Sorry you haven't gotten any replies yet, but that's because no one can ID the problem.

I've never had fungus on my ornamental sweet potato vines, but doesn't mean it doesn't happen. How is the newly emerging foliage looking? Does it have the same problem?

You probably need to take some leaves in a plastic bag to a good nursery to see if they can tell. It looks a lot like chilli thrips damage. However, it seems a little early for them to show up, but with a warm winter in zone 9 they might have stayed active. How about it Houston folks?

Let us know when you find out something. I'm very curious about it.

Texas A&M horticulturalist Doug Welsh advised that the issue on the sweet potato vine above is likely due to stress due to abnormally cool spring we've had. He does not believe it to be a fungus. Sweet potato vine prefers nice warm sunny days. We've had some of those here in College Station, but we've also had a number of cold fronts push through and drop temps quite low for a few days. Doug thins this is the problem and warned growers to try and hold off planting sweet potato vine (or sweet potatoes) too early. You can hear his full response on the Garden Success podcast archive page (04/18/2013).

Hope that is helpful to others too!

I appreciate you letting us know the answer you received!

My sweet potatoes look exactly the same in my raised bed. But it can't be the weather because I just planted them in June. Plus the 2 I planted in my front yard in the flower bed are perfectly fine.

Did yours ever recover??

My purple ones look a lot like that (and have since spring), the green ones look perfectly fine and happy, no clue what's up with the purple, last year they were so vigorous but this year barely hanging in there.

Here's a pic I took this morning of my raised bed with the purple and green sweet potato vine, green looks great, purple really slumping (I'll post a pic in the next post of a close up of the purple).

These aren't fresh plantings this spring, I think this is the second or third year with these same plants, but the first year the purple looks like they do now.

Here's the close up of the purple, darned if I know what the deal is.

I think it is called edema. Another thing to think about is "Mesophyll cell collapse", but I think it is edema. I just heard about the second one so That us just for information and further googling sake.

http://edis.ifas.ufl.edu/pp265

Here is a link that might be useful: Edema article

I took your pic to the local nursery and they said it might be sun scalding