In May, I took some greenwood forsythia cuttings, dipped them in a hormone rooting powder and planted them in rooting mix. The mix was kept moist all year. I had thought they had rooted because they began to grow rapidly and the new shoots hardened off. In October the plants dropped their leaves, and by November they were dead. I dug them up and they had not rooted. There wasn't even a trace of root. What caused it to do this and how do I prevent it?
I don't think the problem is forsythia specific - I've had the same experience with peach cuttings. The first time I tried it I took the cuttings at the wrong time of year (ie. a different unrelated problem), but the second time I took them at the right time and I had one success out of about half a dozen cuttings.
All of the failures in the second batch showed some leaves. Many were either too small (possible a problem specific to peaches where a cutting should be a minimum size); or with the larger cuttings when I dug them up, any roots had rotted away and the stem was beginning to rot. I believe this is your problem. The rot (or mold - rot is after all usually a kind of mold) is due to too much moisture. In your case you describe the mix as being moist all year - so it was probably too moist.
In my case I probably had more variability (outside, Texas, hand watering) - so it was probably too moist at a critical time.