By Jan TenBruggencate
Advertiser Staff Writer

Hawai'i's prized koa trees are under attack by a fungus, which can be fatal to trees under stress, notably in lower-elevation areas subject to drought.

"We are just becoming sensitive to the fact that wilt is becoming more of a problem. It could be due to the five-year drought," that has just ended within the past few months, said University of Hawai'i professor of plant breeding Jim Brewbaker, in the Department of Tropical Plant and Soil Science.

The dry spell exacerbates the problem, killing some koa trees. Often, a standing young tree begins drooping as the fungus expands within its system. The leaves turn yellow before dropping.

Sometimes, it appears that an infected tree exudes sap that attracts rats. Seeking the sap, the rats chew the bark and may girdle portions of the tree, killing off individual limbs before the whole tree dies, foresters say. In these cases, branches die quickly, and the leaves may turn brown and dry up while still hanging on the tree.

Once the fungus takes off inside a tree's tissues, the tree's death is nearly certain.

Brewbaker said the wilt fungus appears to become a problem only in susceptible trees that are in a weakened state. Then, the wilt — a common fungus known as Fusarium oxysporum — is able to choke off the tree's ability to transport moisture.

"It plugs the xylem carrying water into the plant," he said.

"Probably, every koa tree in Hawai'i has it growing happily and not causing a problem, but most of these (disease) organisms sneak up on the weak," Brewbaker said.

The good news is that the fusarium wilt will not destroy the state's koa forests.

There appear to be enough trees with a natural tolerance to the fungus, and enough trees growing in good environments that most will thrive despite having fusarium wilt in their tissues.

Still, researchers are studying the issue. Brewbaker and Hawai'i Agricultural Research Station researcher Nick Dudley are leading research into the problem.

While there seem to be no koa trees that are entirely immune to attack by the wilt, Brewbaker, Dudley and others are trying to identify genetic strains of koa that are more tolerant of it.

They are also seeking ways to grow koa vegetatively, so tolerant strains can be readily reproduced for planting.

If you have a question or concern about the Hawaiian environment, drop a note to Jan TenBruggencate at P.O. Box 524, Lihu'e, HI 96766, e-mail jant@honoluluadvertiser.com or call (808) 245-3074.