Black stem on CLONES.what is it

some of the clones in one of my cloners got their stem black,the roots are fine white and normal the the leaves are good too,is this rot or something else

bubble cloner 1 air stone under a 23w flouoro
water temp-20c
plain tap water

anyone....

pics would help, are the black stems slimy?

m camera broke and i got rid of all the clones from that cloner,none of the other cutting have it but i would like to know what is it so if i get it again i can treat it,the stem gets balck at the tip and the lower part but the roots are white and healthy,one had some slime of 20