Members of the board shall be appointed by the Governor on a nonpartisan basis. Candidates for appointment by the Governor to the initial board may place their names on a candidacy list for the respective district by filing a petition signed by at least fifty growers of such district with the Governor. Candidates for appointment to subsequent boards or to fill a vacancy in either a district or at-large membership position shall file such petitions with the existing board. Qualified individuals residing within their district shall be eligible for nomination as candidates from such district.