Turnip

Turnips are the first official vegetables that ever appear with their first appearance being in Yume Kōjō: Doki Doki Panic. They are frequently found in Subcon.

Yume Kōjō: Doki Doki Panic
Turnips only make an appearance in World 7-2 of Yume Kōjō: Doki Doki Panic. They are produced from the Nightmare Machine, which the Arabian Family uses to throw vegetables at Wart's mouth to defeat him. The Turnips are the only effective items they could use as weapons against their foe to beat him. The bubbles Wart spits will fade way a Turnip if touched once.

Super Mario Bros. 2
Turnips make their first Mario game appearance in Super Mario Bros. 2 in World 7. They now have a different appearnce than in Yume Kōjō: Doki Doki Panic. This time, Turnips are planted in the ground everywhere in Subcon, especially in grassy regions of the world. Turnips can be picked up and thrown at enemies to kill them. The Nightmare Machine can also produce Turnips from it, and they can be used as weapons for Mario to injure Wart, like in Yume Kojo: Doki Doki Panic.

BS Super Mario USA
Turnips make an appearance in the direct sequel, BS Super Mario USA, and are used for the same purpose as in the earlier game. Sometimes, enemies can be turned into these vegetables by certain characters' spells.

Super Smash Bros. series
Princess Peach's down special move starting from Super Smash Bros. Melee is picking up turnips and using them as projectiles. The facial expression of the turnip determines the damage and knockback it causes (e.g. winking turnips cause more damage and knockback than normal smiling ones). The strongest variety of turnip, the sick turnip, is rare, but relatively powerful.

Captain Toad: Treasure Tracker
Turnips make their true 3D debut in Captain Toad: Treasure Tracker. They can be plucked from the ground by Captain Toad or Toadette and thrown to attack enemies. Turnips can also be used to unlock or discover new pathways by having the turnips thrown at obstacles such as POW blocks. In the minecart levels of the game, Captain Toad rides a minecart full of turnips that can be thrown in many angles at his surrounding environment. Some levels also contain cannons that can fire turnips, starting with Turnip Cannon Jungle. Additionally, turnips thrown at Super Gems will effectively allow Captain Toad to obtain the gems without having to grab them himself.

Giant turnips also return from Super Mario Advance during the boss battle against Wingo. At the end of the battle, the final giant turnip thrown at Wingo by Captain Toad causes the bird boss to accidentally swallow the massive vegetable and become too bloated to fly, resulting in Captain Toad's victory.

Paper Mario: Color Splash
A Turnip returns in Paper Mario: Color Splash as a Thing. It appears in the enterance of the Sacred Forest, blocking the Warp Pipe to the rest of the course. All eleven members of the Yellow Rescue Squad must be found in order to pull it free. When it is used in battle, it will become sliced and the slices will fly at the enemies.

Trivia

 * In the Super Mario Mash-Up Pack in Minecraft: Wii U Edition, Potatoes are replaced by Turnips.