NOID Denrobium leaves and stems turning yellow

[Floridan by Choice]

I’m fairly new at this and have no idea what is going on. I have researched for a possible answer to the problem I’m having but found nothing. I have gone over the plant with a magnifying glass and found no pests or damaged areas but I guess something microscopic could be going on.

The blooms (just three of them) just finished on this Den, they are still on the plant, wilted, but they lasted 8 weeks (I think this is great) and the plant seemed healthy until three days ago. I have attached a picture of the blooms when they first opened so someone might be able to identify the plant by those blooms. Three days ago, same time I noticed the blooms were at the end, I noticed some yellow mottling of the leaves. Yesterday, they were even more yellow and today even more yellow. I have four other Dens, but all with white blooms, and none of them mottling in the least.

This is not a new re-pot, no pests visible, not over watered – if anything under watered – and I spray every couple of weeks with either Neem Oil or Thiomyl.

We have been having very cold nights for the last week or so, down into the 40s, but I have been keeping them in a little lean-to hot house at night and the temp in there never got below about 55 degrees.

Could this yellow mottling be from the cold or can anybody spot something else that might be going on. Maybe this is one of those species that drop their leaves? Any help would be appreciated.