Why is this deficiency or toxicity spreading so fast!?

The issue with calcium deficiency is that the symptoms start appearing several weeks after the deficiency begins; meaning the symptoms will not go away right away after correcting the issue. No, the buds quality will not be affected

Go through your system and double check things are in place and working properly. Rust is an air borne fungi. if given the opportunity, it WILL take hold. it usually does during flower as this is when the plants immune system is at its weakest as its energies are concentrating on re-production, not survival. depending on what part of the grow you get it, you can cure/minimize it with either chemical or natural remedies. it enters the leaf via the stomata and then it's off and running. fungacides used are generally copper based and include; thriram, manzecob, maneb or zineb at 10 day intervills. organic; baking soda, 1 tea spoon/ quart of water. spray on. copper sulphate, read lables. fungacidal soap sprays; garlic sprays, 1/2 cup minced garlic, 1 quart of water. let sit 24hrs, strain, spray. sulphur, one of the best natural sprays. it is low in toxicity. micro kill, a citric based killer has great sucess. If my answer helps you please pick it and save your crop please, dont mess with soil, mess the fungus intead. Good luck. Hope you help back for the grow questions contest, its just picking one answer and you help me or anothers.

Looks like calmag problems so you have probably already fixed it. Keep your ppms between 700-1000 though so they dont start getting other deficiencies. Phosphorus is especially important right now. Best of luck mate, hopefully this stops the spread although it won't fix what's already damaged!