A group of Gerawan farm workers, led by 15-year employee Silvia Lopez, began gathering signatures for a petition to the ALRB to vote to decertify the union. A majority of workers were required to sign the petition for a vote to take place.
Late Wednesday, the board rejected those efforts, claiming that too many signatures were forged. It also alleged that the farm itself played a role in promoting the decertification effort. The workers themselves have denied that accusation.
The ALRB confirmed to CNBC that the vote is a no-go, and The Fresno Bee newspaper quoted from its 12-page ruling: "There is no doubt that there are Gerawan workers who genuinely want to decertify the union at their workplace ... (however) the evidence shows that a majority of the current employees at Gerawan have not expressed interest in decertifying the union."
(Watch: Union squeezing cash from forgotten members)
The UFW claimed victory and was planning a news conference in Fresno for Thursday afternoon. It said some workers whose names appeared on the petition told ALRB investigators they never signed it, that there was "compelling evidence" some of the signatures were forged, and that the alleged forgeries "may have been more widespread" than the investigation found.
The ALRB can now move forward and impose a contract on Gerawan Farming, which could mean higher wages and benefits for workers. It could also mean workers will have to pay 3 percent of their wages to the UFW in dues.
(Read more: Farmers raisin' hell over 'Raisin Reserve')
A lawyer for Lopez told the Bee that he may file a federal lawsuit. Paul Bauer claimed that barring Lopez's opportunity to vote on whether to be represented by a union that went missing for years violates his client's civil rights.
"We've been blocked at every juncture," Bauer said. "This is just another example of the board's efforts at thwarting a right to vote."
—By CNBC's Jane Wells; Follow her on Twitter: