Frank,
I hate to be the one to tell you, but it would seem that you have the "Locke Arugual Monster" loose in your garden.
Only kidding LOL
For me it was flea beetles. They can do a number overnight.
1eyedJack and the Dawg

Perhaps leafminers?

If so, you can often see them between the top and bottom layers of the leaf when you hold the leaf up to the light.

I know it's not leafminers, although the leafminers and me are currently going to battle in my chard section of the garden. This is a dry white almost brittle looking patch. It's not confined to the edges of the leaves.

Does that sound like flea beatles and if so do you have any ideas of to get rid of them without using pesticides? I already tried Safer Insect Soap and it didn't help.

Thanks?

Flea beetles make shotgun pellet-like holes in the leaves. What you describe sounds to me like classic sun-scorch. Lettuce planted in full sun, especially in south or southwest facing gardens, get a sudden rise in internal leaf temperature, producing a white patchy scorched appearance along the edges and ribs of the leaf.

Other possibilities are nitrogen burn or herbicide drift but based on your info it sure doesn't sound like any pest problem.

Dave