That is stem rot.

It actually happens faster as temperatures rise.

Cut it off, dip the cut end in any good fungicide.
And try again.

I don't get rot anymore.

Not sure if you skipped post #179, or just didn't understand it?

Maybe go read it again.
Look at the pictures.
For rainwater, read distilled water.
Same thing.

And that's why I no longer get pythium on my cuttings.

Thanks for proving my point, brah.
Like I said, we can all learn from your thread.

Mahalo,
Weeze