Many puppies, and a good number of adult dogs, will grab their leashes while in the middle of a walk (We're looking at you, Retrievers :p ). There's lots of reasons they might do this: It is fun for them, they are really excited and putting their mouth on something makes them feel better, and/or they are frustrated and it makes them feel better.
Here are a few simple ways to reduce this behavior!
Bring a soft tug toy on walks with you. When you notice your pup is thinking of grabbing the leash, offer him the toy. This should be a tuggy that your dog LOVES and that does not "live" in the house. (This keeps it interesting and fresh!)
Some dogs do well with a toy to hold onto. If your dog really likes carrying a ball, I recommend the Fetch It! Brand Breathe Right Ball, especially in the summer. These have special holes in them that allow more air flow.
Don't play tug! When your dog pulls on his leash with his mouth, try to minimize how much “tug” he gets to play by moving toward him (when safe) and keeping the leash from getting taught.
Let them get their ya-ya's out. It's important to remind ourselves that our dogs are, well, dogs! Puppies and teenagers in particular can have a hard time with maintaining self-control all the time. (Heck, humans do, too!) If the above strategies aren't working perfectly, make sure you eliminate the rewards of leash-tugging that you can (by not playing tug, just going really quiet, and not engaging your dog) and calmly wait for your dog to be done. Once he's settled, start walking again!
Try a less-appealing leash. Some leashes are just extra awesome to chew! If your dog is really into that rope leash, it could be time to try a different material. A harder, skinnier nylon would be one option. Many dogs are less interested in something that doesn't "squish" or tug well. If you're feeling stuck, consider a small-link chain leash. (Remember that it's important to choose a size where your dog's teeth are unlikely to get caught.)
Something not working out? Let your trainer know so we can help you troubleshoot!