Wilting water chestnut tree.

I hope it isn’t too cheeky asking this here (and this may not be the right forum), but my girlfriend’s plant is wilting and we can’t think anywhere to go for advice. She’s in Central London, nowhere near a garden centre, and I’ve hunted around on the net to no avail. Basically, she has a water chestnut plant that seems to have a diseased stem. That stem (the tree has five or so stems which twine around each other) is soft and discoloured, and the leaves from that part have fallen off.

Does anyone out there know what this could be, and what we could do to cure it?

It would help if you could tell us exactly what you mean by “water chestnut plant”.

Is it the deciduous tree native to Central America or the aquatic plant that is prohibited in the U.S.?

Tree.
http://www.floridagardener.com/pom/pachira.htm

Prohibited aquatic plant.
http://plants.usda.gov/cgi_bin/plant_profile.cgi?symbol=TRNA
http://infoweb.magi.com/~ehaber/factnut.html

Assuming that you’re referring to the prohibited aquatic plant, European water chestnut or trapa natans, and assuming that you’re dealing with some kind of stem rot, I can’t find any Google hits for it. Looks to me like nobody really cares what happens to a noxious weed with stem rot.

Buy another one? Since it’s an aquatic plant anyway, I’d guess that there’s really nothing you can do about stem rot.

This website says it’s best grown as an annual in Britain.
http://gardenbed.com/t/3926.cfm

So I’d throw it away and start over.

What is she growing this thing in? Just curious.

Could be mechanical, bacterial, fungal or viral. You need to find a local plant pathologist if you want to be sure. The usual cure for this sort of thing is to cut out the diseased portion, and hope you’ve gotten it all.
It is unclear from your description of the plant, whether you have some variant on the chestnut tree, or the water chestnut. Is it a woody plant ?