Rust on apple sapling leaves is typically caused by fungal diseases with names like cedar-apple rust, quince rust, and hawthorn rust. They usually get it if there's a red cedar or juniper tree nearby, but fungi can be airborn...a stiff wind could have brought it.
But it was probably on that gardener's mower blades. That's more likely if other trees aren't nearby.
Yeah, you gotta trim those bits off and then wash the tool you use with soap and water. Wash it before, too!