That's a leaf that will eventually die back. It was bound to do something like that. I wouldn't worry about that leaf.
On most Oncs, when the new shoot gets to a certain size, it starts to produce a pseudobulb. After the pseudobulb forms, the old leaves surrounding the pseudobulb eventually die back. It's part of the natural cycle.
Last edited by King_of_orchid_growing:); 03-20-2013 at 10:37 PM..