Never seen this before. About a quarter of my Tahitian melon leaves are *white*. The white leaves are in groups. That is, they cluster together. Strangely, one cluster of white leaves is way out on the end of a ten-foot vine. So it isn't as if some vines are white, and some aren't. The white doesn't rub off, and it's clearly different than the normal white veining one usually sees (and is also visible in this picture on the green leaves). Not powdery mildew. Nothing evidently "wrong". Everything very healthy, but what ...?