This could be either a bacterial or fungal infection, not uncommon with cactus. This is very hard to treat and may be the demise of the plant. Since it is traveling from the bottom up, we suspect the roots of your cactus have already rotted, and it is just a matter of time for the rest of the plant to catch up. If you want to try to save it, follow the directions for potted canvas here: http://homeguides.sfgate.com/repair-cactus-root-rot-68177.html The rot you see here is caused by overwatering, possibly combined with a soil mix that is not porous enough–it should feel gritty when you rub some between your fingers. Also, there should never be any water left in the bottom of the saucer, as this will keep the soil soggy. We’re sorry we couldn’t give you good news.