A pernicious fungus is wiping out native ohia trees on Hawaii island, prompting scientists to work on ways to prevent its spread to other islands.
By the end of last year, the fungus, known as ohia wilt, had killed half of the ohia lehua trees on about 6,000 acres from Kalapana to Hilo, ecologists estimate.
"We’re trying to stop it initially from getting off the Big Island … and also limiting its spread on the Big Island," said Neil Reimer, administrator of the state Plant Industry Division. "It’s a very serious problem."
There is more at stake than just the ohia trees, which serve as the mainstay of Hawaii’s native forests. If the fungus becomes widespread, entire forest ecosystems and watersheds are at risk, scientists warn.
"A loss of ohia would result in an utter transformation of our forests, not to mention the cultural importance of the trees," said Flint Hughes, a research ecologist with the U.S. Forest Service. "Ohia really forms the foundation of our native forests. It’s the keystone species in terms of forest development."
Hughes said ohia trees comprise about 50 percent of the biomass of a native forest.
Unlike some alien trees that discourage ground cover by blocking sunlight or changing the chemistry of the soil, ohia provides cover for many native plant species that in turn capture rain for Hawaii’s watersheds.
"It’s our ultimate nurse tree for our native species," he said.
The pathogen, formally Ceratocystis fimbriata, disrupts the vascular system of the ohia tree, halting the flow of water. That stains the crown brown and death comes within weeks.
Once infected, a stand of trees will die within three years, scientists say.
It’s unknown how the fungus reached Hawaii island, where the problem arose about five years ago. So far, the disease has not been reported on other islands.
It wasn’t until last fall that Ceratocystis fimbriata was identified as the cause of ohia wilt by Lisa Keith, a research plant pathologist for the U.S. Department of Agriculture in Hawaii.
More research is needed before scientists can devise a strategy to control it.
For now, researchers have determined the fungus spreads in wood, soil and insect excrement, said Keith.
"It is important not to take infected material into healthy areas of the forest," she said. "Proper sanitation and disinfecting tools is critical for disease control. We are currently working to determine if an interisland quarantine is a viable option."
Pigs and beetles also could be vectors, researchers say.
Hughes said scientific literature mentions that beetles have carried similar kinds of fungus elsewhere.
"They may all be pathways," he said.
Sam Gon, senior scientist and cultural adviser to the Nature Conservancy, said preventing the spread of the ohia wilt could be difficult because of the variety of ways it might migrate.
Gon said spores of the fungus might be carried on the tires of vehicles and the boots and clothing of people going through the forest.
"At this early stage it’s very worrisome," he said.
Present in many facets of Hawaiian culture, ohia lehua is mentioned prominently in chants and legends; its blossoms are used as an adornment in hula, and Hawaiians used the wood to carve images of deities.
Four species of the genus Ceratocystis occur in Hawaii, according to the University of Hawaii’s Department of Plant Pathology.
Ceratocystis fimbriata also causes "black rot" on sweet potatoes, and its cousin Ceratocystis adiposa causes black rot on sugar cane. Ceratocystis koepkei causes sugar cane "yellow spot," and Ceratocystis paradoxa causes bleeding cankers on coconut palms and rot on sugar cane, bananas and pineapples.
