I use rainwater on my garden. Algae builds and stops up my drip irrigation system. What can I do to prevent this and still be safe for my garden foods?

Algae requires light to grow. Is your tubing or rainbarrel clear or light colored? Is it possible to set a planter on top of the barrel or to otherwise shield the barrel from light exposure? If you are using clear irrigation tubing, I suggest switching over to black flex tubing or solid tubing that prohibits light when replacing parts next time. In the meantime, you likely need to detach the barrel and use a bit of bleach first to kill what has grown in your system. I'd stick a few piece of old screening near the exit to catch random particles, or maybe set up a gravel/sand filtration process inside your rain barrel. After that is done, then shield the stored tankwater from all sun.