What decimated my kale, collard, turnips, and mustard greens?

This is flea beetle damage. Flea beetles feed on brassicas, which include kale, collard, turnips, and mustard greens. If they feed on the plants when they are young, they will ruin all the leaves. UT Extension Agent Chris Cooper suggests using floating row covers to keep the beetles away until the plants are older. The plants can then tolerate the beetles feeding.

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Flea beetles
How do I get rid of flea beetles in my vegetable garden?How do I get rid of flea beetles in my vegetable garden?