I was hoping somebody who knew what they were talking about would speak up, but since they haven't yet I'll throw out some possibilities. I don't think it's your temperature. I can't grow streps very easily here because it's too warm, tho' they'd be happy this winter. It's our summers they hate. Two possibilities for you to consider--how brieght is your light? Often stems get etiolated without enough light intensity. What are your fertilizer practises? A high-nitrogen fertilizer might contribute to the problem. Don't think humidity would be a contributing factor, at least unless it is really dry, but the dome would have cut down on the light, and it takes a little time to recover from that.