Fillon's campaign offered no explanation for the last-minute change, saying only that a new date for a visit would be published later.
His visit to the massive farm show was supposed to begin early Wednesday morning and several reporters and members of his campaign staff were already on the scene to welcome him.
After beating his rightwing rivals to clinch the nomination, Fillon was favourite to win the French presidency in the two-round election in April and May.
But he has since been hit by a series of damaging allegations that he paid his family for fake jobs and has lost ground in the polls.
Recent surveys suggest that far-right leader Marine Le Pen and centrist candidate Emmanuel Macron are the two most likely to proceed to the second-round run-off on May 7.
Fillon's campaign appearances are frequently attended by protesters supporting Communist-backed candidate Jean-Luc Melenchon.
The annual farm show is a rite of passage for all hopefuls with eyes on the Elysee Palace given the power of the agricultural community in France.
Former president Jacques Chirac was beloved by the farmers at the show but other leaders have had less happy visits to the flagship expo.
Conservative president Nicolas Sarkozy lived to rue the day at the 2008 event when he lost his cool with a punter who refused to shake his hand, saying: "Get lost, dumbass!"
Video of the exchange went viral on the internet. Four years later, when Sarkozy ran for re-election, many of his detractors delighted in turning the phrase against him, and he was turfed out of office.
Last year, his Socialist successor Francois Hollande suffered ignominy at the fair in his turn, when farmers furious over collapsing milk prices heckled him and union activists tore down the agriculture ministry's pavilion.
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