plant a/the seed of doubt (in someone's mind)
To cause someone to have doubts, worries, or concerns (about something); to introduce someone to a doubtful or worrisome idea. The candidate was doing very well in the polls six months ago, but it seems that this smear campaign has been effective in planting a seed of doubt in the minds of voters. Recent economic turbulence in the Eurozone has planted the seed of doubt about the strength of the economy's recovery. Every time you act suspiciously like that, it plants a seed of doubt in my mind about your fidelity.