Single den leaf looks like rot or fungus?

[Girl_With_An_Orchid]

Hello! I have a phal type dendrobium that’s go one leaf that looks really unhealthy. I noticed it started with a couple small black dots. The leaf then turned yellow and softened. I just removed it because it’s now half black and half yellow and very soft and bruised. The spots have spread a little down the stalk but nothing else is affected. I’ve attached pics of the leaf. I have removed the leaf because I was worried about any possible spread.

As far as care, it gets good light, more than my phals but not a lot more, and I water as needed to keep it a little wet but not soggy. I’ve got a small stick in the potting medium that I check regularly for that. It’s my first den so I’m hoping I’m right with that. It is a new orchid so I haven’t repotted it yet but what I can see of the roots is good. I have a new stalk growing so it seems to be fairly happy. Is whatever’s happening to the leaf going to spread?

[estación seca]

It's frightening to see but that is how leaves of this kind of Den. look when they get old then fall. Is it from an older growth?

Den. phal. hybrids should keep leaves 2-4 years, then they often fall, leaving bare stems that still may flower. To keep them healthy and flowering give them warmth all year, even watering and fertilizing, and considerably more light than you give Phals. If this leaf is falling early, causes could be too low temperatures or being too dry. The other leaves look good, but spider mites can also cause leaf damage leading to leaf fall like this.