The most important American composer of tone poem was

Horatio Parker was the most important American composer of tone poem.

Explanation:

Horatio William Parker was an American composer, organist and teacher. He was a central figure in musical life in New Haven. Horatio Parker was a symbolic person remembered as the teacher of the great Charles Ives, who was one of the most famous and highly esteemed composers in the English-speaking world. Horatio Parker wrote two symphonic poems, A Northern Ballad and Vathek.

you mean theperosn charles ives