Whitefly infested plants may be kale or other plants in the cabbage family. If so, they are most likely to be cabbage whiteflies, Aleyrodes proletella. See Cabbage Whitefly.
Cabbage whiteflies typically attack during the cool months, fall through winter. Even though the older leaves of cabbage, cauliflower, and broccoli may be heavily infested, you’ll still get good quality edible parts. But, if kale is the victim, these whiteflies colonize the choice edible portions – the young leaves.
You can rescue your crop with regularly scheduled search-and-destroy trips through the garden. When you see whiteflies, squish them. Periodically spritz the whiteflies with a harsh water spray. Or apply direct hits of insecticidal soap, diluted according to label directions. Repeat as needed. Some folks simply wash the leaves thoroughly just prior to using them