On the mission / quest ‘ Skag Dog Days ‘ you are tasked with collecting 5 Cactus Fruit. This mission can confuse people as shooting the red highlighted plant, does not do anything.
The key to get through this is to shoot it with a weapon that has splash damage. In your inventory, you should have a gun called ‘ The Big Succ ‘. It’s a quest item. Pressing the directional button down will switch the gun to a grenade launcher mode. Fire at the cactus fruit tree and fruit should dislodge from the plant.
*Also, You can ground pound the plant for the same results.
There you have it. Hopefully, this information will help someone that is stuck, like I was.