No, you do not have to file a separate extension for a 2018 Minnesota state income tax return. You can file the state return at any time up through October 15, 2019 to avoid a late filing penalty. Any state taxes owed had to be paid on or before April 15, 2019 to avoid a late payment penalty plus interest.
Go to this Minnesota Department of Revenue website for information - https://www.revenue.state.mn.us/filing-extensions