"Scab Plants" are the little red orbs that you see from the very beginning of the game. You shoot these to terraform the level and sometimes to reveal hidden power-ups.
The total ammount you need for the achievement is obviously 1,000 but this comes extremely quick in the game. Also, this is cumulative over all levels played, not all are needed in one go.
The red bumps in the photo are "Scab Plants"