Is the soil staying wet or are you letting it dry some between watering?

I've been watering when the top of the mix is visibly dry (lighter color). Also feeding lightly, 1/4 of the usual dose for feeding with every watering, with Miracle Gro in the green bottle.

I am not a begonia specialist but always have some in my garden in summer but I start with tubers. As I read your post I thought it might be cooked leaves. The post below, B&L Begonias 2012 has his lights two feet from the plants. I thought that was high and might be because they are shade plants. But with your post today I cringed at Miracle Grow. I have it and use it on older plants but when I clone I like something less chemical. I would use stinky fish emulsion or Liquinox Grow. Both are sold at HD. I hope you can figure your problem out in time.

Feeding is probably okay since you are doing it at 1/4 dosage. It could be humidity - how low does it get? How cold does it get in the room?

I don't have a hygrometer yet, so don't know on the humidity. It's a basement apartment, half buried like a hobbit hole, and damp enough we've had to treat surfaces for mold.

Temps have been in the 60's during heating season. The apartment upstairs has the thermostat that controls both apartments.

Peggiewho, I do have Neptune's Harvest fish/seaweed but will not use it in my living room, which is where these plants are. I used it indoors once and decided that "low tide" scent was best kept outdoors.

I think there are some changes going on they don't like. What I would try is to put them in some kind of glass or plastic container, with a lid, so the temp or humidity changes, whatever is going on, can't reach them. I keep lots of seedlings under glass, so to speak. At about 3 inches the bottom of a soft drink container holds them and top cut off and placed over them makes a neat terrarium. I don't think the lights are too strong and I think fertilizer should be gentle at this stage. That's all I can think of, good luck!

Dug up my old thread because I never reported back. I did cover them as woebegonia suggested and that seemed to help. They were fine once outdoors. They didn't get very big but did flower. So far this year, I have six saved tubers that have survived winter storage and woken up.

Oddly, I also grew Go-Go begonias from seed last year, started in February, and they were right next to the Illuminations but showed no signs of distress.