I've got a small basil plan that suddenly started to get yellowing leaves with fine dark powdery looking marks underneath. I'm not a pro so have been doing some searches and think it might be downy mildew based on that research. Any thoughts based on these photos?

I pulled off all of the affected leaves. Is there anything I can use to treat or to ensure the unaffected leaves don't get the same thing? Or is the whole plant shot?

The pictures are the bottoms and then the tops of two of the leaves I pulled off (same leaves in both pics).