I have a mint and a basil right next to each other. First the mint and then the basil developed a similar leaf disease.
Any advice is appreciated.
It's thrips. Tiny little bugs that usually eat insects by sucking out their insides, but some turn their vampirism to plants, biting and sucking the juices out of the leaves. Sticky traps, neem oil, or even dish soap can help get rid of them.