Cure dark eco infected plants was a mission in The Precursor Legacy that Jak and Daxter completed to obtain a power cell. Jak had to cure plants infected with dark eco after a request from Samos Hagai.
The PlayStation 3 and Vita versions of this mission are related to a trophy named "Green Thumb", completing this mission will reward you with a silver trophy.
You can find the dark eco infected plants in the second area of the Precursor Basin on the very right. You will need to pass through a green eco vent using the A-Grav Zoomer found nearby, and then fly around passing over all of the dark eco infected plants. When you run out of green eco, quickly fly back to the vent and return as quickly as possible to the plants as they will gradually grow back. You will want to keep doing this until all of the infected plants are gone. When this is done, a new plant will grow and release a power cell in its bud.