I am 90% sure it's a fungal problem. Especially with the described circumstances. I'd go with a low dose of broad spectrum fungicide and try it on few of the plants first. Keep in mind that most fungicides don't work well on saving already heavy infected plants, they can slow the growth of the pathogen and stop it to colonize more plants. Still you have to be vigilant.
There are a new breed of fungicides in the recent years - a fungus that feeds on other fungus. I know how that sounds but those are the best ecological solutions right now. If you can get your hands on one of those, it's your best bet and it's very safe! Good luck and ping me in Discord if you need to talk, as my time on Steemit is very limited (an hour a week tops). https://discord.gg/cJUQHG
Thanks, @bobydimitrov
I went to ask for some advice from the people that run the organic farm not so far from my house, and they said it is probably leaf spot disease. A bit of a bummer, because there aren't any biological ways to treat it.
For now, I'm trying to keep it from spreading by spraying every inch of every plant with a thin layer of lava rock and adding lava rock to the soil.
For now, it seems to be working. I know it has only been a couple of days, but some plants that were infected and of which I removed the sick leaves seem to grow healthy new leaves.
I'm crossing my fingers!
I'll definitely read up about the fungus that eats other fungi... interesting 👍