Image above courtesy RapidOhiaDeath.org Distribution page, accurate June 2020 for island of Hawai’i only.
Rapid ‘Ōhi‘a Death encompasses two newly-described diseases, Ceratocystis wilt of ‘ōhi‘a (caused by the pathogenic fungus Ceratocystis lukuohia) and Ceratocystis canker of ‘ōhi‘a (caused by the pathogenic fungus Ceratocystis huliohia). Both fungal species are newly named and were formerly grouped into the species Ceratocystis fimbriata sensu lato.
(‘Ōhi‘a trees are also under threat from ʻōhiʻa or Austropuccinia rust, which has been on all the major Hawaiian islands since 2005; please see separate Gallery write-up on ʻōhiʻa or Puccinia rust here.)
Beginning in 2010, a new, virulent disease has threatened Hawai`i’s most widespread and common tree, ʻōhiʻa lehua (Metrosideros polymorpha) on Hawai‘i Island. Dead and infested trees are now found in most districts of the island and the total area affected has reached 135,000 acres, although most of the total ʻōhiʻa forest is still healthy. Nevertheless, the disease’ rapid spread still causes concern (Friday in West Side symposium). Levels of damage vary; forests in drier environments or at higher elevations (e.g., Volcano, on the border of Hawaii Volcanoes National Park) seem to be less susceptible to the disease (Friday in West Side symposium). Forest managers are now estimating that the disease has killed over a million trees over 180,000 acres of affected forest.
In May 2018, scientists detected the less virulent form of the pathogen, Ceratocystis huliohia (see below), on a second island, Kaua‘i. Several dead ʻōhiʻa trees were found in the Moloa‘a Forest Reserve on Kaua‘i’s remote northeastern side. Scientists do not yet know what impact it will have. https://governor.hawaii.gov/newsroom/latest-news/dlnr-news-release-ohia-fungus-discovered-on-kauai-may-11-2018/
In 2019, infected trees (C. huliohia) were confirmed on Maui and O’ahu. The single infected tree on Maui was destroyed and no further infected trees were located. Five trees on O’ahu have been found to be infected to date (accessed June 2020).
The cause was initially identified by the USDA Agriculture Research Service (ARS) as a new strain of Ceratocystis fimbriata, a vascular wilt fungus (Keith et al. 2015). Further genetic and morphological analysis revealed that the disease is caused by two previously unknown fungal species in the same genus: Ceratocystis lukuohia (which means “destroyer of ‘ōhi‘a” in the Hawaiian language) and C. huliohia (which means “overturns ‘ōhi‘a” in the Hawaiian language). C. lukuohia causes a true vascular wilt and is more virulent; C. huliohia causes cankers which can gradually girdle and kill trees. While the origins of these fungi are not known, C. lukuohia is most closely related to species found in the Caribbean and Latin America; while C. huliohia is most closely related to Asian species. Neither of these new species evolved from C. fimbriata already on islands (Barnes et al. 2018). Research on these fungi continues.
Upon discovery of the disease, the Hawai ‘i Department of Agriculture quickly adopted an emergency quarantine prohibiting movement from the Big Island of ʻōhiʻa lehua flowers, leaves, twigs, and wood (all plants and parts), mulch, and greenwaste of any species in the same genus as ʻōhiʻa (Metrosideros) except under terms of a permit issued by the Department. Soil was added to the quarantine in January 2016. https://hdoa.hawaii.gov/blog/main/ohiaquarantine/ and https://hdoa.hawaii.gov/blog/main/nr-rodsoil/
The origins of these pathogens are still unknown. However, C. lukuohia is similar to Latin American clades and C. huliohia is similar to Asian-Australian clades. This fact suggests that they were separately introduced to Hawai`i. Both are now spread throughout the islands of Hawai`i and Kaua`i. The less virulent C. huliohia has been detected on O`ahu and Maui.
‘Ōhi‘a trees overwhelmingly dominate approximately 80% of Hawai`i’s remaining native forest. Loss of ʻōhiʻa lehua could result in significant changes to the structure, composition, and potentially, the function, of forests on a landscape level. ‘Ōhi‘a forests are home to the Islands’ one native terrestrial mammal (Hawaiian hoary bat) and 30 species of forest birds – especially the unique honeycreeper endemic subfamily. Eighteen of 19 extant Hawaiian honeycreepers in the main Hawaiian islands, including 12 of 13 species listed as endangered by the U.S. Fish and Wildlife Service, depend on ‘ōhi‘a for critical habitat (Loope and LaRosa 2008). Increased light reaching the forest floor following canopy dieback would increase the likelihood of invasion by light-loving non-native species, of which Hawai`i has dozens. Loss of ʻōhiʻa would thus also damage habitat for one-third to one-half of Hawai`i’s approximately 300 endangered plant species (Loope and LaRosa 2008) through encouraging non-native competitors and changing understory environmental conditions.
