Scientists, Resource Managers Seek To Understand, Halt Spread of ROD

posted in: Forests, Invasives, July 2019 | 0
In the Agricultural Research Service lab in Hilo, sections of `ohi`a trunks infected with the Ceratocystis fungus are placed in fabric cages to see what insects emerge from them. Credit: J.B. Friday

So you want to stop rapid ‘ohi‘a death in its tracks?

Probably the very best thing you can do is to remove goats, pigs, cows, deer, and sheep from the area you want to protect – and fence that land to keep them out.

At a two-day scientific symposium held in late May, dozens of experts who have been studying the disease that threatens hundreds of thousands of acres of Hawaiian forests came together to discuss all that they have learned in the four years since the disease was given a name.

Many of the practices that have been recommended to prevent the spread of the two fungi that cause the ‘ohi‘a to die are all-round sensible hygiene, good to do regardless of what impact they might have in curbing the spread of rapid ‘ohi‘a death. Yes, clean boots before going on that trail. Do hose off trucks and scrub other equipment before moving them from one forested area to another. By all means, don’t mark trails or other landmarks into trees with your hatchets. Never take any part of an ‘ohi‘a tree – flowers, stems, wood, whatever – from one island to another without clearing it with state agricultural inspectors.

But their utility in stopping the spread of rapid ‘ohi‘a death isn’t entirely clear.

What is clear is the important role fences play – not that they keep out the fungi that cause ROD, but that they keep out the animals that wound the trees, thereby opening pathways for the airborne spores of the fungi to invade otherwise healthy trees.

Some of the most dramatic slides presented at the symposium were aerial shots that showed fenced areas immediately adjacent to unfenced ones, with dead ‘ohi‘a highlighted. Inside the fenced areas, there may have been a few infected trees, but nowhere near the numbers seen in the forests outside the fence. As Flint Hughes, of the U.S. Forest Services Institute of Pacific Islands Forestry put it, describing one such area in the Big Island district of Ka‘u, “a huge blob of death” was seen outside the fenced area, while inside, “we have yet to find a [ROD-]positive tree.”

WUD and ROD

Driving home the same point was the presentation of Emma Yuen, native ecosystems protection and management manager at the state Department of Land and Natural Resources.

Yuen linked rapid ‘ohi‘a death to an even older and more widespread problem she called WUD, or widespread ungulate damage.

“We have millions of trees that have died because of WUD for more than two centuries now. More than 100 studies have been done showing how damaging WUD is in over two million acres of ‘ohi‘a and other native forests,” she said. Today, even though on the Big Island, 160,000 acres are free of hoofed animals, that number is dwarfed by the more than 2 million acres that are still unfenced, she added.

Now the DLNR is attempting to have some of the funds allocated to fight rapid ‘ohi‘a death used to fence and remove ungulates from forested areas.

Despite the clear link between WUD and ROD, Yuen said, there’s a need to be strategic in identifying areas that should be prioritized for fencing. Also, given the objections that Hawai‘i’s vocal community of hunters is likely to raise against additional fencing, Yuen asked how it might be possible “to translate the community’s ‘ohi‘a love into support for actions needed to protect forests?”

Finally, she threw down a challenge to the state’s conservation community to use the data on the link between ungulates and rapid ‘ohi‘a death “to fence and protect areas with a renewed urgency.”

In the Air

The two different species of Ceratocystis implicated in ROD result in the same event – the death of the tree – but by different mechanisms and over different time spans. C. lukuohia causes death by quickly spreading through the tree’s vascular system, while C. huliohia is slower, causing cankers to form inside the tree. Death is almost certain, but it may take months or even longer.

Thomas Harrington, a professor at Iowa State University who has extensively studied fungal diseases of trees, described the different ways in which he suspects the fungi are spread. C. lukuohia, he said, was most associated with wind, while C. huliohia is found in trees that are more protected.

