From the Super Mario Wiki
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 had begun growing vegetables all over his body after eating spaghetti sauce filled with dehydrated vegetable seeds, called Old McDonald for help. As Young McDonald explained when he arrived at Mario Brothers Plumbing, Old McDonald was unavailable, attending a potato convention, so Young McDonald visited Mario and Luigi in his place.
Brought over to Luigi, Young McDonald began to inspect him to see what was wrong with him, and after looking, said he had never seen anything like this before; asking Mario for some clippers, Young McDonald decided to simply cut and pull the vegetables growing on Luigi off, which appeared to have worked. After apparently curing Luigi, Young McDonald was asked by Mario to take a look at the nearby vegetable garden growing in Mario and Luigi's spare bathtub. As Young McDonald inspected Mario and Luigi's oddly shrieking carrots, Luigi suddenly grew more vegetables from his body and alerted Mario and Young McDonald to this; seeing Luigi in this state, Young McDonald tried to reassure him, saying that maybe he would get used to having vegetables grow from his body, much to Luigi's frustration.
Later, still wanting to help Luigi, Young McDonald began to try and research what was wrong with Luigi; as he researched, Young McDonald managed to stop a fight that was occurring between Mario and Luigi by saying he thinks he's found information on what is wrong with Luigi. Young McDonald revealed to Mario and Luigi that Luigi was suffering from Veggieitis, a rare disease that occures every three-hundred years in Brooklyn, and only when dehydrated vegetable seeds is eaten in spaghetti sauce. Young McDonald also told Luigi that to cure his Veggieitis, he would need to eat a rose, which would cause all the vegetables on his body to instantly fall off. Eating an old rose Mario gave him, Luigi was informed by Young McDonald that eating a rose could cause Roseitis, which lasts a week and causes roses to grow from a person's ears.