A small and informal experiment suggests that the agent/s of the die-off can be reproducibly transmitted from an affected frond to a healthy one

This informal experiment suggests that the agent/s are mobile in water, and will affect healthy fronds sharing water with a diseased frond, especially when the diseased fond is at the "crinkly phenotype" stage (which may be the earliest stage of the presumed infection).

Be warned!  This experiment is exceedingly preliminary, undoubtedly naive, suffers from small sample sizes, has not been replicated, and includes no quantitative data nor microbe-scale assays. They  have been carried out by an untrained enthusiast.  

If that is not enough to scare you off :} you can find lots of detail and time course photography in this report.

Early data on survival rates of 24 ferns at Seward’s Ground Zero versus 12 at the edge of the active die-off zone

Hypothesis:  the hypothetical agent of the die-off  does not persist in an affected area.

We planted 24 fronds at Ground Zero GZ) at Seward Park (where die-off was complete in 2014) and 12 in an active die-off  (AD) zone about 50 meters north.   I watered all 36 ferns throughout the summer drought.  The experiment will run five years and tests the hypothesis that survival at GZ will be significantly better than at AD.  We have possibly interesting results after only eight months.

Warning: this is informal science,  visual more than quantitative; it must be viewed skeptically.   The results are early and very preliminary,  and they may not stand up to either scrutiny or over the full five-year course of the experiment.

See the report.