There is a Rhubarb curculio and also a stalk borer. In both cases the recommended treatment is to remove all weeds/grasses around the area where they insects hide. https://extension.umn.edu/vegetables/growing-rhubarb#insects-923464

Thanks for the speedy reply. I have no weeds or grasses near the plants.  I can not find any worm, bugs, snail, slugs on the plants.The plants are in a raised bed about 2' off ground level.  I did find that a mallard has made a nest between the 2 plants (since yesterday) and just now has laid an egg! Guess I will have to find rhubarb somewhere else this year!.
It is still a mystery to me. Do you have any other ideas and what to do?
Thank you. 

Seeing the photos, it also could be snail damage.

Hi,
If you see no sign of insect damage, I would not be surprised if rabbits are the culprit, especially if the stems appear to be snapped off. There really isn’t much you can do to deter rabbits other than fencing.
Enjoy the ducklings!