Black fungus under leaf?

[katsucats]

Alright, so I got this Cyrtorchis arcuata I'm probably going to throw out. It's been infested with some fast spreading leaf spots that I cut out of the other leaves, as you see in the picture. I leave it alone for 2 weeks and come back to this (see pics). There has never been any water splashed on the leaves, and none of the neighboring plants have this symptom. I do have leaf spots here and there that are clearly delineated black circles on other plants, but they don't take over the leaf like this one. As you can see, there is some black mold looking thing that sticks out from the surface under the leaves where it's heavily infested. I've never seen this before.

I just want to know what it might be so I can formulate a strategy in case I see it again in the future.

[Orchidtinkerer]

that looks highly aggressive and seems to be spreading in the plant. Can't help you with what it is but I share your sentiments that I wouldn't keep that near others.
I've never had anything as serious looking on any of mine before but I have lost a few to mysterious crown rot cases that also seemed aggressive and brought in from elsewhere.
Sorry to see this happen

[SouthPark]

Have you got a collection of goodies, such as copper spray, and thiomyl, and phosacids etc?

Could try copper spray to start with. And provide nice temperature (warm) and gentle air movement around leaves, stem, and media and roots etc.

Also make sure the plant gets enough mag-cal and fertiliser --- not too much though.

[estación seca]

What are your temperatures? Humidity?

[katsucats]

I'll try a copper spray. Room temperature and room... humidity... I don't have any way to regulate the humidity. However, as mentioned, it's not happening to any of my other plants. Is Cyrtorchis arctuata known for this?

[SouthPark]

I don't think any orchid is known for this ----- as in the orchid developing it under very satisfactory growing environments.

[estación seca]

I'm not familiar with this plant other than from reading. High-humidity plants grown in low-humidity environments look like this when they're struggling. I don't know whether that's what's wrong here, but I suspect low humidity is at least part of the problem. Can you fit it into a large vase, jar or terrarium to raise the humidity? I have a few old aquaria I've bought at yard sales I use for high-humidity plants.