She stars in the Disney show "Wizards of Waverly Place." Gomez also has the starring role in the film "Ramona and Beezus," which is currently in theaters.
Officials of the Delaware State Fair say the show could not be rescheduled before the end of the fair, and full refunds will be given to those who purchased their tickets directly from the Delaware State Fair. The show in Ohio was rescheduled for November.
Delaware fair officials said Allstar Weekend will perform in Gomez's place for free.
Disney is the parent company of ABC7.
The Associated Press contributed to this report.