Trouble with Oncidiums - Virus or pest?

[Miyuki]

My Miltassia and Degarmoara have been infected with this rust-like stuff on their leaves. It has only appeared on older leaves. Both plants are actively growing, and the new leaves are fine (so far....) The lighter coloured flecks appear like dust and can be rubbed off. I thought spider mites at first, but there is no movement and no webbing. The rust discolouration is right in the leaf. It is most prominent in the folds of the leaf at the base, and where there is the odd pleated spot. But today I also found some on the underside at a tip. I've been removing infected leaves and spraying with an equivalent to Physan. However, it keeps appearing. Any ideas? Help please!