I've had a Jerusalem Cherry plant for about six months now, and by and large it's doing well in my office window at work. It's got a nice distribution of fruit (the 'cherries' of the title) around the upper leaves, but I'm suffering a great deal of yellowing on pretty much all the leaves, along with some black spots. Weirdly, the plant itself still seems to be thriving, but the yellowness is spreading and I can't figure out why.

I've had a snoop around the internet for likely causes, and most identify over/under watering as probable, but I can't tell which it is! The soil seems to dry out very quickly; particularly at the moment, as the temperatures are picking up in the UK. I've tried fortifying it a bit with some potassium; specifically a banana skin trick my gran taught me, but still haven't been able to stop the spread, and am a bit out of ideas.

Help much appreciated.
Cheers.

http://www.plantphys.net/article.php?ch=t&id=289
Hey Marco, heres a link to fotos of plant nutrient deficiencies.(Tomatos in this case.) Take note if the yellowing is interveinal or entire on the leaves and whether or not it's the new or older leaves first. You may get better results with watering if you get a suitably sized container like a drum or bucket and put the pot in that then fill it with water and soak for an hour. Do this each time the moisture is below an inch under the surface. It may be the plant is too big for the pot too and requires potting up. Do this after the fruit finishes.
A regular foliar feed with a complete liquid fertiliser with trace elements too may help with the yellowing.

I've been meaning to say "Thanks!" for these tips for a while.

Curiously, the plant still fruited superbly this Summer and I found that using a good houseplant feed at slightly above the recommended concentration worked very well in at least making sure that the new leaves stayed green and healthy, while the old ones dropped off in a crunchy pile.

I think it will make it through OK, now, and will prune back hard this Winter to see what next year brings.