STEM PROBLEM PLEASE HELP

My plants look very healthy, the stems were growing strong... The roots are spotless... I am using GH nutes this is my second grow.. Cant seem to figure it out.. Sorry I have no camera, but they are a foot tall very nice except for the stem.. The stem seems to have "hour glassed" on a couple plants in my hydro system.. Approx 1 inch up the rocks... Seems to start with like a light black ring aroung the stem and then seems to dry rot the stem.. Please Any help ..... Again sorry no pics..


I thought maybe it was the black line touching the stem, but now I am leaning towards some type of mold/fungi...