Snow mold?

[Roth]

You may want to consider a couple of things. These type of molds usually proliferate in acid rather than alkaline media, in dry rather than wet media and often in imature media. ie: that the bark (not sufficiently composted) has a low population of decomosing bacteria which out-compete fungi or use them as food. So, drenching the pots in hydrated lime solution with a little fish emulsion MAY help to restore the balance. I have seen it from time to time and always in pots that have been left too dry for a time. If your bark had become water repelent (hydrophobic) no amount of watering will help without dipping in a wetting agent.
Hope you get on top of this!

Those photos sound very familiar. The Dutches used to coin the term 'Water vapor fungus' back to the 90's. It is not related to Orchiata or the bark by itself, but it is a contamination ( like a fusarium would contaminate a plant, this fungus contaminates the potting mix, bark or tree fern, etc...).

It has been correlated usually to a low EC of the potting mix ( not the fertilizer, but the potting mix). In the Netherlands, they usually increase the EC and do not use any 'periodic flushing', which suppress the problem.

As a fungicide, you can try only few that may work. It is not a pathogenic fungus, nor a 'common' fungus, hence many fungides are ineffective. I know as a fact that Dithane drenches can kill it. Physan, Zerotol, etc... will have no effects. Captan works well but can stop the root growth. This kind of fungus can indeed attack the roots.

That's actually something pretty stupid we can read here and there. The 'fungus gnats', some fungus, some insects, some mites, 'live on decaying matters'. This is plain stupid to say that. In fact, anything that can eat 'decaying organic matter' can care of orchid roots nicely, given the starvation, the conditions...

 

[Stone]

It has been correlated usually to a low EC of the potting mix ( not the fertilizer, but the potting mix). In the Netherlands, they usually increase the EC and do not use any 'periodic flushing', which suppress the problem.

Does orchiata treat their bark with Iron sulphate and super phosphate etc? Many fungi do not like P

 

[Roth]

Does orchiata treat their bark with Iron sulphate and super phosphate etc? Many fungi do not like P

We never do, though I am studying if an addition of micronutrients and macros as a start ( like the Dutches do) could not be beneficial to some crops...

As for the high P, I have seen that in my nursery where I am using 10-52-10, so it was not related. Apparently that fungus is disturbed only by a proper EC. There is another fungus that has a very similar behavior, but does some tiny cotton-balls like structures, about 1mm diameter, whitish-yellowish. Both clearly impair the growth of the plants quite strongly after some weeks...

It disappeared for me when I took the habit of never watering with plain water at any cost, always water with fertilizers.

 

[NYEric]

Interesting info.