We do not think that this is a pest or a disease, and it should be fine to eat..
It looks like an environmental issue called edema.
Here is our page on it: https://extension.umd.edu/hgic/topics/edema-vegetables
Interestingly, Rutgers just put out a notice (on cabbage but occurs on lettuce as well) that they are seeing it now too:
I am familiar with edema but wasn't sure about the white growth on the edge of the leaves??
So this growth can be edema related as well ??
Thank you Christine.
Hi - Sorry, we reviewed this question too quickly this morning. It looks like it could be dried latex (sap) and it would be there due to some physical or environmental injury like wind or abrasion. It is normal for lettuces to produce a milky sap, typically when they are developing flowers. Another possibility would be residue if you applied a liquid fertilizer. As with all produce, wash the lettuce well prior to consuming.