NORTH HAWAII — Rapid Ohia Death (ROD), the fungal disease that has killed tens of thousands of ohia trees on the east side of Hawaii Island, is getting closer to the north.
“Unfortunately, we’re seeing it move up the Hamakua Coast. Not a lot of it, but it’s there. It’s in pockets at this point,” said Dr. Flint Hughes, a USDA Forest Service researcher, speaking June 1 at the Waimea Community Association meeting.
While the Hamakua finding is nothing like the amount of disease in Puna or that which is spreading into the Hilo Forest Reserve, “it is moving up the coast someway, somehow,” he said.
His takeaway message for North Hawaii residents was that they need to be vigilant — starting now — to keep the fast-growing fungus out of their treasured forests and watersheds.
The first outbreak of ROD was reported in lower Puna in 2010. Hughes and his peers began attacking the problem in 2014. The outbreak has caught the attention of botanists, ecologists and pathologists from across the state, other parts of the country and even the world.
“Everyone recognizes what a threat it poses to our native forests and our ohia forests in particular,” he said.
Researchers have isolated the pathogen causing the disease as the fungus Ceratocystis fimbriata, and it now has two similar but distinct A and B Species. Species A causes a faster death to ohia once it gets into the tree, whereas Species B has a more gradual effect. To date, researchers have found more of Species A along the Big Island’s windward coast.
Hughes said there is no genetic identity to the ROD on Hawaii Island compared with other Ceratocystis species in the world, so most likely the original species arrived here and then differentiated. The closest match to Species A is in the Caribbean and Latin America, and it most likely came into Hawaii 10-20 years ago. The closest identity to Species B is from Asia. The fungus may have come in on nursery stock, contaminated work materials or work pallets.
Once the fungus gets inside a tree, it essentially strangles and kills it by shutting down its vascular system, halting the transportation of water and nutrients. The crowns of infected ohia trees turn yellow, then brown and die within weeks to days.
ROD may also be seen as dark, nearly black staining in the sapwood along the outer margin of the trunks. Freshly cut wood infected with C. fimbriata may give off an odor similar to ripe or rotting bananas.
Death comes so quickly that often leaves are still on a tree when it expires. The pathogen can remain viable for more than a year in the dead wood of an ohia tree.
A relatively new discovery in the fight against ROD comes in how the disease is spread. It’s been known that traces of the fungus can be found in the soil around infected trees, making it easy to spread on vehicles, shoes, tools and clothing. But botanists have now discovered a new pathway — beetle dust — as a transmitter of the disease.
“We’ve learned that beetles play an important role, not necessarily directly spreading the disease themselves, but because they are attracted to the wood that contains the fungus,” Hughes explained.
Infected wood puts out a signal — a strong smell — that the beetles key in on. They then bore into the wood with the fungus inside and create a boring dust that comes out of the trees. The boring dust is ubiquitous in the forest where there are a lot of diseased trees.
“It’s like talcum powder. You can see it all over the place,” Hughes said.
And that powder is what researchers now think is a main reason for the spread of the disease. The powder gets kicked up in wind, gets on things and gets carried around.
What researchers still aren’t sure about is if boring dust gets kicked up every once in a while, or if it’s there in the environment.
“If the boring dust was very prevalent we’d be seeing it much more, so it’s still a puzzle to us about how it’s getting around; whether it’s a very episodic thing or just low levels but moving around,” Hughes said.
Wounds are also a huge factor in exposing trees to the fungus.
In study sites where there is livestock grazing or people have been injuring their trees, trees are infected. Next to those infected areas, where there are fenced areas with no grazing or no visible mechanical damage to trees, the trees look good, are healthy and not infected.
“It’s a wound enhanced pathogen,” Hughes said. “And where we have caused wounds in some of our control work we’re seeing the wounded trees pick up the disease quite quickly. So one of the key recommendations is don’t wound your trees if you can help it.”
To study ROD, the Forest Service and partner groups have set up inventory plots, about a quarter of an acre each, where they are tracking the health of all the ohia trees in the plot. They install the plots, measure and then go back a year later to remeasure the plot and see how many trees are now dead and how many have the disease. Some plots have now been measured three times and on average have a mortality rate of about 11 percent.
“We don’t know if this average trajectory of mortality will carry through but our data at this point is suggesting that,” Hughes said.
There are variations in the annual mortality rates with some plots having a 44 percent annual mortality rate, while others have little or no mortality even with diseased trees. This may be due to the size of trees on a plot, ages of the trees and the soil. Smaller trees, on average, have much lower rates of mortality versus larger stature trees.
Regardless, “The positive thing is we have a lot of ohia stands out there that are pretty resilient to the disease,” Hughes said. “We don’t know how long that will last or how that will carry out in the future, but it is good that we have some forests stands that are quite vigorous even when surrounded by areas of massive mortality.
“Ohia has been here for 4 million years and has continued to evolve and differentiate across a diversity of environments and places. If any tree species would have the genetic variation that might hold resistance to something like this, my bet would be on ohia,” he concluded.