Sounds like a Magnesium deficiency with the brown spots. Do the leaves curl upwards before they die ? And did it start at the bottom of the plant or the top ?
Maybe you have more than one problem. Get a magnifying glass and look for mites and or webs. I just picked up a product from General Hydro thats organic and its really easy to use. It's will kill them and keep them from returning. It's called Aza Maz. Goodluck
spider mites eventually kill marijuana, they spread from one plant to the next so no they wont be on all the plants yet. thats not mag defientcy causing the white spots, the spots turn brown after a while because the leaf is being sucked dry from the mites. look with a micro scope and have a look at the little pricks! peace