Very large cactus seems to be very sick! Help!

[lmutius]

Hi guys!

I really need your help.
I have a beautiful cactus in my apartment. It is right by the window and gets a ton of sunlight all day.
In the past couple days parts of it has change color to a grayish dust and looks a bit dead. We think it might be frost from the window being left open for a couple of hours.
Below are some pictures. Does anyone know what it could be? Is there anything I can do to make it better? Any help and advice will be greatly appreciated.



Thanks,
Laura

[greenknight]

The top section of that one shoot is dead - cut it off well below the dead part.

The grayish dust is mold, I think. Good idea to dust the cut with sulphur (you can buy it at any drug store).

[lmutius]

Thank you very much for your help!! Will cut it and apply the sulphur.