Question
Thanks for the reply Greg. I've linked to another picture that shows the base, if that helps. The plant hasn't been outside, but as you can see it does sit near a plate glass window and we have had some extremely cold weather in the past few weeks - even though that probably isn't the cause in this case, for future reference is it possible to damage a cactus having it sit too close to a window during extreme cold, even if the room itself never gets below 60 degrees?

http://www.nationhost.com/cactus/cactus013.jpg
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Followup To
Question -
Pictures linked below.

This cactus starting turning black at the top less than a week ago and has now spread to where you see it in the photos. What is the probable cause and is there any hope of salvaging part of it? Thanks in advance.

http://www.nationhost.com/cactus/cactus002.jpg
http://www.nationhost.com/cactus/cactus003.jpg
http://www.nationhost.com/cactus/cactus006.jpg
http://www.nationhost.com/cactus/cactus007.jpg
http://www.nationhost.com/cactus/cactus008.jpg
http://www.nationhost.com/cactus/cactus010.jpg
Answer -
Dear Jason,

Generally there are two reasons why a plant dies from the top down. Either the roots died a while ago (for whatever reason) or the plant was frozen. I can't tell from your pictures what the base of the plant looks like, but if you know it was frost that did the damage, the plant may heal and branch from the base near the soil. Only then trim off the dead parts.

If the plant was not exposed to freezing temperatures, then I am afraid there's not much you can do. Hopefully this helps.

Sincerely,

Greg


Answer
Dear Jason,

Yes, it's similar to what heat can do; only cold affected plants may not show damage for some time after the exposure. Cold travels through glass very well, unless your window is dual pane. Based on the picture, it looks like it is emaciated, or starved for water, which may indicate root damage.

If that is the case, it may be too late for your plant unfortunately. I can see from the picture that it has been watered, if you don't see any improvement in its appearance over the next few weeks or it gets worse, it will be time to replace the plant.

Sincerely,

Greg