Sorry you are having such problems with your basil plants. I live in NE Florida. I always grow mine from seeds. They grow like weeds here. I always put mine out in full sun. I never move it when it rains. I've never had a problem with it.

I would remove the bad leaves. It should send out new shoots whereever you remove leaves

Do you have pictures? Most likely it is downy mildew. Nothing out there organically to control it. Sophie has the best advice but not sure your current plant will survive. Best bet is to pitch it and get a new one.

No, no mildew, just black spot but now....dirt. Killed another group of plants. I am going to get new dirt, scrub the clay pot and try one more time when the weather gets a bit warmer. Thank you again.

So you're saying it's cold outside? How cold is it getting at night time?

Basil leaves often develop black spots when it's cold. I sometimes get it on mine during the winter. You can also see it if you store leaves in the fridge. But it's not a fungus.