From the Super Mario Wiki
Crabbie Grass is an item needed for Mario's sake in Mario & Luigi: Superstar Saga. This is a small, bitter-tasting herb known to cure a disease known as Bean Fever. If this disease is not cured in three days, the victim will be turned into a Bean. Once Mario falls ill with the disease from eating an Invincishroom, it is up to Luigi to go to Guffawha Ruins and retrieve the Crabbie Grass at the end. Luigi only finds one piece of this herbicidal medicine, but it is still enough to cure Mario.
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