black tip

[MT-Phal]

so one of the roots on my phal, it grew... turned slightly brown... then a sprouted a new, healthy one. however, that new healthy root grew very very quickly at first, but suddenly it's very green tip turned into velamin and it's tip turned black. what happened?

i should say that there was a mold issue and i had to repot, again, a few weeks ago. it happened right after. i applied a little bit of isypropl alcohol mixed with water, then dusted lightly with cinnamon.