The team announced the moves before Saturday's game against the Chicago White Sox.
Marcum did not make his scheduled start June 19 because of elbow discomfort. The move to the DL is retroactive to June 15 and he is expected to miss just one more start. In 13 starts, the right-hander is 5-3 with a 3.39 ERA.
Ishikawa has been out since May 27 with a strained left oblique. He is hitting .250 with four homers and 14 RBIs in 32 games.