Disease Spots on Bulbophyllum Carunculatum

[Oanadani]

I have noticed that the leaves on my bulbophyllum Carunculatum get these spots and then slowly turn yellow /brown and fall off. One by one.
It already lost 3 leaves in autumn and now I see 3 more leaves affected. The bulbs seem fine, but I can't help think it's a disease of some sort. I've had the plant for a few years and this only started this autumn. It also bloomed in autumn and now I see it has 3 new growths so I don't know what could be wrong.
Attached are pictures of the leaves, the bulbs and the healthy leaves too.

[Ray]

Whenever I see an issue, I point my browser to this presentation at the St. Augustine OS.

Granted, ID-ing something from photos isn't extremely reliable, but it's a start.

That said, when anything shows up as a large number of little, well-distributed spots like that, I think "mites" first.

[Oanadani]

It's not mites. I looked with a microscope. I had spider mites on some other orchids and treated that, but I can't see any on this

[SouthPark]

At least there's new growth coming out. The orchid has certainly filled up the pot. Does it get the occasional fertiliser and mag-cal?

[Oanadani]

Yes, I do fertilise all the orchids once a month or two at the moment. I'm using orchid focus grow fertiliser.

[SouthPark]

The main thing is that the new leaves and new growth don't get that same condition.

My liquid fertiliser application schedule is once a month too ---- first day of each month. Then in the middle of each month, I apply mag-cal.

Is the circled region under the leaf due to shadow only? The underside of the leaf is healthy and green too, right?

[Roberta]

If the new growths don't show the issue, it may be just a case of aging leaves. (Old ones do drop eventually, force of nature) Also... what is your temperature? This is a relatively warm-growing species, so night temperatures should ideally stay above about 15 deg F/60 deg F, and days at least 21-22 deg C/70-71 deg F, warmer even better.

[Oanadani]

They're inside the house, so temperature is warm 22C throughout the day.

Here are pictures of the underside. First underside of the leaves that look ok on the top and last picture of the affected leaves underside.

[Roberta]

If only the older leaves, I think not to worry. If it spreads to newer leaves then it could be concerning. What is your medium? Bulbos do like to be wet. Shallow pots are ideal, since their roots are short and they tend to spread out.