Black stuff on H. Rubra

A customer asked me what this could be on her H. Rubra and I have no idea. Grown in filtered shade and the leaves do not fall off, she has to pluck them off. But now multipal leaves have it. It's a fuzzy picture but you get the idea....I ruled out sun burn and she see's no bugs. She says " I feed them a light/very diluted weekly feeding of a hydroponics fert I have called Thrive Alive, and B5 Alive or B1 Alive. Every other week, if they are dry enough, I use a bloom booster type fert.
Could it be the fertilizer she is using, too often maybe? Is it a type of mold, maybe?
Any info would be great~Thanks

Looks to me like a fungus that starts where some insect has bitten a tiny bite. I would cut off and burn the affected leaves and then spray with an all purpose fungicide. If she can take the leaves into an Extention Service agent it would even be better....and they can diagnose and specify treatment!!!

Looks exactly like a fungus I had on one of my Krimson Princess Hoyas. I just cut the affected leaf off and I use Safer brand Fungicide that I buy at Home Depot or Wal-Mart ... comes in a blue/green colored spray bottle.