Odd stain on Oncidium leaf

[Ray]

I'm going to disagree with Roberta on this one. I think there was some mechanical damage (insect?) on that left leaf margin, and an infection started there and spread. See my red vectors in the attached image.

I recommend cutting off the leaf below any trace of infection, such as where I indicated with the yellow line. Dust the wound with cinnamon powder immediately after cutting and that should be the last of it.