Killer mushrooms ate my gran - Susan P Gates

Maggot's gran is upset when her fiance, the dashing explorer Jack Dash, stands her up at the altar. All she can find is the last basket of mushrooms he picked, with the words "Help me" carved in one of them. One of the sheds contains a most disgusting mushroom, shaped like a giant brain, red and pulsating - when it explodes its spores enter the human brain and gradually turn the human into a mushroom. Jack Dash has been turned and the spores invade Maggot and Gran. Maggot mysteriously recovers and, with the help of awful African green tea, saves Gran - but the spore continues to grow and must be destroyed.