I read this children's book over 10 years ago (as a kid) and was super freaked out by it.
From what I can remember, a scientist puts a chemical/solution into a town’s water supply which gradually turns people into fruit/veg. The protagonists see people's skin turning green and fuzzy at school and realise they have to avoid contact with water/fresh food.
At some point they find a missing person in a lab (greenhouse?) who’s been turned into a semi-tomato plant. They end up saving the day by pushing the scientist into a vat of the liquid at a town fete, and he turns into a giant head of broccoli.