It looks like basil downy mildew. Look under the leaves and see if you see a black fuzz.
There is little you can do to save them. Bag and trash and dispose. There are no genovese basil that are resistant.
African basil and holy basil have better resistance. African basil has a high camphor content and holy basil has a different taste.
you can report finding downy mildew on the cornell website. Since the spores are in the air, you can't plant basil for a couple of years. If downy mildew is prevalent in your area if your neighbors a half a mile away have it, you may get it too.
Eleonora is a new strain of sweet basil that has been developed with moderate resistance to downy mildew. It is not 100 percent resistant and it does not taste the same, but without genetic engineering it is the best that could be done so far with traditional breeding.
https://www.extension.umn.edu/garden/yar ... ny-mildew/
https://vegetablemdonline.ppath.cornell. ... Downy.html