Any Ideas??

[Cavs n' Orchids]

I have had this Catt for 5 years or so, and I was wondering if anyone knows why some pseudobulbs get this burnt look. It isnt squishy, and it has happened before without hurting the plant, just making it unsightly. It kinda has a burnt look, but I am pretty sure its not sunburn because they leaves have always been fine, its just the pbulbs that are affected. I only fertilize VERY weakly once or twice a month and the roots never get that burned look. Maybe fungal, but I spray 4 times a year with Physan. Anyone ever experience it or have any ideas?
Btw, this guy is C. Hawaii (C. Rembrandt x C. bertii)