A state agency has extended by a month the deadline for California grape growers to return ballots on whether to continue an assessment on their crops to fund pest research.

The state Department of Food and Agriculture said the new deadline to return ballots will be June 10. The deadline had been Monday, but balloting has been plagued by voter apathy.

At least 40 percent of the estimated 6,500 eligible voters must participate for the referendum to be valid. As of last week, only 35 percent of all ballots had been returned in the vote on funding for research to combat Pierce’s disease, the glassy-winged sharpshooter and other pests that can destroy a vineyard.

The department also has sent duplicate ballots to growers who have not yet returned their ballots.

Growers have previously reauthorized the assessment in 2005 and 2010. The vote would extend the funding for another five years.