Dendrobium leaf "trouble"

[stonedragonfarms]

Does anybody know what might be causing this? I'd initially thought thrips, but I can find no evidence of them in the gh. I'd also thought mites, but there is no residue on the leaves. This seems to be localized to one plant in my collection and seems to only manifest on leaves that are in their second season; I've not observed it on any of this years growths.