Pathways for Movement and Rate of Spread of Rapid ‘Ōhi‘a Death on the Island of Hawai‘i

Science Center Objects

Rapid ‘Ōhi‘a Death (ROD) is an emerging and rapidly spreading disease of ‘ōhi‘a (Metrosideros polymorpha), a keystone native forest tree in the Hawaiian Islands. The disease is highly pathogenic in native ‘ōhi‘a and can lead to significant mortality once symptoms become evident. This emerging pathogen is a significant threat to native forests throughout the state because of its potential impacts in high value conservation areas and watersheds managed by Department of Interior agencies, the State of Hawai‘i, and State Watershed Partnerships. In response to this threat, the Hawai‘i Department of Agriculture has recently placed an embargo on movement of ‘ōhi‘a and ‘ōhi‘a products off of Hawai‘i Island to reduce the risk of spread of the infection.

‘Ōhi‘a in bloom

An ‘Ōhi‘a tree covered in bright red lehua flowers in the Kahuku Unit of Hawai‘i Volcanoes National Park. ‘Ōhi‘a trees are one of very few native plants that dominate the forests of Hawai‘i. They are currently threatened by Rapid ‘Ōhi‘a Death, a fungal disease that is spreading across the island of Hawai‘i. (Credit: Christina Leopold, Hawai‘i Cooperative Studies Unit. USGS image)

Overview:

The disease is caused by two members of the Ceratocystis fimbriata species complex, a widespread group of closely related fungal pathogens that are incompletely characterized (Oliveira et al. 2015). Two species of fungi are known to cause infections commonly referred to as Rapid ‘Ōhi‘a Death (ROD) because of rapid progression of the disease once symptoms become evident. Both species of fungi have been detected alone and in combination in native ‘ōhi‘a trees from Hawai‘i Island. Both strains are currently believed to threaten native ‘ōhi‘a forests and trigger the same management actions. The precise pathways for both natural and human associated spread of the infection are still poorly documented, but include movement of infected soil and plant products, use of contaminated tools and equipment for landscaping, and possible spread on contaminated footwear and vehicles. Primary mechanisms of natural spread are still poorly understood, but likely involve production and release of frass (excrement of wood-boring beetles) containing infective fungal spores (aleurioconidia) from infected trees by wood boring beetles and spread of infective aleurioconidia via wind and flowing water. Birds may also contribute to spread of the pathogen by mechanically moving spores either on their plumage or in feces after consumption of infected beetles, but this has not been investigated.  

An ohia tree log showing tunnels created by wood boring beetles

Wood boring beetle tunnels inside of an ‘ōhi‘a log. Researchers are investigating if the spread of beetle frass (excrement) is a pathway for movement of a fungus that causes Rapid ‘Ōhi‘a Death.(Credit: Carter Atkinson, USGS. Public domain.)

Resources Management staff at Hawai‘i Volcanoes National Park (HAVO) are responding aggressively to detection of symptomatic infected trees by cutting and covering them with tarps to reduce potential spread of infective beetle frass, but the effectiveness of this approach is still largely unknown. Early detection and removal of infected trees before they become sources of infective spores is particularly important because it may slow spread of the infection. We will use a combination of sensitive diagnostic assays recently developed by colleagues at USDA-Agricultural Research Service (qPCR targeting fungal ceratoplantanin gene) and our laboratory (recombinase polymerase assay targeting fungal ribosomal ITS region) and newly developed environmental sampling devices for capturing wind-blown spores to monitor spread of the pathogen, determine pathways for both natural and human-mediated spread, and monitor effectiveness of management actions developed by HAVO management staff. Because this is a rapidly emerging issue with no clear guidelines for protecting native forests, our efforts will be focused on providing up-to-date information about spread of the infection, assisting resource managers to identify high risk areas for spread of the disease, and assisting resource managers monitor effectiveness of evolving management actions.

Project Objectives:

A spore trap deployed in the Kahuku Unit of Hawai‘i Volcanoes National Park to monitor for Rapid ‘Ōhi‘a Death

A spore trap deployed to monitor for Rapid ‘Ōhi‘a Death in the Kahuku unit of Hawai‘i Volcanoes National Park. (Credit: Carter Atkinson, USGS. Public domain.)

Objective 1: Measure rate of spread along the eastern boundary of Hawai‘i Volcanoes National Park that are adjacent to infected forests of Lower Puna.  

