The mother of Afrikaans singer Robbie Klay said on Monday she only learnt about her son's sexual abuse during an argument years after it had allegedly started.

Jenny Liebenberg is testifying against Klay's alleged abuser, Jurie Els.

Els pleaded not guilty to the three charges against him.

Liebenberg said despite her best efforts to find out what was bothering her him her son refused to tell her.

She said she even spoke to the deputy principal at his school and sent him to a psychologist for several sessions because she was worried about changes in his behaviour.

Liebenberg said the first time she heard about what had happened to her child was when he broke down during an argument they had over his girlfriend.

Klay's mother said although she knew something was amiss she never even thought of the possibility that her son was being molested.

Klay claims Els abused him over several years from the age of 11.

He admitted under cross-examination last week that the molestation could not have started before his 12th birthday as he earlier testified.