The severity of the ohia rust disease in one of Hawaii’s critically endangered plants can be reduced by using a “smoothie” of fungi found in leaf tissue, a recent study has shown.

In a study published in September in the Phytobiomes Journal, scientists affiliated with the University of Hawaii at Manoa’s Department of Botany were able to cut the lesions found on infected plants by more than half.

Ohia rust, also known as myrtle rust, strips infected plants of their leaves and inhibits reproduction. The pathogen’s ability to kill host plants makes it an existential threat to the nioi, a critically endangered plant species endemic to Hawaii.

But with the help of healthy leaves from wild plants related to nioi, such as rose apple, that have been blended into a slurry or “smoothie,” the impact of ohia rust can be reduced.

“We found healthy leaves of the rose apple, we threw those in a blender … and then you end up with a smoothie, basically,” said Anthony Amend, a UH Manoa associate professor of botany and one of the study’s authors. “And we just pass that through a filter to get the chunks out, and then put it in a spray bottle. And we spray that on the (nioi) leaves because it’s the leaves that get impacted by the rust.”

The study said that some of the plants, which also can be hosts for ohia rust, were asymptomatic when infected. Other research, meanwhile, has shown that the microscopic fungi that live in the leaves of all plants are important to plant health.

It’s analogous to humans’ use of probiotics for health benefits, Amend said.

“The same thing sort of applies to plants. They have microbiomes, also, that live on and inside their leaves, and these do a lot of things: They produce chemicals that deter pests and herbivores, they can help them uptake water,” he said. “And one of the important things they do is they help them fend off diseases.”

The study is of particular importance in Hawaii, where many plant species are endangered and are often grown under ideal conditions like greenhouses. Individuals raised in such “bubbles,” when planted in the wild, can be more susceptible to diseases because they haven’t been exposed to wild and beneficial fungi.

A collection of nioi plants, for example, is being maintained by the Oahu Army Natural Resources Program, and individuals were used for the study.

Using plant mixtures that contain microbes from wild plants can, in turn, help “re-wild” the greenhouse plants.

“It’s sort of a way to reintroduce the natural complexity of these natural microbiomes,” Amend said.

The transfer of microbes isn’t new, but Amend said the use of the fungi as a disease suppression method is fairly novel, and its success is promising. He said that in a previous study researchers found that applying fungi to the Hawaiian mint reduced the severity of powdery mildew disease, which is also caused by a fungus.

Practically speaking, the method is also promising because it’s easy and inexpensive.

“The advantage of that is that it’s very low-tech — it just involves a blender and a leaf,” Amend said. “But it’s sort of a way to reintroduce the natural complexity of these natural microbiomes that’s otherwise difficult to replicate in a lab system.”