Black root rot/stem rot

My cardon. (The oldest) died. 
Er.. was unsavable.

If you look at the pictures the yellowing at the root wasn't enough to cut off and save..
The two others survived. Which I replanted seperate. And they are fine. :-)


I attempted to cut out the black spot I first noticed after some research with stem rot....

But a day later I came home to see this (below)

I knew it might not be able to be saved even though I repotted seperate and in a faster draining 50/50 cactus soil and sterile sand mix for faster draining (so maybe the root could dry out?)

but when I took out of the soil to check how bad the yellowing of the stem was it wasnt enough to keep it up..

ABOVE: closer to my thumb is the rot spot.. You might be able to see the yellowing (my mobile pictures arent that great) 

Above: turned about 20%, you can see the yellowing more

Above: Circled black spot (EEP!)

 I ended up having to quarantine this cactus for that 24hour test period before throwing it out when I checked the root after cutting the black spot. I didnt want it to infect the others..