From the Super Mario Wiki, the Mario encyclopedia
Young McDonald is a young farmer and the son of the well-known Old McDonald.
In The Super Mario Bros. Super Show! episode "Flower Power", Mario, wanting to help Luigi, who has begun growing vegetables all over his body after eating spaghetti sauce filled with dehydrated vegetable seeds, calls Old McDonald for help. As Young McDonald explains when he arrived at Mario Brothers Plumbing, Old McDonald is unavailable, attending a potato convention, so Young McDonald visited Mario and Luigi in his place.
Brought over to Luigi, Young McDonald begins to inspect him to see what is wrong with him, and after looking, says he has never seen anything like this before; asking Mario for some clippers, Young McDonald decides to simply cut and pull the vegetables growing on Luigi off, which appears to work. After apparently curing Luigi, Young McDonald is asked by Mario to take a look at the nearby vegetable garden growing in Mario and Luigi's spare bathtub. As Young McDonald inspect Mario and Luigi's oddly shrieking carrots, Luigi suddenly grows more vegetables from his body and alerts Mario and Young McDonald to this; seeing Luigi in this state, Young McDonald tries to reassure him, saying that maybe he will get used to having vegetables grow from his body, much to Luigi's frustration.
Later, still wanting to help Luigi, Young McDonald begins to try and research what was wrong with Luigi; as he researches, Young McDonald manages to stop a fight that occurs between Mario and Luigi by saying he thinks he has found information on what is wrong with Luigi. Young McDonald reveals to Mario and Luigi that Luigi is suffering from Veggieitis, a rare disease that occurs every three-hundred years in Brooklyn, and only when dehydrated vegetable seeds are eaten in spaghetti sauce. Young McDonald also tells Luigi that to cure his Veggieitis, he needs to eat a rose, which will cause all the vegetables on his body to instantly fall off. Eating an old rose Mario gives him, Luigi is informed by Young McDonald that eating a rose can cause Roseitis, which lasts a week and causes roses to grow from a person's ears.