In addition to the regular election under Section 50730, upon receipt of a recommendation of the board of trustees or upon receipt of a verified petition of 20 percent of the landowners who, together, own at least 20 percent in value of the lands in the district, as determined from the last tax roll of the county, the board of supervisors shall appoint a time, not less than 75 days or more than 90 days after receipt of the petition, and place for holding a general district election for the unexpired term of the trustees then serving.