Western Sword Fern Dieback
Western Sword Fern (Polystichum munitum)
Western sword ferns (Polystichum munitum) are distributed from southeast Alaska to the central coast of California. Sword ferns are an integral component in the understory of Pacific Northwest (PNW) forests, holding cultural, ecological, and economic value.
The Problem: Sword Fern Dieback
Recent observations have indicated a dieback phenomenon affecting western sword ferns, particularly in the Puget Sound region. In 2013, reports from Seward Park in Seattle highlighted a concerning decline in fern populations, with areas that previously supported healthy ferns now exhibiting extensive dieback, leaving only a few surviving specimens.
Fern Dieback Symptoms
The symptoms of this dieback include foliar browning, tissue rot, the presence of fern stumps (remnants of the ferns’ short stalks), and eventual death.
Fern Stump 
Foliar browning 
Thinning Fronds
The Approach
With the WSU Ravenholt Urban Forest Health Lab, we initiated a research project to sample affected western sword ferns. The goal is to determine whether soilborne plant pathogens are contributing to the observed dieback in these ferns.
Observations of unhealthy sword ferns recorded on iNaturalist were used as GPS coordinates for soil sample collection. At each GPS location, soil samples were collected from three nearby ferns. Priority was given to sampling ferns displaying symptoms, with healthy-appearing ferns sampled as needed to obtain three samples per GPS point. In some locations, fewer than three ferns were sampled due to the absence of additional ferns. In other locations, more than three symptomatic ferns were present.
A total of 51 soil samples were collected between November 2023 and February 2024.
Soil Sampling
The fern that was selected had the leaf litter and duff removed on the top of the soil before collecting the soil. Soil was collected from 3-4 spots around the fern within a meter of the base. About 500 grams of soil and small roots were collected and placed in a plastic bag.
Sampling equipment was sterilized before collecting each sample. Soapy water and a brush were used to clean the soil particles from the sampling shovels. The residual soap was removed by sticking the shovel into the soil near but not in the spot that the next fern was going to be collected.
Soil Baiting
Soils being vented 
N. Willers sorting soils 
N. Willers flooding soils
Soils and Organisms
Soil and fine roots were baited to isolate Oomycetes and investigate their associations with root disease–causing microbes. Microscopic organisms, such as Phytophthora and Pythium, are plant pathogens that can spread through the movement of infected plants, water, and soil, facilitating their dissemination within introduced areas. Bags of soil were left open to vent until the baiting process began.
Setting Up Baits
Each soil sample was placed into a bin and flooded with water. Rhododendron leaves were then added to the flooded soil as baits for Oomycetes. The leaves remained submerged, allowing Oomycetes to migrate through the water and colonize the leaves. Subsequently, the rhododendron leaves were removed, washed, and stored in individual bags to maintain moisture.
Results
Isolating Oomycetes
The rhododendron leaves remained in their respective bags until symptoms showed. Leaves that showed symptoms of infection were hole punched and placed in agar plates containing PARPH+V8 media to grow. Multiple rounds of isolation were performed to get a clean sample.
Findings
Forty secondary isolations resembling Oomycetes were obtained from the original bait leaf isolations. Of these, 16 were discarded due to fungal contamination or lack of growth. Results for 14 of the remaining secondary isolates are still pending, while 10 secondary isolates were confirmed as Oomycetes.
- One isolation of Phytophthora pluvialis was recovered from soil beneath an unhealthy fern at Swan Creek.
- One isolate of Pythium macrosporum was also recovered from beneath another unhealthy fern at Swan Creek.
- Eight isolates of Pythium intermedium were recovered from seven plants at six sites from four locations.
- Of these plants, six were either unhealthy (4) or dead (2), while one isolate was recovered from beneath a healthy sword fern.
Get Involved
Fall 2024 Soil Sampling
More sampling is planned for Fall 2024! If you would like to be involved, please let us know with the form below.
Sword Fern Soil Sampling
Please share your details below if you are interested in helping advance knowledge about the dieback of sword ferns in the Pacific Northwest.Collect Soil Samples
If you want to collect soil samples, join the iNaturalist project called ‘Sword Fern Health Watch,’ add observations of unhealthy ferns, and take soil samples!
Soil Sample Collection Guide
