Rapid ‘Ohia Death Threatens the Forest

‘Ohia flowers on the Big Island. Photo by J. B. Friday, University of Hawai‘i

What does a Big Island tree fungus have to do with Kaua‘i? A great deal, where the iconic native ‘ohia tree (Metrosideros polymorpha) is concerned. This endemic tree and the entire forest ecosystem it supports is under threat from rapid ‘ohia death, or ‘ohia wilt.

On the Big Island, numerous adult ‘ohia trees have died in the last two years from a recently discovered fungus called Ceratocystis. It has affected more than 6,000 acres from Kalapana to Hilo, with tree stands showing greater than 50 percent mortality.

The significance of the ‘ohia forest to the Hawaiian Islands cannot be overstated.

The Ceratocystis fungus produces radial stains in the sapwood of infected trees. This sample later was shown to have the disease. Photo by J. B. Friday, University of Hawai‘i

The tree is beloved for its lehua blossom and has many cultural uses, including construction of houses and canoes as well as use of the blossom in leis. The Hawaiian legend of ‘ohia and lehua, a story of two lovers who were changed by the Goddess Pele into tree and blossom is well known. It is the dominant tree in Hawai‘i’s native forests, comprising approximately 80 percent of forest stand.

“If we lose ‘ohia, we’ve lost the forest,” says scientist JB Friday, of the University of Hawai‘i College of Tropical Agriculture on the Big Island. This could mean the loss of habitat for native songbirds, watershed health, and the entire forest structure; so all efforts are being put towards controlling the disease’s spread.

Ceratocystis wilt causes part of the crown or the entire crown of the tree to die quickly, leaving dead leaves on the branches for a few weeks. The branch on the center shows typical symptoms and is probably infected with Ceratocystis; the branch on the left is just beginning to show symptoms as the leaves are turning yellow, and the branch on the right is healthy now but will likely die within a few weeks. Photo by J. B. Friday, University of Hawai‘i

‘Ohia wilt, first observed a couple years ago in Puna and Hilo districts, kills trees within weeks. The symptoms include tree crowns turning yellowish and then brown within days to weeks.

Scientists J.B. Friday, Lisa Keith and Flint Hughes have been working to identify the cause. Last Spring, Keith, a pathologist from the United States Department of Agriculture, revealed through DNA analysis the new Ceratocystis pathogen. This fungus’ signature is a dark, nearly black, staining in the sapwood along the outer margin of the tree trunk that is often radially distributed throughout the wood.

The disease jumps from forest to forest in a non-contiguous fashion, begging the question of how it spreads. They have ruled out air dispersal, and are starting to do data collection on insects to see if they may be the culprits. Another potential cause being investigated is mud dispersal, from vehicles as an example, since the fungus has been found in mud samples.

A healthy ‘ohia in Koke‘e. Photo by J. B. Friday, University of Hawai‘i

In the meantime, Friday cannot stress this point enough: Do not move ‘ohia around, either alive or dead, including seedlings, dead trees, posts, poles, flooring, firewood, etc.

Put simply, do not bring ‘ohia from the Big Island to Kaua‘i or any other island.

Kaua‘i’s Amanda Skelton from Plant Pono emphasizes how important it is to be vigilant on Kaua‘i to ensure the disease does not spread to our forests.

Because it is unknown how the disease is being transmitted, it is difficult to track its movement. Therefore, early detection is imperative. The Hawai‘i Department of Agriculture has agreed to take samples on Kaua‘i if any indication of the disease is found. If you see evidence of the disease on ‘ohia, it should be reported quickly and properly to the plant quarantine office in Lihu‘e at 241-7135.

Ruby Pap

All this is also a good reminder to be mindful of how we conduct ourselves in our sensitive native forests. Shoes, tools and tires can easily spread plant seeds, fungi, pathogens, etc. While we don’t know exactly how ‘ohia wilt is spreading, as a precautionary measure clean your gear before entering the forest. As Skelton says, “you never know when next pest will come in, what it will be, and how it will move.”

Visit ohiawilt.org for more information.

• Ruby Pap is a Coastal Land Use Extension Agent at University of Hawai‘i Sea Grant College Program. She can be reached at rpap@hawaii.edu.