A 66-year-old farm worker accused of murdering seven colleagues in a mass shooting last month at two mushroom-growing sites near San Francisco has pleaded not guilty to all charges.

Chinese citizen Chunli Zhao is the lone suspect in the January 23 massacre in the seaside town of Half Moon Bay.

San Mateo County District Attorney Steve Wagstaffe said Mr Zhao entered the plea himself, speaking through a Mandarin-language interpreter, during a brief court hearing in Redwood City.

The next hearing was set for May 3.

That proceeding was scheduled to set a future date for a preliminary hearing, where prosecutors present their case to persuade the judge they have sufficient evidence to proceed to trial.

Mr Zhao is charged with seven counts of premeditated murder and a single count of attempted murder in a criminal complaint that also cites "special circumstance" allegations accusing him of "personally and intentionally" shooting to kill.

Under California law, defendants convicted of murder with "special circumstances" are eligible for the death penalty, though Governor Gavin Newsom in 2019 declared a moratorium on executions.

The state has not carried out an execution since 2006.

Authorities said they have yet to establish a clear motive for the killings, the second of back-to-back deadly gun rampages in California last month in which a total of 18 victims were killed two days apart.

Mr Zhao had been employed by one of the growers, Mountain Mushroom Farm, and had resided at the property along with some other employees, according to its owners.