I am not 100% sure if this is the same thing that is happening in Seward Park or not, but I noticed some sickly, dying, and dead sword ferns in this area recently. It's disturbing in that it's a very rural area without a great deal of human impact. There is an abandoned road grade which elk evidently use in the immediate area; perhaps the elk carried the pathogen in on their hooves.
The worst area (pictured) is about 50' across. There are sickly/dead/dying ferns scattered through the area here and there (not just where pictured). There are no large areas hundreds of feet across with no surviving ferns.