“This pathogen poses a serious threat to Hawai`i’s flagship native tree species whose loss would be catastrophic for the diversity, structure, and function of Hawai`i’s remaining native forests and the services they provide.”
— “First Report of Ceratocystis wilt on `Ohi`a”
Hughes has worked on some pretty bleak subjects – invasive grasses, shrubs, and trees (including albizia), among others – but Ceratocystis fimbriata, a fungus that has been killing `ohi`a trees on the island of Hawai`i for the last five years tops all the others, he said in a recent interview.
Hughes and colleagues have watched as the fungus, known now as `ohi`a wilt, has spread rapidly from what they believe to be ground zero – the Leilani Estates subdivision in lower Puna – to forested areas throughout Puna and beyond. Remote sensing surveys in 2010 estimated the spread of the infestation at 1,000 hectares (around 2,500 acres). By 2014, 6,000 ha, or 15,000 acres, of `ohi`a stands had been infested. So far, the disease has not been detected in Kona and Kohala, Hughes said, but it has spread northward to Hilo and westward as far as the residential subdivision that backs up against Hawai`i Volcanoes National Park.
“We have a highly virulent strain of the fungus. We’re looking at a worst-case scenario,” he said.
The research that Hughes and colleagues from the institute (an agency of the U.S. Forest Service’s Pacific Southwest Research Station), the U.S. Department of Agriculture’s Pacific Basin Agricultural Research Center (PBARC), also in Hilo, and the University of Hawai`i’s College of Tropical Agriculture and Human Resources (CTAHR) undertook to identify the fungus has been summarized in an article, “First Report of Ceratocystis wilt on `Ohi`a,” published online in the scientific journal Plant Disease. That has been a major step forward in pinning down the exact cause of the widespread death of `ohi`a, Hughes noted. “The Plant Disease report is the benchmark in establishing that this is the pathogen killing the trees,” he said.
Until recently, there had been some suspicion that the trees were dying as a result of “classic cohort senescence,” Hughes said, referring to a phenomenon noticed decades ago that occurs when trees in large stands of similarly aged `ohi`a die off within a relatively short time.
But what is occurring now has nothing to do with cohort senescence and is instead the result of the newly described fungus infecting the trees.
Even determining the extent of infestation is difficult. Trees may be infected long before showing signs of wilt. That confounds efforts to control the disease. While removing a tree showing signs of the wilt might reduce the chance that it could infect other trees, if it is surrounded by other trees that are infected but still apparently healthy, little would be achieved by that effort.
What’s more, little is known about the way the disease spreads through the forest, Hughes says. “It’s hop-scotching, almost like a forest fire,” he noted, with new outbreaks detected at some distance from old ones.
One of the mysteries yet to be solved is how the fungus arrived in Hawai`i in the first place.
Genetic analysis of the wilt showed it was very similar to strains found in arrowhead plants (Syngonium) in Hawai`i, Florida, Brazil, and Australia. However, it is not yet known whether this fungus is the introduction of an exotic strain or whether it is an existing strain infecting a new host. The range of Ceratocystis hosts “is scary,” Hughes said, and includes fruit trees, understory plants, and crop species.
While stopping the spread of the disease on Hawai`i island may be difficult, it may yet be possible to keep it from moving to other islands. “If we’re smart, we need to take action quickly. Any number of things could carry this to other islands. There’s an immediate need to do something.”
A Forest Whodunnit
J.B. Friday, extension forester with CTAHR, elaborated on the difficulty of finding a cure for the disease when so little is known about it.
Friday was one of the first to raise the alarm about Ceratocystis – although until last December, neither he nor anyone else had a name for the disease they were seeing.
He first became aware of a problem in 2010. “I visited a landowner in lower Puna,” Friday said. “He couldn’t figure out what was wrong” with his dying `ohi`a trees.
“You see a dead tree,” Friday said, “and there’s a heck of a lot of things going on. Bugs are getting in, disease – but most of that is secondary.”
