Last night with the temperature at 32F and the sky dry and clear, we had a radiative frost (heat in the plants, ground and streets escaping to space) and sweet potato 'Marguerite' looked like this in the morning. The old timers called this "frost burn," as if a torch blackened the leaves, not cold. Same outcome. This plant has branched so many times that the growing tip of the vines has to point up to grow above the carpet of leaves and the dense shade they cast. But here's the poor growing point (with its small, young leaves) killed and blackened by radiative frost. Some leaves that were protected by the leaves above them will keep their color but the growing points like this one have come to the end of their lives.