I repotted a younger plant into a pot still containing leftover soil from a male plant I just killed, and now it looks like this. I had recently put nutrients in the soil for the old plant so did those cause this to happen to this plant?
are the older roots still intact or did you break them all up and let them dry out before using the old soil? I had this problem before because I was being lazy and it strangled the roots and eventually died...be careful when using old dirt that has roots in it that are 'fresh'.
Try re-transplanting if possible or at least getting the old roots out.