What is causing this?

https://drcannabis.io/symptoms Here is a good guide with pictures. I have found it very helpful myself. Good luck :)

Never take advice from a stoner.

I'm with grannie on this one, you have a pest problem. The Amazing Dr. Zymes works very well at fighting not just pests, but any sort of fungi as well. It takes care of powdery mildew and the works. But, it's a little expensive. You can get free samples though, you just have to pay shipping! Plus, you get a sweet wizard pin, to boot. www.doctorzymes.com/wizard If you don't have much money, you can bum a few cigarettes from a friend/neighbor and dissolve them in 90%+ isopropyl alcohol. After that, strain the leftovers of the cigarettes and tobacco out of the alcohol and then mix it with a pint-quart of water. Spray liberally! The nicotine will be absorbed by the pests and kill all that bump into it or get sprayed. Good luck!

Welcome to summertime growing! You've got an infestation of some kind of leaf sucking insect - either thrips or spider mites or both... Grab yourself some Spinosad - you can get it on Amazon or any local garden center should have it, too - it's an organic insecticide and is the best one for killing thrips, spider mites, aphids, whiteflies, leaf miners.... and the nice part is that you can use this all the way to harvest if need be (and it will probably be needed!). Oh - and btw, spray it on your other plants as a preventative! Good luck!