Rapid ʻŌhiʻa Death (ROD) is a devastating fungal disease rapidly spreading across Hawaiian forests, threatening one of the island chain’s most significant native trees. The ʻŌhiʻa lehua (Metrosideros polymorpha) is endemic to Hawaiʻi and is the most common native tree, making up approximately 80% of the native forests.
It is a pioneer species, often the first to colonize new lava flows and help create soil. The tree’s canopy is integral to watershed health, effectively capturing fog and mist to replenish aquifers. Beyond its environmental function, the ʻŌhiʻa holds deep cultural value; its fiery red blossoms, the lehua, are used in lei making and are sacred to the goddesses Laka and Pele. The emergence of this vascular wilt disease poses a serious threat to this foundational species and the entire native ecosystem it supports.
The Pathogen and Its Origin
The cause of Rapid ʻŌhiʻa Death involves two distinct, non-native fungal species from the Ceratocystis genus. These are Ceratocystis huliohia (“changes the natural state of ʻōhiʻa”) and the more aggressive Ceratocystis lukuohia (“destroyer of ʻōhiʻa”). Genetic analysis suggests both species are recent introductions to Hawaiʻi, lacking the genetic diversity of long-established native populations.
The fungus enters the tree only through an open wound, such as those caused by pruning, strong winds, or feral animals. Once inside, the fungal spores germinate and grow within the sapwood, the outer layer that conducts water. This growth clogs the xylem, the vascular tissue transporting water and nutrients from the roots to the canopy.
The two species cause death differently. C. huliohia is less aggressive, causing cankers beneath the bark that slowly girdle the tree over months or years. C. lukuohia causes a systemic wilt that spreads quickly, choking off the water supply and killing the tree within weeks. This results in a sudden and widespread dieback of the crown.
Identifying the Signs of Infection
The most visible indication of a Rapid ʻŌhiʻa Death infection is the sudden browning of the tree’s canopy. Leaves on a previously healthy tree turn yellow and then quickly brown within days to a few weeks. This characteristic sudden dieback is why the disease is named Rapid ʻŌhiʻa Death.
Unlike the gradual decline seen with other tree ailments, the leaves typically remain attached to the branches after browning, giving the canopy a “crispy” appearance. When the wood of a dead or symptomatic tree is cut open, a dark-brown to black radial staining is often visible in the sapwood. This discoloration confirms the fungus has invaded the vascular tissues. Because the fungus grows inside the wood, confirmation requires laboratory testing of a wood sample, as external symptoms can be confused with other causes of tree death.
Mechanisms of Spread and Transmission
The fungal spores are not actively released from the bark of a live tree but rely on various vectors to spread to a new host. Long-distance transmission primarily occurs through human activity, including the movement of infected ʻōhiʻa wood (firewood or construction materials) and the contamination of tools, vehicles, and clothing.
The Ceratocystis spores survive in the soil and are easily transported in mud carried on shoes, gear, or vehicle tires. Ambrosia beetles attack dead or dying infected trees, creating spore-laden sawdust (frass) as they tunnel. This frass can be carried short distances by the wind. The beetles also carry spores on their bodies, and their boring creates new entry points for the fungus. Feral ungulates, such as pigs and goats, contribute to the spread by wounding trees and moving contaminated soil.
Current Strategies for Containment
Management efforts focus on containing the spread of ROD, as there is currently no cure for an infected tree. Biosecurity protocols are a major strategy component, urging people to clean gear and vehicles thoroughly after leaving a forest. This involves brushing all soil from shoes and tools, spraying them with a 70% rubbing alcohol solution, and washing vehicles to remove clinging mud.
Authorities advise against moving any ʻōhiʻa wood or parts, including flowers and soil, from infected to uninfected areas; inter-island transport is regulated by quarantine rules. Early detection relies on regular aerial surveys using helicopters and drones to spot symptomatic trees, followed by on-the-ground sampling and laboratory testing for confirmation. Researchers are identifying ʻōhiʻa trees with natural resistance to propagate resilient strains for future forest restoration. Targeted treatments, such as insect repellent paste that deters ambrosia beetles, are also being explored to protect individual trees and limit spore spread.