Was ist das?

I had the same problem myself, and I managed to fix it quickly using water and baking soda. Just mix 1 teaspoon of baking soda in 1 liter of water and spray it on the affected areas. It’s best to spray just before the lights go off, so you don’t risk burning the leaves.

mildew times 4!

Powdery mildew Sorry dude make sure to keep humidity low durring flower like never turn on a humidifier during flower

looks like mildew

white powedery mildew. There is no effective product out there that you can buy or anecdotal shamanistic nonsense like the 'cornell formula' lol... or milk.. don't spray your plant with weird shit that won't work. Cut it off, and pray it hasn't spread. Once you can see it, it's already producing spores, unfortunately. This is almost always caused by climate issues -- I'd wager you are hitting dewpoint after lights go out... Temps drop, absolutey humidity remains the same, but RH% spikes. If there is a big enough temperature drop, you are very likely causing condensation every night, which makes WPM almost inevitable. a wireless temp/rh probe, this makes reviewing data easy and no need to potentially interrupt a photoperiods dark cycle in flower, if that is the case. When i scaled up a few years back, i assumed my dehum was doing its job. It was fine during light hours. I grow photoperiods, so looking at a rh reading inside the tent after lights out in flower is not possible. Only after i had a wireless probe did i realize my RH was spiking each night and when the plants were large enough, condensation was consistently occurring. I was getting WPM like clockwork about 5-6 weeks into flower phase. The timing was very similar each time. The point of that anecdote is that the climate is the likely culprit here. Once that was fixed, i've never seen wpm since. Not only that, i was so pissed and demoralized, i didn't even clean my tent after a grow cycle.. slimmed down number of plants, and due to less moisture and dehum keeping up, that cycle was wpm free... there were definitely spores in that tent, too. It never got a foothold and grew because the environment didn't allow it. So, even if you think it's fine, you need to look into your temps and RH and what's happening throughout 24-hour cycle.. not just when you look at a thermometer or a hygrometer. It's a 10 USD device, so they are cheap. Something you can read outside the tent so you don't disrupt photoperiods etc... or if oyu have autoflowers, interurupt their dark cycle all you want and read our probes, lol.