Tree-Killing Fungus Continues to Spread on Hawaii's Biggest Island

At least two previously unknown species of the plant pathogen Ceratocystis fimbriata are killing large numbers of Hawaii's native ‘ohi’a (Metrosideros polymorpha Gaudich), the state's most widespread and ecologically important trees. Despite efforts by scientists to stop their spread, so far, the fungi appear to be winning. But scientists working together with land managers have learned enough about this tenacious pathogen to begin effectively fighting back. Scientists suspected, but have not been able to conclusively demonstrate, that species of Ceratocystis that eventually infected ‘ohi’a sneaked into Hawaii on imported plants more than 20 years ago. However, the fungi's effects went unnoticed until 2010, when a few isolated cases of dead and dying trees were reported to foresters J. B. Friday, at the University of Hawaii's College of Tropical Agriculture and Human Resources, and Flint Hughes, at the US Department of Agriculture's Forest Service in Hilo. Reports of diseased trees were escalating by 2012, and in 2014, the damage that C. fimbriata could inflict became obvious on Hawaii, the largest island in the Hawaiian archipelago. That year, Lisa Keith and her team at the US Department of Agriculture's Agricultural Research Service (ARS) in Hilo, along with Friday, Hughes, and colleagues, identified a new tree-­killing C. fimbriata. The scientists warned then that the disease it causes, which was named rapid ‘ohi’a death (or ROD) because of the short time it takes for trees to die once their crowns turn yellow, would prove “catastrophic to the diversity, structure, and function of Hawaii's remaining native forests and the services they provide.” A second C. fimbriata species was discovered in dead and dying ‘ohi’a later that year. “As of 2016, aerial surveys showed more than 50,000 acres containing hundreds of thousands of otherwise healthy trees on Hawaii Island had been killed, most likely by the fungus,” says Keith. She notes, though, that it is difficult to arrive at a true estimate because trees killed by C. fimbriata look the same as those that die from other ailments or drought. Furthermore, the fungi can persist in a tree for more than a year before there are visible symptoms of disease. Once the first symptoms emerge, the ‘ohi’a die quickly. An infected tree can be strangled to death in weeks as the fungus colonizes its sapwood, blocking water flow from the roots. “Mortality of ‘ohi’a at this scale is of great concern as the understory in these forests is often occupied by invasive species capable of severely limiting ‘ohi’a regeneration,” wrote Leif Mortenson and colleagues in the October 2016 issue of Forest Ecology and Management. Originally caught infecting sweet potato, C. fimbriata are members of a diverse group of host-specialized fungi that cause wilt, cankers, and rot diseases in many economically important plants. Scientists consider it likely that the group contains many undescribed “hidden” members. C. fimbriata distinct from the ‘ohi’a-killing variety have been found on Hawaii Island infecting kalo (also known as taro) and Syngonium (a common ornamental plant), but these species have never infected the ‘ohi’a. It has now been firmly established that the Ceratocystis species killing ‘ohi’a are true invaders. DNA sequencing places each C. fimbriata strain into one of three host-­specific broad geographic groups: the North American, Latin American, or Asian clades. One of the two new ‘ohi’a-infecting Ceratocystis pathogens is closely related to those in the Latin American clade and the other to the Asian group. ‘Ohi’a are critical to the ecology of Hawaii, providing habitat for rare and endangered native insects and birds. The trees’ extensive root system guides rainwater underground; therefore, the loss threatens Hawaii's fresh water supply. So far, the ‘ohi’a-killing C. fimbriata have been contained on Hawaii Island by strict export regulations. In 2014, Hawaii's Department of Agriculture quarantined all ‘ohi’a products—wood, leaves, shoots, and seeds. Because the spores of many Ceratocystis species remain viable in soil for extended periods of time, it, too, is quarantined. The restrictions became permanent in 2016. Unfortunately, quarantine alone is not enough. “The problem is quite simply the ease with which spores are carried on the worldwide transport of virtually anything,” says Anthony Trewavas, from the Institute of Molecular Plant Science at the University of Edinburgh. Meanwhile, scientists are screening ‘ohi’a for genetic sources of host resistance, evaluating biofungicides and natural products as alternative strategies to control C. fimbriata, and investigating ways to sanitize sawdust and sterilize logs and poles. Importantly, a new diagnostic assay has already been developed by Keith and colleagues that decreases the time it takes to detect C. fimbriata from 2 to 4 weeks to less than 24 hours.

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