It looks like Phyllosticta Leaf Spot to me but then I can only judge from your pictures. It can occur on Cymbidiums, Oncidiums and Ondontoglossums but is perhaps more serious on Dendrobiums. It is usually not a severe problem but rather cosmetic. Spots may start anywhere on the leaf or pseudobulb as small yellow lesions. As they continue to grow, they become round or oval and are slightly sunken. The black spots in the center are the spores of the fungus. This disease is usually not that serious but if you want to attack it, try a mancozeb fungicide as a foliar spray.
Jim
