Ian Gordon, 66, also from Troon, is alleged to have placed a pillow over the face of 63-year-old Patricia Ann Gordon and restricted her breathing.
At the High Court in Glasgow, his QC Gordon Jackson entered a guilty plea on his behalf to the reduced charge of culpable homicide.
But he will stand trial for murder after prosecutors rejected the plea.
Advocate depute Iain McSporran QC told the court: "That plea is not acceptable to the Crown."
A date of June was set for his trial.
Bail for Mr Gordon was continued.
A pensioner has admitted killing his wife at her home in Troon in April last year.