dendrobium aphyllum with odd marks on leaves

[Petyr88]

Hello

I was recently given x2 divisions (all new canes ) of a dendrobium aphyllum from a friend. When I received the divsions, the canes were all small and were just sprouting leaves. As the weeks have passed, the divisions have grown.

I was recently looking at the divisions and it appears that some random marks have appeared on the leaves of one of them. It is like they appeared out of the blue. The other division (grown in the same pot) does not have any marks on it. The marks can be seen on the underside of the leaves as well.

The plant is grown under artificial lights in semi hydro however it is a fair distance a way from the lights (one shelf below them and barely getting the full intensity of the lights which are T5s) - my cattleya`s are the closest to the lights.

I am a novice with dendrobiums so would be keen to understand if this looks like a burn or something fungal? I do not spray or allow water to sit on the leaves. I have however recently started using a new fertilizer. As far as I am aware, no insects are present.

Any help most welcome