‘Ōhi‘a forests and Rapid ‘Ōhi‘a Death

Hawaii’s most important native tree, Metrosideros polymorpha (Myrtaceae; ‘ōhi‘a in Hawaiian) is suffering devastating levels of mortality due to two new introduced pathogens in the genus Ceratocystis. ‘Ōhi‘a forests are critical to Hawai‘i for watershed protection, native species habitat, and cultural uses. Over a million trees are estimated to have died since the outbreak started. Since the pathogens were first identified in 2014, teams of plant pathologists, entomologists, ecologists, remote sensing scientists, and forest land managers have developed ways to diagnose the disease, identify means of spread, and protect forest health. Pathologists have described two new diseases, a vascular wilt and a canker, caused by these fungi. The fungi spread through wind-blown beetle frass produced by ambrosia beetles attacking diseased trees, by feral animals in the forest, and by humans moving wood from place to place. Beetle repellents can reduce beetle attack and frass production. Ceratocystis typically needs a wound to infect a tree, and remote sensing has shown that levels of disease may be 50x higher in forests with high populations of feral pigs or cattle than in protected forests. A disease resistance program has tested thousands of seedlings and rooted cuttings and is finding that a significant fraction of the natural population seems to be resistant to the diseases. Native forests that have lost much of their ‘ōhi‘a canopy suffer from invasions of non-native weeds. Restoration efforts are underway involving fencing out feral animals, controlling weeds, and planting of disease-resistant ‘ōhi‘a seedlings.