Possible disease on cattleyas

[rbarata]

Did it have the same black spots on the underside of the leaf?

Some but not so pronounced. From the photo it seems the leaves are covered by timy reddish spots that together form a larger spot. That is not related with the yellow stains. The reddish spots are, probably, related with anthocyanin which is, in humans, the equivalent to skin tanning.
Some more info here.

Heres some photos of my tenebrosa taken just now (sometimes the pigmented area is not evenly distributed, some spots seems to have more pigmentatation that others, which creates large spots):





A close up photo of the larger spot would be really helpful.

[Orchid Whisperer]

Originally Posted by rbarata

My Laelia tenebrosa also had this issue (same symptoms) and I assumed it was Mg deficiency.
Watered it with Epson salts and, while the leaves never recovered from those "stains", the new ones grow without it.
I was advised to never water with fertilizer and Epson salts together. I used them for a month, then one week without fert or salts, then resumed fertilization.

Some info here.

For what it's worth, the advice to not use Epsom salts with fertilizer applies more to concentrated solutions that place a sulfate (Epsom salts) with fertilizer that may include nitrate. The linked article talks about a "sludge" produced when the two are mixed. The sludge is a precipitated, less soluble sulfate (probably will be gypsum if calcium is present). In lower concentration (undersaturated) solution, you won't have this problem.

The linked article discussed using 1/4 tsp/gallon CalMag fertilizer and 1/8 teaspoon Epsom salts. That is unlikely to produce any precipitated "sludge".