root tips turning black

[pinett]

Hello,
this is my dendrobium amabile, put in S/H since june. It was doing well almost till november as you can see in this photo, with active root growing.
at the moment, some root tips are turning black and one leaf in the middle height of the cane is now completely and uniformly yellow.
there have been no recent change in the growing condition, good humidity 60-70%, just I think lower night temperature as it's winter here.
is it normal? thank you