Not so! I found today that if it is removed then no charge goes to the battery!
That's a bit weird. The only reason I can see to have a diode in the mower harness is as a blocking diode, to prevent the battery running the hour meter. The diode on the engine side of the plug is already performing the rectifier function.
Unless there's more than two terminals in the diode plug, and it has more than one circuit?
That wiring diagram isn't really clear enough to be a good circuit diagram, so I had to speculate a bit about the exact circuitry in that area of it. Whoops! That owners manual isn't in the ODK Ride-On manual library, either.