It’s a common problem, especially with puppies and young adult dogs. What is is? Leash chewing.
You go for a walk and they turn and chomp on the leash, or maybe you’re in group training class and they begin to chew while the trainer explains the next task, or maybe you need to tie them up for a minute while you clean up a mess and they begin to naw while they wait. If this sounds familiar have no worries there is an easy, inexpensive fix.
And we don’t mean those sprays that come in a million different flavors because, well, some dogs ACTUALLY like the flavors. That’s the whole reason why there are different flavors. While one dog may hate the sour spray another may enjoy it and think you just made the leash taste EVEN BETTER! And let’s not forget that the sprays only work if you remember to keep reapplying them to the leash.
So what’s the easy fix? A small piece of PVC pipe (see photo). 
All you need to do is go to your local hardware store and buy a small PVC pipe to slide over the leash. Usually 6- 12 inches of pipe will do the job (Home Depot will even cut the pipe to size for you!). Be sure the width of the pipe isn’t too big and the pipe isn’t too heavy for your pup. Don’t worry they come in sizes that work for even the smallest of pups (Maggie in the photo is only a few pounds).
Most dog’s do not enjoy the feeling of the pipe when they turn to chew and will stop the behavior pretty quickly. Not to mention the pipe will also protect the leash so you aren’t dropping $20 on a new leash every time your pup chews up another one.