Idk exactly what you should do but did your plant look like this when you first got it? Also it may need more light.

Just to clarify - my question is mainly what I should do with the biggest stalk? It's not very attractive since it started losing leaves, and it is so much taller than the other stalks. Can I prune it back?

@cactusboss - it never looked like your plant. The big stalk was the only plant originally and it was very small, about the same size as the little all-green plant in there now.

Well now that i think about it it is a Dieffenbachia and lower leaf loss as they get older is inevitable so enjoy a maturing plant or you can air layer it. somebody else can explain explain air layering cause idk exactly how it works. Your dief looks to be the same kind as mine(Camile) i just think mine has more than one plant in it. Also what humidity do you grow your plant in? It may want more humidity however mine tolerates direct hot/cold air from the vent even though sources say they hate that.

This post was edited by CactusBoss on Sun, Sep 15, 13 at 17:37

Stunning! Diefs propagate easily although I wouldn't cut this beauty! Should you want to cut it, the stump should grow one or more new tops, the cut top can be stuck back in the pot or its' own pot. Repotting should make it happy again, back to losing leaves much more slowly (which I would do whether cutting the tall one or not.)

I like it as is..but if you want, do as purple advise..be carefull about over watering tho.

This looks like a dieffenbachia 'tropic snow', or a similar cultivar. It looks nice and healthy and growing well, so I think your watering and light are spot on. As purple said, if you don't like the bare trunk look, dieffs are easy to cut and re-root. Though you can fuss with air layering if you want to, not at all necessary.