We've grown mustard in our raised beds with row covers on them all winter. They are going great guns and looking good now. However, we noticed a few leaves that had this on (in?) them. See picture. At first I thought they might be insect eggs. But on further thought maybe not. First, the row covers are pretty tight and I don't think bugs have gotten in. Second, no kale in the bed. Though I like to eat it, it attracts more bugs than I want to deal with. And third, it's right along the veining and no where else. Could it maybe be a (almost) freeze when it was taking up water? Anyway, just curoious. Michaela