Den Phal's leaves mysterious fade and color loss

[NewBloom]

Hi gang,

My NoID den phal has lost leaves starting around December. This occured even on the newest cane. It still flowered as usual though. The problem started one leaf and a time with a fading of the green color then going to yellow before falling off.

I've had the plant for 2 years and this is not the traditional shedding of an old leaf. It's been in the same shaded spot so sunlight isn't responsible for whatever this is.

The photos were taken this week. You can see there's a pattern and there's also a texture change in the leaf. The black spots on one cane have been there for months and never progressed.

Could this be a virus or nutritional difficiency? It's putting out new canes and the time to repot is here. I am holding off in case it's a virus and I have to toss the plant.

Btw, I switched back to an old batch of Better-Gro fertilizer and rested the DynaGro Grow formula. I think DynaGro has calcium while the BG does not. I started adding espon salts (1/4 tsp per gallon) to my fertilizer routine. Could that cause a problem? I noticed a similar "green fade" on an oncidium but that occured long after the Den Phal exhibited leaf changes.

You guys' help is appreciated.