Hawaiian Islands and Rapid Ohia Death

When visiting the beautiful Hawaiian islands, it does not take long to understand the great importance of ohia trees to the islands. Not only do these hardwoods have significant beauty, but they also have great cultural, historical, spiritual, ecological, and economic value to the islands and the people of Hawaii.  For these reasons, Adjunct Associate Professor Jenny Juzwik is collaborating with scientists from the USDA-ARS PBARC and the University of Hawaii – Hilo and Manoa to find ways to protect these beloved trees from recent losses experienced from Rapid Ohia Death (ROD).

ROD-affected ohia trees in forest landscape near Hilo, Hawaii

Ohia lehua is the most abundant and ecologically important native tree species of Hawaii and holds significant biological and cultural value. The wood of ohia is also highly-valued and supports a local economy for its use as decorative poles and posts in buildings and other structures.

Unfortunately in recent years, the ohia tree populations on the Big Island have faced increased losses from two new fungal diseases. ROD is the general term used to describe these two new diseases that have killed hundreds of thousands of ohia lehua trees (Metrosideros polymorpha) on Hawaii Island since its first discovery in 2014. Initial reports of ROD in 2014 documented 16,000 acres of affected forest in the southern districts of Puna and Hilo on the east side of the island. Now ROD is known to occur on over 75,000 acres on all corners of the island.

Originally little was truly known about ROD from a biological standpoint, and it was not even certain what the disease-causing organisms were. During Juzwik’s first field introduction to ROD in January 2017, it was apparent that at least two different diseases were present – and classified as ROD. Since January, Juzwik and colleagues in Hilo, HI (USDA-ARS PBARC and University of Hawaii – Hilo and Manoa) have conducted field studies that are the basis of the first full descriptions of the two new Ceratocystis diseases.  Two, “new to science” Ceratocystis species currently known as Ceratocystis A and Ceratocystis B were classified as being responsible for ROD,  a systemic wilt and a canker disease, respectively of this keystone forest species.

While the species have been identified, new treatment methods and management strategies are needed, as there are currently limited options available.

Plant Pathology Adjunct Associate Professor and Graduate Student Kat Sweeney

Adjunct Associate Professor Jenny Juzwik (back) and graduate student Kat Sweeney (front)

“For roundwood, the only treatment recognized, but not well-tested, is to subject wood to kiln drying at 133 °F.  New treatment methods are needed to kill the pathogen in firewood, logs, poles and posts to prevent further movement of the pathogen on Hawaii Island and prevent human-aided introduction to other islands. Strategies and tools for managing the pathogen in ohia forests and urban trees are also urgently needed,” says Juzwik.

HI colleagues and Juzwik have initiated studies to evaluate different methods for their ability to kill the Ceratocystis pathogens in wood originating from ROD-killed trees. Methods under investigation or being considered include: dip diffusion of poles and posts with borate and quaternary ammonium compounds, kiln-heating, and vacuum steam treatment. Systemic fungicide injections of living ohia are also being considered for preventative or therapeutic treatment.  

Of course, much more basic knowledge about the two diseases is needed to support development of other disease control strategies. One of Juzwik’s graduate students, Kat Sweeney, recently initiated studies on tissue-level colonization of ohia wood by each Ceratocystis pathogen using light and fluorescent microscopy techniques.  In addition, Sweeney will investigate responses of ohia to infection by the two pathogens using histochemical and microscopic techniques. Results of her studies will complement recently completed greenhouse seedling trials by a University of Hawaii graduate student who documented relative susceptibility of the five varieties of ohia found on Hawaii Island.  

Other plant pathology colleagues from Hawaii, Iowa State University, and the University of Pretoria in South Africa, are leading efforts to describe and name the new pathogens, study phylogenetic relationships with other Ceratocystis species, and investigate the epidemiology of both pathogens. Juzwik and Sweeney are excited to contribute to the overall pathology research effort and, ultimately, contribute to viable management options for these new devastating tree diseases.