Lease negotiations to build a $50 million minor league baseball park on a former National Grid gas distribution site on Commercial Street in Malden have taken longer than expected, and will probably delay Opening Day to April 2015, one year later than originally planned, the developer said.

“We had a challenging timetable anyhow, but things have gone slower than we would have wanted,” said Alex Bok, chief executive officer of Boston Baseball Field of Dreams LLC, the development group proposing to build the ballpark. “We’re working our way through many issues — rent, credit, environmental.”