‘Ōhi‘a also has significant cultural values to the Hawaiian people through its connection to the deities Ku, Pele (volcanoes) and Laka (hula) (Loope and LaRosa 2008) and use in lei for hula.
‘Ōhi‘a can become infected only if the fungus can enter through a wound. Such wounds can be caused by damage resulting from wind, animals (especially domestic or feral ungulates which strip bark from the trees), construction equipment, and pruning.
Dispersal pathways for the two Ceratocystis are unknown. The leading hypothesis is through movement of contaminated ambrosia beetle frass. However, there was little data to support this theory. Viable spores of Ceratocystis lukuohia have been detected in beetle frass. Frass can be moved by wind or in contaminated soil, or on machinery used to cut ʻōhiʻa. Fungal spores can also be moved directly on infected cutting tools. The ambrosia beetles themselves may also directly transmit the disease if they attack healthy trees. Scientists from the U.S. Geological Service are intensively studying the ambrosia beetles associated with ʻōhiʻa to determine their role in spreading the disease. Lastly, human transport of infected wood in the form of posts, poles, or firewood can probably also spread of the disease.
Atkinson and Roy (2023) tested use of a network of passive environmental samplers to detect the presence of these pathogens through measuring airborne environmental DNA (eDNA). They found that passive sampling of spores transported by air currents is not an effective method to monitor the presence of Ceratocystis lukuohia or C. huliohia. C. lukuohia was detected a total of three times at three locations. Two of the detections might have been the result of spores transported on vehicles. C. huliohia was not detected at all.
Atkinson and Roy list several reasons why long-distance aerial dispersal of Ceratocystis spores is unlikely. First, the fungi are probably better adapted to dispersal by insect vectors. The spores’ size and intimate association with the wood would lead to rapid falling out of the air column. And the spores cannot infect a tree in the absence of a wound. They recommend further study of the possible role of ambrosia beetles, feral ungulates, and contaminated tools and vehicles.
As far as is known now, the two fungi attack only ʻōhiʻa.
Considerable effort has been put into mapping the spread of the disease. These efforts have used a wide range of tools, including annual airborne observation from fixed-wing planes (Asner et al. 2018, Vaughn et al. 2018) and bi-annual helicopter surveys across the entire Big Island. Drones are proving quite useful for surveying areas as big as 200 acres – especially since the terrain is usually very rough. Drones can pinpoint suspect trees’ location so scientists doing the essential ground-based sampling can find them most easily. Drone-based surveys can also track individual trees’ decline over time and facilitate analysis of links between the disease and land use, environmental conditions, and other factors.
These surveys have shown that following high levels of tree mortality during 2013-2015, tree death is still increasing but not at as great a rate. Perhaps conditions during the earlier period were very conducive to infection – including widespread damage to trees in late 2014 caused by hurricane Iselle and high levels of frass blowing in high winds. Despite the slow-down, scientists and conservationists are worried that the disease has spread so fast and continues to infect additional trees.
Currently, there is no effective treatment to either protect ʻōhiʻa trees from becoming infected or to cure trees that exhibit symptoms of the disease (College of Tropical Agriculture 2015). Arborists manage Dutch Elm disease, a similar disease, by injecting fungicides regularly into infected trees. It is as yet unknown whether similar injections of fungicides could arrest the development of disease in infected ʻōhiʻa trees. Experience with other similar diseases indicates that the protection might be effective for 1 – 2 years and that the tree can be re-injected repeatedly. This procedure might be able to protect a tree during high infection periods. However, such injections create wounds and weaken the tree (Hughes 2018).
Scientists at the USDA ARS and the University of Hawai‘i have detected promising indications that some ʻōhiʻa trees might have genetic resistance to the pathogens (Luiz 2017). Scientists have formed a subgroup to pursue research into defining levels of resistance and integrating that resistance into programs to breed trees for reforestation efforts.
