Without being able to see and touch the leaf, this is difficult but I will give you my best guess. The line patterns on the upper leaf look like a mosaic virus. Just which one is hard to say. If it is reddish in color, it is probably tobacco mosaic virus. The white power like substance on the lower leaf looks like salt residue, perhaps from hard water or fertilizer? The two black spots on the lower leaf could be Cercospora Leaf Spot. Is the under side of the spot yellow? This is a common fungus on Dendrobiums.
Jim
