Undenge, who is also the deputy minister of Economic Development and Planning, allegedly made the remark during a campaign meeting held at Charter hall in Chimanimani last week.
According to sources who attended the meeting, Undenge, who is seeking his third term in office, insulted Sako, a white Zanu (PF) political activist who is challenging Undenge in the primaries.
Efforts to get a comment from Undenge were unsuccessful, and Joshua Sako said he was not at the meeting so was unable to confirm the accusation.