To reduce the spread of Ceratocystis, officials urged landowners to avoid transporting wood of affected ʻōhiʻa trees to other areas and to clean pruning tools, chain saws, vehicles, and shoes used off-road in infected forest areas (College of Tropical Agriculture 2015).
USFS scientists and managers developed a conservation priority-setting framework for forest tree species at risk from pest & pathogens and other threats. The Project CAPTURE (Conservation Assessment and Prioritization of Forest Trees Under Risk of Extirpation) uses FIA data and expert opinion to group tree species under threat by non-native pests into vulnerability classes and specify appropriate management and conservation strategies. The scientists prioritized 419 tree species native to the North American continent. The analysis identified 15 taxonomic groups requiring the most immediate conservation intervention because of the tree species’ exposure to an extrinsic threat, their sensitivity to the threat, and their ability to adapt to it. Each of these 15 most vulnerable species, and several additional species, should be the focus of both a comprehensive gene conservation program and a genetic resistance screening and development effort. Rapid ‘Ōhi‘a death is not known to be a threat to any of the 15 most vulnerable species.
Sources
Asner, G.P, R.E. Martin, L.M. Keith, W.P. Heller, M.A. Hughes, N.R. Vaughn, R.F. Hughes, and C. Balzotti. 2018. A spectral mapping signature for the Rapid Ohia Death (ROD) pathogen in Hawaiian forests. Remote Sensing 10:404 – 428. doi: 10.3390/rs10030404
Atkinson, C.T. and K. Roy. 2023. Environmental monitoring for invasive fungal pathogens of ‘Ōhi‘a (Metrosideros polymorpha) on the Island of Hawai`i. Biological Invasions (2023) 25:399–410
https://doi.org/10.1007/s10530-022-02922-3
Barnes, I., A. Fourie, M.J. Wingfield, T.C. Harrington, D.L. McNew, L.S. Sugiyama, B.C. Luiz, W.P. Heller, and L.M. Keith. 2018. New Ceratocystis species associated with rapid death of Metrosideros polymorpha in Hawai‘i. Persoonia 40: 154-181. https://doi.org/10.3767/persoonia.2018.40.07
College of Tropical Agriculture & Human Resources, Hawaiian Forestry Pests and Diseases. 2015. Rapid ʻŌhiʻa Death | Ceratocystis Wilt of ʻŌhiʻa https://www2.ctahr.hawaii.edu/forestry/disease/ohia_wilt.html Accessed August 24, 2015
Hughes, M. 2018 Rapid ʻŌhiʻa Death Symposium -West Hawaiʻi March 3rd 2018, https://vimeo.com/258674532 Accessed April 4, 2018 (see also full video archive at https://vimeo.com/user10051674)
Friday, J. B., L. Keith, and F. Hughes. 2015. Rapid ‘Ōhi‘a Death (Ceratocystis Wilt of ‘Ōhi‘a). PD-107, College of Tropical Agriculture and Human Resources, University of Hawai‘i, Honolulu, HI. URL: https://www.ctahr.hawaii.edu/oc/freepubs/pdf/PD-107.pdf Accessed April 3, 2018.
Friday, J.B. 2018. Rapid ʻŌhiʻa Death Symposium -West Hawaiʻi (“West Side Symposium”) March 3rd 2018, https://vimeo.com/258704469 Accessed April 4, 2018 (see also full video archive at https://vimeo.com/user10051674)
Loope, L. and A.M. LaRosa. 2008. ‘Ohi’a Rust (Eucalyptus Rust) (Puccinia psidii Winter) Risk Assessment for Hawai`i
Luiz, B.C. 2017. Understanding Ceratocystis species A: Growth, morphology, and host resistance. MS thesis, University of Hawai‘i at Hilo.
Potter, K.M., Escanferla, M.E., Jetton, R.M., Man, G., Crane, B.S., Prioritizing the conservation needs of US tree spp: Evaluating vulnerability to forest insect and disease threats, Global Ecology and Conservation (2019), doi: https://doi.org/10.1016/
Vaughn, N.R., G.P. Asner, P.G. Brodrick, R.E. Martin, J.W. Heckler, D.E. Knapp, and R.F. Hughes. 2018. An approach for high-resolution mapping of Hawaiian Metrosideros forest mortality using laser-guided imaging spectroscopy. Remote Sensing 10: 502-519. Doi: 10.3390/rs10040502