Grapes are poison to dogs. My boy Sage had cushing's disease and therefore died from it's complications, including cancer But, I can tell all that Grapes were probably an included part of Sage's demise. We have a winery and vineyards. The grapes are delicious even to dogs. Sage ate these little fruit for a few years until I discovered the facts. 
There can be a dangerous build-up of residuals in the renal system, more so in the kidneys if dogs consume grapes. I saw Sage's left kidney when it was removed. The internist iterated that he had never seen a canine kidney with such a particular condition. I am almost sure the residuals in that kidney was a result of my allowing Sage to eat grapes from our vineyard. In the beginning I was ignorant of such a canine renal poison, but it will be with me forever that I allowed this. I know so much about researching canine diseases now, that I wish I had known before getting my babies. I urge everyone to study their breed of dogs and be on the vigil for known conditions of that breed.