I have two christmas cacti and weird things happening to them. They developed yellowing branches from the side. Now i know its not usual yellowing from the tip that is associated with lack of humidity or lack of water, but from the sides of the branch. I gave them lots of direct light and kept them in the cool location because they go into dormancy after flowering, but this hasn't detered yellowing. The surface of the leaves where yellowing occured is limp, but so far the problem didn't spread, i've put them in a much warmer place and i am ready to increase the watering, but unfortunately i cannot correct the light, because i can provide only so much with my east orieneted window? So my question, does it something to do with environment? is it a virus?