An aerial survey of some 20,000 acres of Oahu forest has yielded the island’s fourth detection of rapid ohia death, the fungal disease that has killed millions of ohia trees on Hawaii island.

But state officials said Thursday that they’ve tagged 41 more trees that could be ailing from rapid ohia death. Crews are expected to conduct a follow-up examination of the suspect trees on foot.

Rob Hauff, state protection forester with the state Division of Forestry and Wildlife, said fortunately the latest confirmed tree is infected with Ceratocystis huliohia, the less aggressive of the two fungal pathogens associated with rapid ohia death.

Ceratocystis huliohia is known to take months to years to kill ohia trees, while the more aggressive species, Ceratocystis lukohia, can kill a tree within weeks and is responsible for wiping out 80% of the trees destroyed by rapid ohia death on the Hawaii island.

While the more aggressive form of the disease also has been found on Kauai, it has yet to be confirmed on Oahu and Maui.

The latest infected tree was spotted in the Honolulu Watershed Forest Reserve using a helicopter-mounted camera system developed by the University of Hawaii- Hilo, Spatial Data Analysis and Visualization Lab.

It was the first time the new technology was deployed on Oahu, Hauff said, and the collected images were analyzed by Oahu Invasive Species Committee and state forestry officials who were trained on the Big Island.

The disease was first discovered on Oahu July 31 on a dead ohia tree on Kamehameha Schools land in a remote area in the Koolau mountains above Pearl City. Two more became known in residential areas of windward Oahu.

Nathan Dube, manager of the Oahu Invasive Species Committee, said both of those trees were reported by landowners who noticed their ohia were either dead or dying.

A field team cut down the trees, dissected them and sent samples to a lab in Hilo for positive confirmation and for research purposes, he said.

The recent aerial survey was focused on an area in the vicinity of the earlier Waiawa detection. Hauff said surveyors spotted an ailing branch of the tree and then had it confirmed as suffering from rapid ohia death.

With the advice of experts from Hawaii Island, crews decided to remove the infected branch and use a sealant on the scar to prevent additional disease infection, he said.

Surveyors identified more than 40 additional trees that need to be tested for the disease, Hauff said, but that will take some time because the area is rugged and difficult to reach. The ailing trees also are widely distributed.

“It will probably take several months because they are all in different places,” he said.

A coalition of public and private organizations has been conducting regular helicopter and ground surveys on Oahu to look for rapid ohia death since 2016. Scores of dead or unhealthy ohia trees have been sampled for the disease in the past three years and all were negative until the discovery this summer.

The range of ohia lehua, famous for its red flower, is spread across Oahu on some 50,000 acres in the Koolau and Waianae mountain ranges. The keystone species serves to recharge aquifers, provide habitat for endangered species and is used for cultural purposes.

At least one way the ohia comes down with the disease is through open wounds. A recent study found evidence that protecting habitat by fencing out pigs and goats reduces the prevalence of the disease by reducing damage caused by those animals.