A very helpful email exchange with Dr. Rodrigo Valverde of the LSU Department of Plant Pathology and Crop Physiology, November 8th, 2015.   Rodrigo published a discovery of a novel fern virus in 2008:  New Virus Causing Disease in Japanese Holly Fern.

 Questions from me are in italics, and Rodrigo's answers are in bold.

Finally I have some time to answer your questions. I talked to other plant pathologists and in general they agree with my thoughts about the fern problem. Ok, below are your questions and my answers.

The ferns appear to die over about 12-24 months — that is, they go from good health to full death in that length of time. Is this faster than what you saw in your 2009 paper?

The time is similar but the fern virus does not kill the ferns (entirely) only several leaves followed by a slow decline which may end up in death.

We do not see any insects around the plants, but I will look again knowing now that the usual suspects do not seem to be present.  Have you any tips (or suggestions of things I can read) in order to do a good job on insects?

Unless they are vectoring a pathogen, it is unlikely that insects will kill the ferns. And if that is the case, they (the insects) will be very visible.

Our local plant pathologists are stumped.  Are there standard strategies for tracking down as yet unrecognized fungal, bacterial or nematode causes?  

Yes and no! Most new pathogens are old on pathogenic agents that evolved into a pathogen due to changes in the environment. It is very difficult sometime to differentiate between a pathogen and a saprophyte.

Maybe things like:

  1) look at a completely dead fern for lingering pests,  or
  2) look at an only partly affected fern (under the assumption the pest is still operating, and can be more easily found)
  3) culture in the lab for fungus & bacteria, hoping they will grow until they can be recognized.

These points (2 and 3) make a lot of sense and I am sure that the plant pathologists there are considering them. However, if an abiotic factor is involved as primary cause, they will not work. 

or anything else, any other strategies?

My impression is that it could be a soil-borne pathogen or a soil abiotic factor (pH, salinity, etc) or a combination of both. The abiotic factor allows the "new" pathogen to thrive!  Soil nutrient analysis including pH will may provide some information.

Good luck and keep me posted of any new development of the fern problem.

Rodrigo