Basils have yellow powder on underside of leaves -- what could it be? & harvested/pruned

Hmmm...  The plants look super nice and green on the upper sides, but noticed today while pruning/harvesting that most leaves have subtle patches of yellow powder.  Sort of reminds me of last year's downy mildew except totally the wrong colour, and not fuzzy.  

Oh well, it washed off well, so fingers crossed!

It's on both varieties -- the hybrid I'm trying for the first time that's DM-resistant, and the classic Genovese that I bought from the store.