To support the notion that the more devastating fungus is carried by wind and not, for example, by insects, Harrington showed two aerial photos of the same area, Wao Kele o Puna, before and after Hurricane Iselle, which hit Puna in August 2014.

In 2008, there’s no sign of any diseased trees, but 18 months after Iselle, “the forest is transformed.”

Near Hakalau Forest National Wildlife Refuge, C. lukuohia hit an area that, Harrington said, is undisturbed and has no ambrosia beetles, a type of insect that’s suspected of carrying spores of the fungus from infected trees to uninfected ones, thereby spreading the disease. This, he said, “could not be human-associated inoculation. It had to be the wind.”

Genetic analysis of the C. lukuohia found in infected Kaua‘i trees showed it to be close to strains found on the Big Island and is very likely to have come from there, “probably in a wind event,” Harrington said.

“Trees that are exposed to the wind, trees above the canopy, seem to be the most susceptible” to infection with this species, he said.

Why are those taller trees more likely to be infected with C. lukuohia?

Harrington has a theory. “I suspect it’s because of the upright architecture of the tree. In between the stems that grow together is old bark. In the wind, they’ll open up the tree where the included bark is,” he said. “The heaviest stain” – which is symptomatic of infection by C. lukuohia – “is often in the included bark.”

Wade Heller, with the U.S. Department of Agriculture’s Pacific Basin Agricultural Research Center in Hilo, noted that air samples from more than 20 sites had tested positive for either or both species of fungus as far as 20 miles from the closest known infected tree. DNA was present in trace amounts – although he cautioned that the mere presence of DNA did not indicate that the fungus was viable.

Sheri Mann, Kaua‘i manager for the state Division of Forestry and Wildlife, said she, too, suspects that wind has been a big factor in the spread of Ceratocystis on that island, where both species have been found.

Are Beetles to Blame?

Ambrosia beetles, sometimes called “fungus farmers,” have also come under suspicion as a means of infection. Kylle Roy and Kelly Jaenecke, researchers with the U.S. Geological Survey’s Pacific Island Ecosystem Research Center, described the research they had done on the possibility that the beetles’ frass – the sawdust-like waste the beetles produce – might carry the fungus from one tree to another.

Less than 10 percent of the frass samples they collected contained Ceratocystis DNA, but no spores were detected.

The beetles themselves may be able to carry the fungus from one tree to another. Roy and Jaenecke are just starting to test this possibility.

Franny Brewer, communication director with the Big Island Invasive Species Committee, reported on a study she conducted last year at trailheads on the Big Island that revealed the presence of Ceratocystis DNA in dirt.

She and co-workers collected dirt from boot brushes and nearby areas, tested that for the presence of Ceratocystis DNA, and then attempted to grow it out on carrots, the culture medium used most frequently to determine fungal viability.

Trace amounts of DNA from both species were present in every sample, although most of it was not able to be cultured.

“The surprising thing is where we actually were able to culture Ceratocystis from samples – neither of them came from places where there’s a lot of ROD-infected trees nearby,” she said, adding that the inoculum probably came from far away on someone’s boot.

Brewer added that she had tested a limited number of samples from off- island. “All have been negative so far,” she reported.

Harrington spelled out what he sees as the long-term future of Hawai‘i forests: “There’s DNA floating around everywhere, just looking for a wound. I think it’s eventually going to spread through all ‘ohi‘a forests on the Big Island, although the percentage of trees dying is actually small.”

He defined success as “a low level of infected trees. You’re not going to get rid of it on the Big Island.”

Eventually, he added, “it may take decades, but the fungus will be in ‘ohi‘a forests all around the islands. Maybe five to ten percent may go out over a decade.”

One positive aspect: “It doesn’t spread rapidly from one tree to the next. That’s what is different about this disease as opposed to, for example, chestnut blight or oak wilt. … We’re not seeing it wipe out the whole forest. I’m more hopeful than most, but I think we have to accept some level of disease.”

— Patricia Tummons

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