So my next lesson is humidity

I cant grow weed safe with 70-80% humidity i guess.

My oldest plant is hit with mold.
Should i take the whole thing down? Buy a dehumidifier, and keep the 3 plants that are few weeks behind in flower?
Is there anything i should worry about like lingering spores?

Cut off affected areas. Lower humidity will rendering the germination impossible of the spores. More heat if possible(makes my house a sauna so argh), exhaust? Have to find something to hang over inline fan for less vibration(stealth) for improved airflow
And never 80% again
At least the wait is over

Lowering rh and increasing air flow like with box fans was how I got rid of it. No cleaning needed. When tents are too packed cant really do nothin but pray. I low key love that and aim to push it next run because if youndo make it youll get rewarded lol.

The following is about basil, maybe it applies to cannabis. The quick summary is decrease N and increase K and Ca. I personally would avoid the fungicides mentioned, at least until I had researched whether they are safe on cannabis, which I doubt. KCl should be fine. As previously mentioned, dehumidify and increase airflow.

Kind of depends how profuse it is, if your near the end of flower and it’s not already spread through everything, then you might get luck and keep on top of it. But I would isolate the infected one for now if possible, get a dehumidifier and drop that RH down low.
Some pictures of where you are at and what the damage is would help

Remove infected areas, and lower RH promptly.
At end of flower dont go above 60RH
Trichomes love dry cold environments at the end to fully express.
At veg there's no problem, high heat and high humidity is where you wanna be
At flower, until week 5-6 you should keep RH high, something like 70+
At week 6-7 and beyond, drop RH as low as you can with dropping temps aswell
Dont drop temps or RH without the other, it will fuck up your grow.