My son is in college now but he has a nagging memory of a book he listened to long ago. I wonder if anyone here can name it for us? He says I checked out the book on cassette at the library for him. It was about a boy whose grandmother is infected(?) by mushrooms and becomes zombielike. I think the main character realized that she was infected when they were in her barn so maybe she lives on a farm or out in the country. So boy-grandmother-mushrooms-zombie(ish)-barn. I've searched and searched and can't find it.

Anyone know it? :)

Is there any chance he's not recalling a book, but a series called Ray Bradbury Theater?  There's an episode called Boys! Raise Giant Mushrooms in Your Cellar! that has a similar premise.  The story also appeared in The Machineries of Joy by Bradbury, so it may have been available as a book on tape.