Need help with Brassavola nodosa rot

[Conjuration]

How the rot happened (I think):

It all began with a repot.

More than a week ago, I repotted the B. nodosa I bought early November. The nodosa was in bloom but I spotted bugs in the substrate and, with the visible roots not in the best of shapes, I thought I'd take her out and have a closer look.

She was potted in, mostly, bark chips. There was moss involved (of which I shall elaborate upon later). I removed the as much organic media as I could then repotted her into a mix of leca and pumice (both soaked beforehand) in semihydro setup.

Then I made the mistake of upending a plastic bag over her to increase ambient humidity (and eliminating airflow). The plastic bag did little to increase humidity during the days, which were hot and measured around 30C/86F in temperature and 49% in humidity. The nights were much cooler at around 22C/72F and 68%.

A few days later, I noticed white fuzz (mold) growing on the canes and the exposed roots. All the growing root tips were dead or dying. I immediately removed the bag.

A day later, I discovered a brownish soft spot (pictures attached) at the edge of an old leaf that's in contact with wet pumice. The pumice was moved away, but the soft spot continued growing. I cut off the infected leaf two days after the soft spot was first discovered.

Yesterday, I found another infection (pictures attached). This time it's on the newest growth (which was slowly growing in length and roots before repot) and had, seemingly, spread upward from the cane. This soft spot spreads much faster than the previous one.

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I've tried comparing my nodosa's infected area with pictures on the St. Augustine Orchid Society website. I think the rot is, and was, either brown rot or black rot.

I have no experience on rot and my amateur diagnosis includes both bacterial and fungal infection. So I'd very much appreciate a more experienced opinion on this.

What's the disease my nodosa's infected with? How should I treat it? Since the newest soft spot spread up from the canes, does it mean the rhizome's infected? Do I need to cut up the rhizome? What can I do to help my nodosa recover?

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More on the repotting:

The bark was in okay condition (as in not mushy or rotten). But the roots had adhered to the bark chips. I had to gently prise the chips off roots, accidentally tearing off a few roots in the process .

Overall condition of the roots wasn't so good. The roots ranged from dry to desiccated despite the moist condition of the bark. As I unravel the outer roots, I discovered that the roots of some older pseudobulbs were around 2 inches in length, had signs of being previously rotted, and were wrapped around soggy moss.

I don't know if those roots in moss were still viable, so I didn't cut them off. Aside from the roots I accidentally torn off, the only roots removed were the ones which velamen were falling off.

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Regarding attachments:

First two photos -- Front and back photos of infection on newest growth today

Next three photos -- Front and back photos of infection on newest growth yesterday

Next five photos -- Front and back photos of infection on the leaf that was cut off.

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I really really like this orchid and very much want to keep her alive. So if you've got any suggestion/advice/personal anecdote, feel free to throw them at me. I need as much information as I can get!

And thanks for bearing through my lengthy, slightly panicked rambling!

[Ray]

Your excellent description of the process and conditions suggests a variety of potential issues.

An immediate action I'd recommend is lifting the plant out of the medium about 1 cm or so, as it appears to be potted too deeply, which can lead to the rots you are seeing.

Also, it is important to understand how plant tissues grow, especially roots, in this case. As they grow, the meristem cells differentiate into a variety of "root cells", arranging their numbers, location, and size so the roots will funtion optimally in the environment into whch they are growing. Once they have grown, they cannot change, so when you move them from one environment to another - in this case bark to semi-hydroponics - you have immediately rendered them to be sub-optimal and they will begin to fail.

That's why the best time to repot is right when new roots are emerging from the base of the plant; they will take over the support of the plant while the old ones die.

I will add that the rotting of the old roots is not a "disease" as we typically think, but is the normal decomposition of the dead and dying tissues.

Having said that, any root damage caused by the repotting process itself, including prying bits of bark off of them (which was not totally necessary), provides potential sites for invation of pathogens.

At this point, you're in recovery mode, so in addition to elevating the plant a bit in the medium so that the root/leaf junctions is at-, or above the medium surface, you're going to want to keep the plant very warm to induce root growth (seedling heat mats are good for warming the container of medium, which favors that). Application of a root growth stimulant would also be good, but I don't know what's available to you.

[Conjuration]

Thank you for your reply! I'll remove the topmost layer of pumice (should do the same job as lifting the plant out a bit).

Originally Posted by Ray

That's why the best time to repot is right when new roots are emerging from the base of the plant; they will take over the support of the plant while the old ones die.

I did repot while new roots were emerging (she was blooming and growing a new leaf at the same time). It's just... the new tips all died.

Originally Posted by Ray

including prying bits of bark off of them (which was not totally necessary)

Oh, I actually thought organic medium inside a semihydro setup would rapidly degrade and rot the surrounding roots and was quite meticulous about removing the organic matter...

Originally Posted by Ray

At this point, you're in recovery mode, so in addition to elevating the plant a bit in the medium so that the root/leaf junctions is at-, or above the medium surface, you're going to want to keep the plant very warm to induce root growth (seedling heat mats are good for warming the container of medium, which favors that).

I can add heating to the bottom of the pot. Should I also place a fan somewhere in the vicinity? It would increase airflow, but also cool down the plant (probably too much with the dropping winter temperature). Airflow or warmth, which one should I aim for?

Originally Posted by Ray

Application of a root growth stimulant would also be good, but I don't know what's available to you.

Rooting hormones (NAA, more specifically) are popular and cheap here. But I can definitely find seaweed extract too.

What about the rotting new leaf? I believe I ought to cut it off (at the base I think). Should I do anything about the rhizome? I don't know how the rot spread from the sick old leaf to the new leaf (they weren't even touching) or if indeed the two leaves contracted the same disease.