Mandevilla vine in pot NOT happy

I am hoping to use this in a wedding NEXT week (9/16/11). there is a problem in one area - one whole strand has been producing yellow leaves, now the whole strand is starting to wilt -- !!!! Any suggestions??

Here's another picture -- it is yellowing in one area (the same area where the whole strand is now starting to wilt).

here is another photos -- anyone have any suggestions?

Another photo: this one shows ONE leaf (only one leaf!) that is deformed & has tiny black specks on the underside. This is the only one I can find. The other leaves that have been rapidly yellowing do not show this.

I have this potted in a big pot, along w/ sweet potato vine. It sits in full sun, have watered it daily, have fed w/ Miracle Grow once/week all summer. It is to be used in an outdoor wedding on 9/16/11. I have read all about how 'rapidly" this vine grows - -you couldn't prove it by me! I don't think it grows quickly at all. I have fed it, pinched it back, done whatever i could to stimulate it, and now this. Sigh!!

I forgot to say: This is a white Mandevilla vine.

If it's just one branch/strand that's wilting, I'd examine it and see if it got bent/broken somewhere--that could cause that particular branch to wilt. Watering issues are a common cause of yellowing & wilting (too much water and too little water can both do this) but I would expect that to have a more widespread effect over more of the plant.

I thought of that when it first started showing yellowed leaves, b/c they all seemed to be confined to that strand of the vine. I traced it all the way back down to the pot it was planted in, but I cannot see anything pinching or impinging on it. I am baffled.

My fear is that by Friday 9/16/11, the WILT will be much worse -- I've been babying this thing along all summer for this wedding! :(

I was googling around on http://hortchat.com/info/mandevilla-care-and-tips and someone described something similar, and the response was that maybe it was a fungus, and they suggested Zyban, a fungicide. So I googled around about that & it is apparently carcinogenic, teratogenic, and mutagenic. !!!! Not exactly what I want my kid to get married next to -- a pot of lethal chemicals. Sigh!!

(there is also an anti-smoking medication w/ the same name (Zyban); different substance altogether than this fungicide).