I recently posted a question asking why my watermelon leaves are starting to turn yellow, and I treated them with Immunox last week, but mine werent as wilted near the ends as yours, only yellow so far. Its only affecting 1/2 my plants so far too.

But yours could be Fusarium Wilt judging by the dead looking brown areas at the ends. Here is a link, and fro the little I read about this disease, there may not be a cure, but you can do things to prevent it next year. Preventative measures...

I am not saying that is definitely what you have, as I cant say for sure, you will have to bring leaf samples to your local cooperative extension service for evaluation. Look it up online.

Here is a link about Fusarium Wilt:

http://www.ars.usda.gov/is/pr/2006/060814.htm?pf=1

This post was edited by HighlanderNorth on Sun, Aug 3, 14 at 23:09

That looks like spray drift from a weed killer.

My first question would be what's with the soil being dark on one side and light on the other side? (wet vs dry?) as that appears to be the dividing point of healthy vs unhealthy vines.