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Rapid ʻŌhiʻa Death

Project Collaborators

  • Christian P. Giardina, Institute of Pacific Islands Forestry, USDA Forest Service
  • Rich Sniezko, Dorena Genetic Resource Center, USDA Forest Service
  • Lisa Keith, Daniel K. Inouye Pacific Basin Agricultural Research Center, USDA Agricultural Research Service
  • Blaine Luiz, Daniel K. Inouye Pacific Basin Agricultural Research Center, USDA Agricultural Research Service
  • James B. Friday, Department of Natural Resources and Environmental Management, University of Hawai‘i, Mānoa
  • Rob Hauff, Hawai‘i Division of Forestry and Wildlife
  • Douglass F. Jacobs, Department of Forestry and Natural Resources, Purdue University
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‘Ōhi‘a(Metrosideros polymorpha) is Hawaii’s most abundant native tree and the most important ecological and cultural keystone species in Hawaiian forests. A recently introduced fungal disease, caused by the vascularwilt fungus, is creating widespread mortality in ‘ōhi‘a. Healthy trees often die within a few weeks after infection and so the disease is known as Rapid ‘Ōhi‘aDeath (ROD). The disease has potential to kill ‘ōhi‘a trees throughout the Hawaiian Islands and its spread is being closely monitored. The loss of ‘ōhi‘a due to ROD is an unprecedented threat to the ecological integrity of native Hawaiian forests and culture.

There is wide genetic variation in ‘ōhi‘a and multiple varieties of M. polymorpha are recognized (newellii, incana, glaberina, macrophylla, polymorpha, pumila, psuedorugosa, and tremuloides). Even in heavily infected stands, some individual ‘ōhi‘a trees do not contract the disease. If genotypes that show resistance to ROD can be identified, then this may form the basis for a disease resistance improvement program to restore large tracts of ‘ōhi‘a forest impacted by ROD. The genetic basis for disease resistance by ‘ōhi‘a against ROD is poorly understood. Preliminary research (Luiz 2017) shows wide variability in susceptibility of ‘ōhi‘a genotypes to ROD, yet only across a limited set of genotypes.

Our goal in this project is to continue to quantify the presence and strength of resistance for various ‘ōhi‘a genotypes on Hawai‘i Island. This knowledge will help to inform restoration strategies including decisions about how much ‘ōhi‘a should be considered for restoration, and if so which genotypes. To accomplish this goal, we will 1) identify disease resistant genotypes of ‘ōhi‘a; and 2) challenge these genotypes through established disease exposure protocols (Showalter et al. 2018) to identify genotypes that show resistance to ROD. The use of such resistance screening assay has been key in other resistance programs (e.g., Phytophthora root disease in Port-Orford-cedar, white pine blister rust). Especially important in this early phase work is refinement of a quick greenhouse assay to find genetically resistant parents and to improve ‘ōhi‘a greenhouse culture to speed up assay timelines (Sniezko and Koch 2017).

Trees that have been identified as ROD resistant will be planted in common gardens for second generation progeny research. From this work, genotypes with putative resistance will be planted into seed orchards in year three for follow up work to identify whether resistance is heritable. Concurrently, we will support established and ongoing M. polymorpha conservation work (state-wide seed collection and banking) with information on genotypes showing resistance (Sniezko and Koch 2017). This research will lead to improved understanding of ‘ōhi‘a disease resistance, identify the most resistant genotypes for propagation, and create a source of disease-resistant trees that will form the foundation for a disease resistance improvement program (Sniezko and Koch 2017) to optimize the restoration of ROD-impacted Hawaiian forests.

References

Luiz, B. 2017. Understanding Ceratocystis Species A: Growth, Morphology, and Host Resistance. MS Thesis, University of Hawai‘i at Hilo. 63 p.

Showalter D.N., K.F. Raffa, R.A. Sniezko, D.A. Herms, A.M. Liebhold, J.A. Smith, P. Bonello. 2018. Strategic development of tree resistance against forest pathogen and insect invasions in defense-free space. Frontiers in Ecology and Evolution 6:124.

Sniezko, R.A. and J. Koch. 2017. Breeding trees resistant to insects and diseases: putting theory into application. Biological Invasions19:3377–3400.