How could I save my orchids? Brassavolas with black rot and bacterial brown rot?

[meriadeliade]

Hi orchid growers!

I ordered some orchids a few weeks ago, and two specimens were sick at the time they arrived. I quarantined all the new orchids, but I am new to the hobby, and I would like to ask for your help.*

I don't have a proper growing space or a separate quarantine room... so I placed them in a room that has only one small eastern rooftop window. I can't measure the exact humidity, but since I live in a continental climate in a city, I think it is rather low... I'm pretty sure these conditions couldn't be more different from those in the nursery, but all my other rooms have plants, and I don't want to risk their health.

I think the Brassavola nodosa is very sick. It had a few dark-brown/blackish rotten spots on the leaves when it arrived. After the first watering, these rotten spots started to become larger, and also, new spots developed. The plant came in tightly stuffed, pure sphagnum moss and remained wet for a long time. So I tried to handle the problem, but I am sure I made some serious blunders...

Since the rot started to spread after the first watering, I decided to remove the wet media. The plant was out of the pot for a week, but I left some moss around, leaving it very airy and slightly moist. I noticed that most of the roots were gone, but I didn't want to bother them for a few days.

When I repot this orchid, I cut the rotten roots, but when I was removing the dead tissues and the leftover moss, I saw that some of these dead roots even had small tubers, which is certainly not normal (sorry, I couldn't take any photos of that, but they were the same color as the rot, solid, and about the size of a pea).*This is when it became obvious to me that even some rhizomes are affected, and the infection is more serious than I expected.*

I sprayed the remaining root system with hydrogen peroxide, put the plant in fresh media and watered it, probably making a big mistake. One week later, the media seems to be dry, but the rot is still spreading. I also realized that I put the older rhizomes too deep in the media and lost the older leaves due to that... What a stupid step...

I suppose the disease may be black rot. I am not sure, but I think the symptoms indicate that. Also, I found a paper (https://edis.ifas.ufl.edu/pdf/PP/PP26000.pdf) that shows the white mycelium of the fungus on wounds. I have the same issue. I made a picture of that as well.

I'm a bit less worried about my Brassavola David Sander (on the blue tray in the picture). It has some small brown spots, but for me it seems to be a different infection. These spots started to spread after repotting, and by now, most of the affected leaves have dropped. The plant is a young specimen and has only four leaves left, and I am afraid to cut them because the only healthy leaves are still developing.

Could you please give me any suggestions on how to treat these infections?

Do you think it is necessary to find a new quarantine with better conditions to help the orchids cope with the disease, or can I try to heal them where they are now?
Should I treat the B. David Sander with a systemic fungicide? Or is it a bacterial problem?
Shall I remove the B. nodosa from the pot and cut away all the infected areas? I have read posts on Orchidboard and many growers suggest doing so and giving the healthy part a chance for a new life without the rotten tissues.


I would be happy to read any kind of advice from you. Many thanks!