polymorpha on Hawai‘i Island and should be investigated more intensively.