From the Super Mario Wiki
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Dr. Toby is a somewhat absent-minded, yet friendly doctor, who possesses the superhuman ability to sense whenever a plant is dying. When the basil plant belonging to Mama Mario (which Mario and Luigi were looking after) is dying, Dr. Toby can sense it and comes running over to Mario Brothers Plumbing to help.
Dr. Toby presents Mario and Luigi with his experimental Super Plant Growth Formula, which he claims would be able to save the basil bush, and warns them to follow the instructions before leaving.
At some point later, Dr. Toby is called by Mario, who reports that his plant formula worked, but ends up transforming the basil bush into a Plant Monster that is attacking Luigi. Dr. Toby tries to give Mario some advice over the phone, but has to ultimately return to the basement with an antidote for the formula, which he uses to shrink the monster back down to its original basil bush form.
As an apology, Dr. Toby then offers the Mario Bros. his new fast-acting Super Yeast formula to use in their pizza dough, but they quickly refuse his offer.