I was given a small Jerusalem Cherry plant a few months back which has since been moved to my office window at work. It's flourished for a while but has lately developed a distinct curl and yellowish tinge to many of its leaves, along with some black spots.

This seems to have spread to most of the leaves now but not, strangely, to the cutting I took a fortnight ago which is thriving in a nearby pot.

Any ideas? Am I over/under-hydrating, and what can I do to counter this?