It is unclear what substance within these fruits is toxic to dogs. There are several theories: mycotoxin, salicylate, or tartaric acid (including tartrates like potassium bitartrate), are all naturally found in grapes and decrease blood flow to the kidneys.
The cause of kidney failure in dogs due to grapes and raisins is still unknown. It is known, however, that the danger occurs with both commercially-grown and home-grown grapes, so it is probably something in the grapes themselves rather than an insecticide or something like that.

A leading veterinarian is issuing an urgent warning to all dog owners. He highlights three common foods that can cause immediate and severe harm to pets.What they are: All forms, including seedless grapes, red and green varieties, and raisins.

Furthermore, visual representations like the one above help us fully grasp the concept of Can Green Grapes Cause Harm To Dogs.
The Unexplained Mystery of Grape Toxicity. Despite extensive research, the precise toxin in grapes (and raisins, which are dried grapes) that causes harm to dogs remains unidentified. Some theories suggest that it could be due to: Mycotoxins (toxins produced by fungi) on the grapes.
Grapes and raisins may seem harmless fruits but they pose a significant threat to dogs health. The exact substance within these fruits that causes toxicity in dogs is unknown; however, even a small amount can cause harmful effects such as kidney failure.