Is black spot on Catasetum rot or sunburn?

[sonorlite]

I have a Catasetum division that has grown beautifully this year and has a spike about to open. I also have a large black patch on the newest pseudobulb. The spot is very hard and slightly depressed but does seem to be getting larger. Is this black rot or some other fungal disease?

For most of the growing season it had been under compact fluorescents but I moved it to a South facing window when the spike got too big. I do get full sun in this window and I try and move the plant when it's sunny so it might also be sunburn.