Dendrobium Nobile possible fungal/bacteria infection
 

I recently went on vacation and came back to find my Nobile dropped most it's leaves and has a couple of canes turning yellow. Prior to leaving its leaves we're yellowing and dropping. . I moved it to semi-hydro almost 9+ months and was doing well. In SH it produced several canes and flowered prior to the issue starting. Before I left I carefully unpotted it to inspect the roots. I did not find any rot or pests. Now I'm thinking I have a fungal or bacteria infection and would like help identifying what it is and what I should do. I have have this one for at least 5 years and would like to save it if possible.

As winter approaches, these naturally drop leaves. Cut back on the water. How much depends on humidity... if in a humid greenhouse, maybe water once a month. If outdoors in a dry climate, once a week or 2 weeks may be appropriate. No fertilizer until you see new growth in the spring.

It looks extremely healthy. Lack of water and cool temperatures caused it, as Roberta mentioned, to drop its leaves. While they can be grown with full sets of leaves with winter watering, some people intentionally withhold water to the point leaves drop.

I am used to it dropping leave in the winter as I have had it for years, but it is actively growing new canes and the leafs are dropping off the new canes. The new leaves have small spots and don't look healthy. I'm also starting to lose caned for the first time in years and the same time.

When was the last time it was repotted? This isn't a good time for repotting, but if the medium is really bad you may have to do it.

About 9 months ago and it is in Leca.

So medium is not the problem... perhaps someone else can come up with some ideas!

With any thin-leafed Den. I always worry about spider mites. They could cause problems like in the second set of photos. The yellowing stems I see look quite old; they could be dying naturally.

I took a closer look and noticed what seemed like a red powder on the leaves and then took this image with my macro lense.

I think yoi have spider mites. I would spray all surfaces of the plant and pot with either 70% isopropyl alcohol (rubbing alcohol) or a solution of 1/2 - 1 teaspoon of liquid dish detergent per quart of water. Your other plants have them too.