Turnip
A Turnip is the first official vegetable that ever made an appearance before the rest, starting in Yume Kojo: Doki Doki Panic. They are frequently found in Subcon, yet are originally found in the Real World. These veggie-like vegetables have smiling faces.
History
Yume Kojo: Doki Doki Panic
Turnips only made appearance in World 7-2 of Yume Kojo: Doki Doki Panic. They are produced from the Nightmare Machine, which the Arabian Family will use to throw at Wart's mouth to defeat him, as the Turnips are the only effective items they need to use as weapons against their foe to beat him. Wart's bubbles he spit will fade way a Turnip if touched once.
Super Mario series
Super Mario Bros. 2
Turnips made their first Mario game appearance in Super Mario Bros. 2. They have now different version than that from Yume Kojo: Doki Doki Panic. This time, Turnips are planted on the ground everywhere in Subcon especially in grassy regions of the world, including for some of the rest of the vegetables. Turnips can be picked up and be thrown at enemies to kill them. The Nightmare Machine can also produce the Turnips from it, and can be used as weapons for Mario to injure Wart, like in Yume Kojo: Doki Doki Panic.
BS Super Mario USA
Turnips makes appearance in the direct sequel, BS Super Mario USA, and used for the same purpose as the prequel. Sometime, enemies and such could be turned into these vegetables by certain characters' spell. They are even found from the Nightmare Machine.