Question
I have what I think is a barrel cactus that was given to me as a gift.  Recently the
base has started to shrink and I've noticed browning near where the spines
protrude.  Today I noticed this browning seems to be spreading.  I assume this
is bad.  What's wrong with my cactus?  How can I save it?

Answer
James,

Browning sometimes occurs when a cactus ages. However, yours may be caused by rot. One way to be sure is to push on the brown part. If it is soft, then you have rot and unfortunately with barrel cactus, there is nothing you can do. If it is hard, then it may be the normal aging process.

If you cactus is OK, remember to water only when the soil is dry. Hope this helps.

Sincerely,

Greg