The Problem: Rapid ‘Ōhiʻa Death
The ‘Ōhiʻa tree is named after a Hawaiian legend that tells of the love and separation of the young couple ʻŌhiʻa and Lehua; Jealous of their love, the goddess Pele turned the warrior ʻŌhiʻa into a tree and Lehua into the tree’s flower. If you pluck the red lehua blossoms of the ʻōhiʻa tree (Metrosideros polymorpha), legend says the lovers’ tears fill the sky with rain as they are separated again.
Today, the tears of many others are being shed over the future of the ʻōhiʻa tree because it is threatened by microscopic fungi that recently invaded the islands of Kaua‘i and Hawai‘i. These invasive fungi, Ceratocystis huliohia and Ceratocystis lukuohia, are responsible for the phenomenon named “Rapid ‘Ōhiʻa Death” or ROD. Since 2014 when first identified, the fungi have infected thousands of acres of forest and if unstopped, could irreversibly change Hawai‘i’s ecosystems and culture by eliminating the beloved ʻōhi‘a.
Currently, the only known visual cue of fungal infection is when a tree’s leaves suddenly turn brown, and the tree begins to die. Current detection methods are labor intensive, frequently requiring sample collection over challenging and rugged terrain, as well as lab analysis, either on or off-site. Field sampling has been limited to trees exhibiting symptoms of infection and mortality, and there is no clear methodology to identify asymptomatic trees (i.e., trees without signs of infection). Once the leaves begin to turn brown, it’s already too late in the infection process to save the tree or prevent spread of the infection.
This fungal pathogen can affect individual trees as well as entire forests. There is a tremendous urgency among land managers, cultural practitioners, and private citizens to halt the spread of the disease. Loss of ʻŌhiʻa would have irreparable effects on the biological diversity, hydrology, cultural traditions, and quality of life in Hawaiʻi. Monetary losses would be in the billions of dollars; cultural losses would be priceless.
Two newly discovered invasive fungal pathogens Ceratocystis lukuohia and Ceratocystis huliohia (formerly Ceratocystis fimbriata), are killing hundreds of thousands of ʻŌhiʻa (Metrosideros polymorpha) on Hawaiʻi Island. First observed in 2010, these fungi are responsible for Rapid ʻŌhiʻa Death (ROD). ROD can affect individual trees and entire forests, but is only known on Hawaiʻi Island, where currently, over 100,000 acres of forests are affected. While there is widespread support for research and management to halt the spread of ROD, many unanswered questions remain. For example, we do not fully understand how trees become infected, or how the disease spreads through forests. Understanding the spread of ROD is critical, yet the difficulty of detecting the fungus presents a significant barrier.
A key hurdle to the effective management of ROD at all scales is the difficulty and high cost of ROD detection efforts. The fungus is invisible, making it impossible to visually track along invasion pathways, including wind currents, soil samples, and possibly rain. Trees may be infectious yet asymptomatic for over a year, and the only visual cue of infection occurs when a tree’s leaves suddenly turn brown, and the tree begins to die as the fungus chokes the tree’s vascular system. At this stage, sampling teams are deployed to collect wood samples from the tree and submit these to a laboratory for analysis. This is often a costly endeavor for trees on remote sites or challenging terrain. When the tree’s leaves turn brown, it’s also too late in the infection process to save the tree and/or prevent spread of the infection. Field sampling has been limited to trees exhibiting symptoms of infection and mortality, and we have no clear methodology to identify asymptomatic trees. We also have very limited tools for detecting fungal spores in environmental pathways such as wind, water, and soil.
Efforts to aggressively manage ROD are critically important to protect Hawaiian ecosystems and cultural traditions. Strong private and public partnerships have been formed to address the threat posed by ROD, and in 2016, a multi-stakeholder strategic response plan was created to outline a unified approach to gather information and manage the disease and its impacts. This plan underscored the need to limit the spread of this fungus and “deploy technologically advanced surveillance, monitoring, and early detection approaches” to protect ecosystems and cultural traditions.
The inability of land managers to detect the ROD fungus at early stages of infection in trees and elsewhere in the environment presents a unique challenge. This challenge is compounded by the vast landscapes on which ʻŌhiʻa trees reside, including remote and often steep areas not easily accessible by foot or vehicle. To address this challenge head on, it is critically important to develop tools that enhance the detection of ROD. Such tools promise to increase our understanding of how the disease spreads and pave the way for eventual elimination of this disease from Hawai'i Island before it can spread to other islands. A solution may be worth billions of dollars to the Hawaiian economy, and contribute immensely to the perpetuation of the cultural values and identity of the Native Hawaiian People.