To add to the above if you hit the left mouse button you can attack and destroy healthy carrots, onions, and turnips. If you attack barley or flax it just harvests it if its ready.

Also note that if you have planted long rows and are harvesting that row in a continuous action after a certain number of consecutive pickings the sound will cut out and the picked crop icon will not drop to the ground or auto gather (this is a bug) and one or more plants, if you continue picking crops, will disappear. If you stop picking when the sound cuts out and let the program catch up you can prevent this bug from affecting you.