In 2011, more calls came in from homeowners in the Leilani Estates area. “There are always `ohi`a dying in people’s lots. They bang their roots, do other things. We can’t help homeowners with sick trees, though.”
By 2012, Friday and his colleagues were seeing trees “going down” and in 2013, they began surveying areas to pinpoint the infestation and taking samples from dying trees in an effort to identify the pathogen.
“The samples we took didn’t turn up anything unusual. There are a lot of organisms in a dying tree. But do any of them cause the disease? You can do a swab of your tongue and find a lot of pathogens there, but you’re still not sick. There’s a disease triangle. You have a pathogen, an organism, and an environment where the pathogens can cause disease.
“So there was no answer as to why we’re seeing this widespread disease with regular run-of-the-mill pathogens.”
For years he and his colleagues struggled to identify what was wrong with the dying `ohi`a. “We were casting a wide net, trying to figure out what was going on. Some of the areas lined up with a rift zone, so I even talked with Don Thomas, a volcanologist, about elevated subterranean levels of CO2. Some folks suspected geothermal was causing it. We just didn’t know what this was,” he said.
Finally, in 2014, Brian Bushe, diagnostician with CTAHR’s Agriculture Diagnostic Service Center in Hilo, recovered a pathogen. Lisa Keith, a plant pathologist with the USDA Agriculture Research Service, “nailed down what it was,” Friday said. Never before had it been found on `ohi`a.
While identifying the pathogen as Ceratocystis fimbriata marked a huge step forward, in many respects, that only multiplied the questions facing Friday and the rest of the team working on this issue.
“To me, the biggest question is, how does it spread?” Friday said.
The fungus is related to Dutch elm disease, he noted, “and that gives us some insight into where it could be going.”
That disease ravaged elms in Europe and North America in the last century. Treatment of individual trees is possible with injections of a fungicide, but at an annual cost of hundreds of dollars per tree. “But that’s landscaping,” Friday said, “and is out of the realm of real forestry. We’re not going to be flying helicopters over the forest, spraying fungicide.”
Knowing how it arrived in Hawai`i might yield important clues about how it disperses. “It didn’t get here on an `ohi`a seedling, since we’re not bringing in `ohi`a seedlings,” Friday said, “but it could have come in on an infected piece of wood.”
Then again, he continued, “it could have moved on an alternate host. Fungi have complex life cycles and have different hosts at different points in their life cycle.” As an example, he cited rust of pine, which has an alternate host of gooseberry. Although gooseberry plants can be infected, they are not seriously damaged by the rust. When white pines are infected, however, they die.
“If there’s an alternate host for this,” Friday said, referring to Ceratocystis, “and we’re happily moving this other host around, then we don’t know what we’re doing.”
Friday has set up a website, http://www.ohiawilt.org, where he and colleagues can post the latest information on the disease.
Keith, the plant pathologist, is more accustomed to working with diseases of commercially grown plants than native species. But the process of narrowing down pathogens is the same in both cases.
“We look for signs and symptoms, and how the host is responding to this organism,” Keith said in describing her work to the Coordinating Group on Alien Pest Species at its meeting last month in Honolulu. In this case, she continued, “signs and symptoms were rapid browning of leaves and complete foliar death. The leaves remain attached, though, and it seems to take only a couple of weeks from the time of notice to the death of a tree.”
About a year ago, “we started collecting samples from a variety of areas,” she continued. Field sampling involved taking down entire dead trees and slicing up “cookies” – cross sections of the trunk. “Once we started looking at the cookies, we noticed vascular discoloration, everything from mild to a ‘flower burst’ pattern. There’s streaking when you remove the bark. All this led us to believe we would find an organism associated with this,” she said.
