see AZ1399

http://cals.arizona.edu/maricopa/garden/html/pubs/pubs.htm#Desert

Might want to read 1048 and 1225 as well.

Looks like a bacterial or fungal rot to me. Whether primary or secondary I don't know since it started at the top and is working its way down. But I am no expert.

Were it me if I can find no further info on such infections in organ pipe cactus, I'd sterilize a machete (best to make a single cut) or tree saw and remove the arm as close to the base as I could and then dob the wound with a fresh (1:10) 10% bleach dilution and then let it dry and heal over. This is a two person job however. One with gloves and an old piece of carpet to hold the arm from falling as it is cut. Discard the infected arm in the trash and any debris from the cactus in the area.

I would perform an autopsy on the plant arm before discarding. Inspect the wounded base. Is it nice and white or brown or black discolorations noted? I would cut off the end (well away form the now wounded organ pipe so as not to spread the disease) and inspect it for same. If no discolorations are noted you may have saved the cactus.

Here is a link that might be useful: Desert Plants Info

This post was edited by Fascist_Nation on Wed, Oct 29, 14 at 14:17

Yup, it's an organ pipe cactus for sure. I'm with FN: I'd cut that sickly arm off WELL below the green part, probably all the way down to the base, and discard it and then sterilize the cutting implement with 10% bleach just to be safe.

Sometimes these things just happen, or it could be a sign of something else. I'd check any irrigation nearby to make sure the cactus isn't getting watered very often. Once a month in spring/summer/early autumn and none in winter is plenty in my opinion.

Let us know what you do and how it works out.
Good luck!
Grant