what is happening to my cactus

[notexan]

I have had this plant for 4 years. Just recently I have noticed the outer ends of all the fins are turning soft and black all the way up. It appears to me that it is dying. Is there anything I can do?

[templegatejohn]

I am not sure what you mean by the "fins" but if you mean the ribs and the plant is a columnar cactus rather than a globular one check if the top of the plant is OK, if it is cut it off with a clean sharp knife, if where you have cut through looks green and healthy keep the top of the plant for a week to 10 days to allow the cut end to dry and "skin" over and then place it upright on some sand or fairly dry compost and hope that the plant re-roots. Depending on the temperature this could take anything from 2-3 weeks to 6 months or more.
The problem with your cactus sounds like black rot, wich is usually attributable to over-watering and there is very little that can be done to eradicate it once it is there.
I'm afraid most of us have had a plant die of this problem so don't be disheartened.
John

[daiv]

I agree with John's suspicion based on the available data. Is it possible to get a picture posted of it? We could narrow it down much more easily for you.