Passion fruit leaves yellowing and falling off - what to do?

Hi there!

I have a young Nellie Kelly grafted passion fruit plant growing in a pot. It's been growing vigorously over summer and now has 4 fruit on it - see the pictures. But from about two weeks ago (when it started getting colder here - it's autumn now) it stopped growing altogether and its leaves started yellowing from the middle of the plant outwards, and bit by bit they are falling off.

I then very carefully transplanted it to a larger pot without doing any damage to the roots, because I thought it might have outgrown its pot, which seemed to be the case as the rootball was very dense. I watered and fed well straight after transplanting. Nothing has changed since then though, the yellowing continues.

Is there anything wrong with this plant, or is this normal in the colder months? If there's something wrong, what can I do to save it?

A couple of "stats":

- Plants is about 1 - 1.5 years old

- I live in Sydney, Australia

- I water the plant well, at least once a week but more, even daily for a while, during the hot months. I also feed regularly with a seaweed product and "powerfeed".

- It gets plenty of sun and is in a fairly sheltered spot.

- Possible causes I am thinking of: 1. Over-watering? 2. Applied too much powerfeed, causing toxicity?

Any advice would be greatly appreciated as I love this plant and would hate for it to die.

Cheers,

Armand