White powder on lettuce

If it is on bottom side of the leaves, and a high level of humiditiy, it is Bremia lactucae(latin). Mostly use of systemic fungicide as quadris, aliette or avi handle this. BUT if lettuce is near time when you plann to pick it, I advise to only force the picking instead of use of fungicide.

Most of people use fungicides on copper base, but it has shown at some sorts can affect in a way that lettuce won't form head, or it can be deformed.

But I strongly advise to use chemicals only if necesarry.

Apology for Tarzan english (not my language).

Is that just the lower leaves where they are in contact with the soil? Usually you'd discard those tough outer leaves, anyway, they are too bitter to eat. I guess it's a fungus from the soil.