The quest Days of the Skags-Dogs will ask you to collect Red fruits to collect from cacti. To recover them, you will need a weapon that does damage by projection, such as a grenade launcher or a rocket launcher.
Once the cactus is touched, a red fruit should fall. Another solution exists if you do not have weapons of this type. To do this, you will need to get high and perform a crush attack towards the ground as close as possible to one of these cacti to be able to drop its red fruit.
Where to find the cacti with the red fruits?
The first two cacti to find will be quite easy to see since they will be close to you as soon as you enter the mission area. The cacti containing the red fruits will be glowing red.
The first will be on the left next to a lair from which Succulent Skags will escape and the second will be right in front of the first near a path that leads higher.
The others will be marked on your map. All you have to do is go to the places indicated. The area is quite small so you should have no trouble spotting them.
The only problem for these is that the markers don't disappear until you've completed the mission, so remember the places where you've already harvested the previous fruits.