Answer: If you've positively ruled out insects, I'd suspect a cultural problem. Leaves can be damaged by cold weather or an insect snack attack as they are just forming. The damage often won't show up until the leaves have completely unfurled, at which time the damage grows as the leaves grow. (One small bite can appear gigantic when the leaf has reached mature size.) Hope for warmer weather and subsequent leaves should be just fine.
Q&A Library Searching Tips