Citizen-scientists are still trying to figure out why Seattle’s sword ferns are disappearing

SEATTLE’S CITIZEN FERN detectives haven’t yet caught the culprit behind mass die-offs in Seward Park and elsewhere — but they might have it cornered.

The small group of volunteers who first sounded the alarm over disappearing sword ferns got lucky this summer. After trying to cobble together experiments and surveys with little money, they won a $7,750 award from 100 Women Who Care, a band of local residents who chip in money and give it to worthy causes.

The ferns’ plight also attracted the interest of a real scientist with time on his hands. Dylan Mendenhall recently finished his M.S. degree in forestry at the University of British Columbia and moved back to Seattle, his hometown. When he heard about the acres of once-luxurious ferns reduced to brown nubs, he offered to lend his expertise to the search for the killer.

The result was a set of honest-to-goodness experiments — modeled on a shoestring affair conducted by Paul Shannon, a leader of the citizen-scientists who have been working for five years to solve the mystery. Shannon put fronds from sickly ferns and fronds from healthy ferns together in a couple of beer bottles filled with water. The healthy ferns shriveled and died.

Mendenhall replicated the experiment on a larger scale, with dozens of fronds and 50 Mason jars. He got the same results — twice. “It is very good evidence that there is pathogen present in the foliage of the ferns in these die-off zones, and it can be transmitted through water,” he says.

The next step is to identify the bug, which could be a virus, bacterium, fungus or a type of water mold called phytophthora — whose name means plant-destroyer. Mendenhall hopes to use advanced DNA sequencing on sick and healthy ferns to narrow the field.

Since 2013, the area called “ground zero” in Seward Park where the shriveling ferns were first spotted has grown to 20 acres. More than 70 similar die-offs have been reported across Western Washington.

Mendenhall is optimistic about finding the answer. But he also suspects there might be more than one factor at play. For example, drought might weaken the ferns and make them more susceptible to infection.

“It could be very complicated,” Mendenhall says. “Which could explain why it’s been so hard to figure out.”