The sword fern die-off was first noticed by Catherine Alexander (naturalist and daily visitor to Seward) in the autumn of 2013. Here is her account, offered on October 28th, 2014:
The Sword Ferns near the hatchery and along the northern end of the hidden trail above that area began to fail about a year ago, though the first signs were quite subtle - just a sense that they were somehow sagging, as if they'd been covered with cardboard for a week or two. At the time I estimated that perhaps 40% of the population was affected.
I kept watch over the winter, thinking that what I was seeing was a temporary thing, that the ferns would recover when their fiddleheads appeared in spring. That was not to be. None of the affected ferns produced fiddleheads this spring, and by late spring the remaining fronds withered and died.
By July I noticed that whatever was happening had spread beyond the initial 40%, though it could be that I just hadn't seen the first signs of failure in the seemingly unaffected population. I monitored spore production in and around the affected areas this year, and noticed that many of the apparently healthy individuals either failed to produce sori, or that their sori were sparse and underdeveloped. Many of these individuals failed over the summer.
I haven't a clue as to the cause, though I have been thinking about it. Initially I looked for signs of
Aplodontia (mountain beaver) activity, thinking that a new and active colony might be destroying the fern roots. I found none of the signs I would expect from such activity - no burrows, no bitten off fronds - so think that Aplodontia is in the clear, at least this time.
November 2014: Seattle Parks staff visit to the affected area
Several members of the Friends of Seward Park (Paul Shannon, Paul Talbert, Al Smith) and Seattle Parks staff (Gail Takeoka, Jeanne Schollmeyer and plant ecologist Jillian Weed) visited the affected area. General puzzlement was shared. Jillian proposed an informal monitoring project: we planted two young sword ferns in December; almost one year later these two ferns are growing slowly and seem to be healthy. I watered them weekly over the dry summer.
October 13th 2014: Video walkthrough of the area
https://www.youtube.com/watch?v=hX0p_nzL2gU
December 2014: Report from Jenny Glass, Plant Diagnostician, WSU Puyallup Plant & Insect Diagnostic Laboratory
With funding from the Friends of Seward Park, Jenny Glass analyzed two entire ferns (one dead, one still with a few signs of life) in her laboratory in Puyallup. Her report:
Microscopic observations and incubation of the sample in moist chamber have not revealed any evidence of a primary pathogen in the root zone or crown tissue at the soil line. A composite sample of tissue from these areas tested for the Phytophthora pathogen via an Agdia immunostrip test kit was negative for the presence of this disease group (nor do soil conditions seem to be poorly draining, conditions that promote the growth of Phytophthoras).
Microscopic investigation of the fronds of the sword fern also have not shown any primary pathogenic activity that would be responsible for the occurrence of this dying patch of plants.
May 23rd 2015: Lab report from plant pathologist Dr. Olaf Ribiero
Without charge, Dr. Ribiero analyzed diseased sword fern samples in his Bainbridge Island lab, and visited Seward Park to consult with us. His report:
The dieback is more typical of herbicide injury or some other chemical injury. Was any herbicide applied on this area? [We have no evidence for, or report of, herbicides applied here.] Rhizoctonia is a difficult pathogen to control.
Autumn 2015: Dr. Tim Billo (UW Environmental Studies, field ecologist)
Tim, a frequent visitor to Seward Park's old-growth forest, noticed the die-off early in 2014. Catherine Alexander put us in touch. Under Tim's direction, with contributions from other UW faculty, crucial guidance from Seattle Parks plant ecologist Lisa Cieko and Nelson Salisbury of Earth Corps, two undergraduate students (Tristan O'Mara and Kramer Canup) have begun a systematic monitoring program.
October 2015: Video and slideshow, with closeups of healthy, affected and dead fronds:
view
October 2015: Additional Scientific Contacts
Dr. Robbin Moran, Curator of Ferns & Lycophytes at the New York Botanical Garden:
I am not sure what could be causing the die-off of Polystichum and have not seen anything like this in other extensive fern populations. I hope you can find out what is happening. It is a curious phenomenon, and worrisome that it might spread elsewhere. The world's expert in the taxonomy of the genus is Dr. David Barrington (dbarring@zoo.uvm.edu), from the University of Vermont, Burlington. Although not a plant pathologist, he might have some insight into the problem.
Dr. David Barrington, Professor of Plant Biology, University of Vermont:
I understand your concern about the sword fern, given the stark pattern you are observing in your part of the world. I have never heard of a blight impacting species of Polystichum so unfortunately I have nothing to offer in the way of insights into what might be at the root of your dieback.
Dr. Rodrigo Valverde, LSU Plant Pathologist and author of a 2009 paper describing a dsRNA virus infecting ferns, generously responded to my inquiry, and after viewing the video, suggested:
I have over 30 years of experience dealing with plant diseases, most of them viral diseases and diseases of unknown cause. After looking at the photographs, my first impressions (in order of most likely to less likely) are: 1, fungal, bacterial, or nematode disease; 2, physiological stress (lack/excess of water, pH imbalances, acidic most likely caused by acid rain); 3, viral disease. As you can see, I placed viral disease last because it is rare that viruses kill plants. However, the fern virus I worked with tend to cause a slow decline and eventually death on ferns. I did not mention insects or other arthropods damage but that is something that cannot be ruled out. However, a careful inspection will answer that.
I would rather make a "guess" based on actual/live inspection of the problem, but if I have to make a guess from pictures, I will guess a fungal or bacterial pathogen. This is because of the apparent pattern/distribution of the dead plants.
