Aphids. Lots of them.

Squish, wash off, or direct shots of insecticidal soap.

I'm familiar with several types of aphids but the bottoms of the leaves are completely clean of insects or eggs. The purple color does go all the way through, and just looks like a slightly raised bump on an otherwise healthy leaf. In very advanced stages it did seem to be detrimental - the one plant I found that was almost covered in it was curling up at the edges and wilting, but still no visible bugs anywhere. Could the aphids be hiding that well and still doing damage?

Hmm. part of the leaf? Can't find any info about that. Perhaps someone else will chime in.

Some things are regional. Where do you live other than the US, zone 5b?

If only a few leaves, I'd remove & discard.

but it surely looks like something else is dining on the leaves.

Hey mine too!! Some on the Quinoa and some on the Lambsquarters (what you are calliing goosefoot?) And I'm in Zone 3 Canada. Some of the spots are green but most are the same purple that yours have in the picture.

think of it as variegation.. and ignore it.. lol ...

that is what i have always done.. and never really cared to know what it is ...

hows that for bettering myself thru education.. lol ...

ken

Looks similar to the damage caused by Crimson erineum mites

Ian

I'll have to check out the mites as a possibility. I never found out what it was, but it's gone now. I pulled all the affected plants and got ahead of it. It did spread regionally throughout the yard, and did affect plant health when extremely severe although the plants stood up to initial stages pretty well. I haven't seen a newly affected plant in at least three weeks now.

It's amazing the variety of critters and things we find in our gardens.

Goosefoot - Lambsquarter's - same thing, different name.