Answer: Based on your description I am not certain what this is. It could be a fungal problem such as anthracnose or it could be a wilt disease such as verticillium (although I would expect this one to be visible early in the season and then clear up), it could be related to overwatering or growing in a less than well drained location. Curling and distortion are often related to herbicide damage as well. You might take a look at the following information on anthracnose and see if it seems similar. You may need to cut and paste the complete url into your browser to make it work correctly.
Another possibility is that there is a wilt or canker disease problem. You might want to consult with a professionally trained and certified nurseryman or your UMass extension and see if they can identify it for you. I hope this helps you trouble shoot.
Q&A Library Searching Tips