General:
* Immature
* Green (often used for fruit)
* Underdeveloped
* Young
* Raw
* Unripened
* Unseasoned
* Tender (sometimes used for fruits and vegetables)
More Specific:
* Hard (for fruits)
* Sour (for fruits)
* Tart (for fruits)
* Blighted (for plants that have not developed properly)
The best word to use depends on the context and what you are trying to emphasize.