New growths turning brown and withering from the bottom up
 

This has been a rapid onset issue with a few of my plants, others not pictured here. Some of them weren't even growing in the same area when it started.

I've been admittedly neglecting most of my orchids for the past year or so, rarely fertilizing and underwatering for sure. But I'm confused by this particular pattern of die-off. It starts with the NEWEST growths and from the bottom up, not from the leaf tips. In the case of the Prosthechea cochleata, I cut off all of the affected portion of the rhizome to prevent spread, but that did nothing. It has progressed unabated. This doesn't seem to be a problem of rot because, if anything, I'm way underwatering these specific plants. I barely ever bother to humidify this space, and it's been dry as a bone down there.

Is this a virus or just a plain sad depressed plant that needs some ordinary TLC? It's just so weird and rapid. Even the dying parts of the plants are putting out new roots (you can't see this in the pics).

Prosthechea cochcleata is a wet growing plant. If it stays dry too long this can happen. There are also ascending rots on Cattleyas that happen when it's hot and wet, but I don't think that is the issue now.

I've been growing this cochleata for about 8 years and I can usually tell when it's dry. This issue started before I moved it to the drier growspace (I thought drier might help, especially as it recovered from its surgery). I know it's a thirsty plant ordinarily. It grows in natural light and as the days have grown shorter, I water less, but it was getting an amount of water and light its accostumed to. It was perfectly happy and thriving, putting out lots of new growth, until this suddenly started happening on one end of the rhizome (it's growing in a few different directions). Now I'm seeing it with other plants. The other plant pictured is an encyclia that shows the same pattern–ONLY the new growths are turning brown and dying. This one is getting the same treatment it always does. I've just never seen this before, where only the newest growth begins dying and it's not pulling energy from backbulbs if it's thirsty. The encyclia's new growth all died over the period of about a week.

What is your water source? What fertilizer do you use?

I haven't been fertilizing with as much dedication as I used to, but ordinarily I'd use rain water with Ray's K Lite and a kelp supplement in the spring/summer. A couple of the affected plants have had that regimen continue, and others I'm just using tap, and I use tap most of the time in the winter with no fert.