She and colleagues at PBARC were able to quickly isolate the fungus. “Right away, we knew we had Ceratocystis, which is a large, diverse complex of species that causes wilts.” Lab tests they conducted over the remainder of the year confirmed that this fungus was the disease-causing agent. By clogging up the tree’s vascular system, it prevents water from reaching stems and leaves, resulting in first the wilted leaves, then the dying branches, and finally the death of the tree. From the time the infection becomes apparent in inoculated seedlings, with the first wilted leaves, to the time the seedling dies is a matter of days, which led researchers to call the disease rapid `ohi`a death up to the time the fungus was identified.
At the Honolulu meeting, the response to presentations by Keith, Friday, and Hughes was a near-unanimous sentiment that drastic action should be taken, quickly, to prevent the spread of the fungus to other islands.
Christy Martin, CGAPS director, suggested that a name that carries more terror than “`ohi`a wilt” might be a good place to start. “From the public’s perspective, `ohi`a wilt is much less descriptive of what is going on. Should we call it instead rapid `ohi`a death?”
Friday replied that `ohi`a wilt “is the term used by experts” when referring to this class of diseases.
Martin noted wryly that this may be one of the reasons “scientists usually have a hard time communicating these issues to the public.”
Hughes agreed. “Wilt doesn’t quite convey the magnitude of the problem,” he said.
Bryan Harry, a member of CGAPS and former director of the National Park Service in Hawai`i, argued that even in the absence of clear information, when dealing with a risk as grave as this, immediate action was warranted.
“This group should try to go forward with limited knowledge while things are small, rather than wait until it’s scientifically documented and we’re writing an obituary,” he said, eliciting applause from those in attendance.
Hughes asked for guidance from the group: “What would really be helpful for us is to know what kind of questions you all need to have answered before you stick your necks out to do something…. Tell us what kind of information you need.”
The Plant Quarantine Branch of the state Department of Agriculture has the legal authority to put emergency rules into place. Such rules are in effect for no more than a year, but they can give resource managers and scientists a breathing space while they learn more about a pest and develop more narrowly tailored permanent rules.
Amy Takahashi, acting head of the Plant Quarantine Branch, was not encouraging about prospects for an emergency rule. “We need to have the science to back it up,” she said.
If there was a ban on the inter-island movement of `ohi`a firewood, she said, “we need to demonstrate that there is fungus in the dead wood.” Also, she said, “we have to have an option for treatment. We can’t put a total ban on transfer” of suspect items. “There needs to be a way for commodities to move.”
Also, before products from any island can be quarantined, the pest that is the target of the quarantine has to be shown to be absent from the non-quarantined areas.
Proving the absence of a disease is difficult, however.
In addition, Friday pointed out a “big wild card” – if the fungus is being transported by an insect or a plant other than `ohi`a that no one is yet aware of.
The group agreed that surveys of `ohi`a on other islands would be a good first step toward determining if the fungus is confined so far to the Big Island. Members volunteered to join an ad hoc working group to address the issue and cooperate on allocation of limited resources.
In the meantime, the disease continues to move, swiftly, inexorably, across the island of Hawai`i. Whether it will spread to other islands, or whether it already has, are questions time alone will tell.
— Patricia Tummons
If You Have Infected Trees…
Despite the lack of information on the specific ways trees become infected, the website http://www.ohiawilt.org has common-sense suggestions for anyone who lives near an outbreak.
- Wood from affected trees should not be carried to other areas, since the fungus may remain viable in dead wood.
- Any tools used to cut infected `ohi`a trees should be cleaned, “either with a Lysol spray or a 70 percent rubbing alcohol solution. Chlorine bleach can rust steel tools, but a 10 percent solution of chlorine bleach and water can be used as long as tools are oiled afterwards. Chain-saw blades should be brushed clean, sprayed with a cleaning solution, then run briefly to re-oil the chain.”
- “Vehicles used off-road in infected forest areas should be thoroughly cleaned underneath so as to not carry contaminated soil to healthy forests. Shoes and tools used in infected forests should likewise be cleaned.”