Current efforts to monitor spread of the infection are based on repeated aerial surveys by helicopter to identify trees in later stages of infection where leaves have started to turn brown. While this approach can provide a measure of changing patterns of ‘ōhi‘a mortality over time, it does not have the resolution that is needed to distinguish Ceratocystis infections from other causes of death that may mimic the symptomology of the disease.

We have proposed to coordinate with aerial surveys to help locate and test symptomatic trees to confirm infection with Ceratocystis and to also develop a network of “sentinel” ‘ōhi‘a trees that can be visited by foot and monitored and tested on a regular schedule to detect asymptomatic early infections and to determine incidence of infection or numbers of new cases per unit time. This information is essential for making accurate estimates of how rapidly the disease is moving.

Objective 2: Determine pathways for natural and human mediated dispersal of the fungus

Ceratocystis produces spores that can be moved through contact with insects and possibly birds or are moved to the outside of infected trees in the frass (excrement) of wood-boring beetles where they can become either airborne or subject to passive spread in contaminated soil, on footwear, vehicles or landscaping tools. Both types of spores can produce new infections by entering through wounds in healthy trees that may occur naturally as a result of wind and insect damage.

Geneticist in lab using a pipette to extract a sample from a centrifuge tube

Geneticist, Kylle Roy, extracts DNA from samples to detect Ceratocystis, a fungal pathogen that causes Rapid ‘Ōhi‘a Death. Samples have been acquired using aerial spore traps, by collecting wood shavings from‘ōhi‘a trees, or by collecting frass (excrement) of wood-boring beetles.(Credit: USGS. Public domain.)

We will investigate possible pathways of movement of the fungus by establishing a network of passive “spore” traps based on capture of windblown spores and beetle frass. The number of detections/unit time and location can be used to map high risk areas for new infections. Preliminary sampling with these traps indicates that they are capable of collecting particulate matter that tests positive for Ceratocystis by qPCR. Efforts to actually culture the fungus from these samples are currently underway.

Objective 3: Evaluate effectiveness of management actions for controlling spread of the fungus

The best available guidance about reducing spread of Ceratocystis is to aggressively locate, cut, and cover infected trees to prevent infective beetle frass from entering the air column and moving by wind to new locations. The effectiveness of this approach has not been proven by pre- and post-removal monitoring. We plan to work closely with resource managers to monitor air samples before, during and after tree felling to determine if there are decreases in numbers of airborne spores after tree removal,  intensively monitor nearby trees for evidence of new infections, and also monitor wood samples from trees that have been cut and covered by qPCR and culture methods to determine how long viable spores can persist.

Objective 4: Early detection of incipient infections in on Maui and Moloka‘i.

Early detection and management of trees infected with Ceratocystis on other islands is essential for preventing establishment of the disease throughout the Hawaiian archipelago.  We propose to work with resource managers at Haleakalā and Kalaupapa National Historic Parks on Maui and Moloka‘i and the Maui and Big Island Invasive Species Committees to coordinate laboratory testing of suspect trees with an early warning network of passive air samplers to detect airborne spores. We will also be working closely with resource managers on state and private lands,  colleagues at USDA-Agricultural Research Service and USDA-Forest Service, and a newly funded Early Detection and Rapid Response Team on Hawai‘i Island to assist with sample and environmental testing. We hypothesize that environmental monitoring with spore traps may be more effective at detecting incipient infections in remote locations where aerial and ground surveys are infrequent or difficult.

Highlights and Key Findings:

Research is ongoing. Work to date has focused primarily on environmental monitoring of air samples to determine whether movement of windblown frass/spores occurs. Two studies are underway to assess short and long distance transport of the fungus.

A portable lab or "Lab in a Suitcase" has been developed to allow for rapid detection of the fungal pathogen in the field. Samples can be collected from trees and tested on site with results in approximately an hour.  The "Lab in the Suitcase" is already being used by the Big Island Invasive Species Committee to detect infected trees. A full report on the methods and field application is available.

Two technicians sit at table using lab equipment to extract DNA

Partners with the Big Island Invasive Species Committee in training at the USGS Kilauea Field Station to use the "Lab in a Suitcase" to test samples for Ceratocystis, a fungal pathogen that causes Rapid ‘Ōhi‘a Death.(Credit: USGS. Public domain.)