The first thing that came to mind was wildlife damage (i.e., deer and rabbits). However, if this were true the damage should only be where the varmints could reach. Thus, if the damage is all over the vine, it must be an insect of some sort, such as the grape cane girdler (Cornell University)
The most likely culprit is grasshoppers. Grasshoppers are not that difficult to kill, but they keep moving in after you treat. Contact your local Extension office for pesticide recommendations. Always read and follow the label directions.
Bath is an attractive, productive black grape with a mild
labrusca flavor. It ripens about a week before Concord and
may require careful pruning or thinnning to prevent overcropping. It is very subject to mite infestation. American type, dessert variety, slipskin. (Cornell University)
