Tropicals & Tender Perennials: Black stuff on H. Rubra, 1 by plantattraction

plantattraction wrote:
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