This treasure is actually a strawberry. It's found on a ledge in the Awakening Wood, behind the sleeping Red Bulborb in the main field area of the Awakening Wood. It's best to defeat the bulborb first, then just throw some Pikmin up onto the ledge to pick it up. This treasure is identical to the Combustion Berry, which is just a slightly larger version of the Sunseed Berry.
 Olimar's Journal
"I found this fascinating fruit while out exploring today. The skies cleared up, and I felt peaceful watching the pink petals flutter in the wind. During this perilous expedition, it seems strange to have such softhearted, romantic thoughts. If I told my wife about this, I'm sure to get laughed at... again."
 Sales Pitch
"This fruit is born of bright sunlight and cheerful warmth. Come, all you gloomy naysayers! One bite and even sad sacks will become foolhardy optimists. Eat one today and change your life!"