Will the frequency of the sickle-cell gene occurance decrease if malaria were eliminated in africa?

Very slowly decrease: If there was no selective pressure to retain the mutation causing sickle cell disease (= no malaria or other rarer diseases), the mutation would be selected against over time. This could take 10's of thousands of years. It is estimated that the first sickle cell mutations arose 75, 000-150, 000 years ago.

Sickle cell: The malaria-sickle cell association began thousands of years ago at a time where medical therapies were very limited eliminating malaria now will have no forceable effect on the incidence of this congenital disease.

Can't predict: Selection Evolution clearly played a part in the genetics of Sickle Cell trait. If it was no longer adaptive against P. Falciparum- it is difficult to say how low it would take for diminished #'s of peoples with this issue. Evolution tends to evolve over many thousands of years.