Rhubarb and the stage
How did the rhubarb plant, popular for its tart stalks used in pies, come to mean a loud dispute on the baseball field? Forget the garden. A rhubarb, or "squabble," comes from the stage. Years ago, actors who participated in mob scenes, especially angry ones, commonly intoned the sonorous "Rhubarb! Rhubarb!" to give the impression of a loud fracas. (Try it!)
Therefore, rhubarb came to mean a row.
At about the same time, baseball was coming into its own in America. It didn't take long for rhubarb to reach the diamond. Sportswriters insisted on using the colorful theatrical term when there was an argument on the field that involved both teams.
Calamity and crop failure
One possible theory behind the origin of this word traces it to farmers in ancient Greece. They planted an important crop, calamus, which is a kind of wheat.
When the calamus harvest was successful, it meant prosperity for the community; when it failed, there was hardship.
When droughts, floods, or pests damaged the calamus crop, the disaster was called "calamitas."
Over time, the word "calamity" came to mean grave misfortune anywhere.
SOURCES: 'Why You Say It,' by Webb Garrison; 'The Encyclopedia of Word and Phrase Origins,' by Robert Hendrickson; 'What's in a Word,' by Webb Garrison.