Just as some has mentioned... just clean it off after your hunt. Dry it with a rag. If not planning on digging again anytime soon... put a little wd40 or light oil on it. Try to keep the sheath clean and dry.
This tool will last years. I love my Lesche. I have dug over 10,000 coins in the past 4 1/2 years (counted) and at least half that many other targets and expect to continue digging with no issues of rust.
....just keep digging each day and don't give it a chance to sit idle and rust.... LOL
But seriously
keep it clean and dry you'll have no issue. Good luck hunting.