Dendrobium Spectabile - yellowing leaves
 

I purchased a big Dendrobium Spectabile about a month ago. Since then I've had it in a greenhouse tent with my vandas.

The vitals: the tent has a ton of light - so much so that I had to switch off two of the LED bars because the vandas' leaves were turning purple - constant 65-70% humidity, 14 hours of light per day, a deep watering every 7-10 days (with distilled water), complete fertilizer every other watering, and was getting a light misting once or twice a day - its pot is below the vandas, so when those get their daily misting(s), the excess water drips into the pot. It's it in fir bark, charcoal, and perlite in a 5" plastic pot (inside a 6" terracotta). The temperature when I first brought it home was ~62* at night, but the weather's been warming up and now it barely falls below 68*, into the low 80s during the day. I have two 4" desk fans in the tent for air circulation and it sits beside a sundew+ping for passive gnat control.

Over the past two-three weeks, the leaves on the smallest canes have started yellowing, and the pace seems to have increased over the past few days. I have a Den Thanes Blue that was doing the same thing, but once I increased its watering, the yellowing stopped. I've started directly misting the spectabile's pot every morning, but it isn't producing the same results. Now I worry it's getting too much water. A new growth has poked up through the medium this week (the last picture), so maybe it's just drawing resources from the old leaves to fuel this growth and it's nothing to worry about, but I don't have much experience with dendrobiums so I figured I'd ask. Any thoughts are appreciated.

That looks like a pretty coarse mix. And these need lots of water when they are in active growth (which is happening now) Don't spritz, WATER.

Misting doesn't do anything useful for most plants. Pretend you never heard of it. It is a good way to humidify an enclosed terrarium.