To musical fans, Mr. Sundgaard was best known for collaborating with Kurt Weill on the 1948 folk opera Down in the Valley; with Fritz Kreisler and John Latouche on the 1944 operetta Rhapsody, in which he co-wrote the book with Leonard Louis Levinson; and with Victor Ziskin on the short-lived The Young Abe Lincoln, which began Off-Broadway and transferred to Broadway in 1961 for a short run.
A young T. Edward Hambleton produced his plays The First Crocus, in 1942, and The Great Campaign with ANTA in 1947. Each lasted only five performances. Mr. Sundgaard's first Broadway play was Everywhere I Roam, a collaboration with playwright Marc Connelly. The large-cast production, also directed by Connelly, ran 13 performances in 1938 and 1939.
Mr. Sundgaard is also responsible, for better or worse, for the enduring popularity of the song "The Yellow Rose of Texas." According to The Dallas Morning News, he discovered the forgotten tune while researching an episode about the Civil War for the CBS television series Omnibus. The copyright had expired and the song was in the public domain. It was used in the show, and bandleader Mitch Miller happened to hear it. Miller recorded a new version of the tune, which became a national sensation and remains a virtual anthem for the state of Texas. Miller then copyrighted the song and made a mint.
A shy, retiring man who was born of stolid Norwegian stock in St. Paul, Minnesota, on October 31, 1909, Mr. Sundgaard came closest to being a radical in 1938, when the Chicago Federal Theatre Project produced his play Spirochete, about the spread of the disease syphilis, as part of its "Living Newspapers" series. He was encouraged to write the socially-minded drama by Susan Glaspell, former Provincetown Playhouse cohort of Eugene O'Neill. The show was a hit in Chicago, but raised a firestorm of protest when it moved to Philadelphia. Attempts to shut it down did not succeed, however, and it was praised by critics. It never made it to New York City.
While in Chicago, he met and made friends with author Studs Terkel. He was also friendly with actor Gregory Peck and played high school football with Warren Burger, who went on to become U.S. Supreme Court Chief Justice. He attended college at the University of Wisconsin.
Mr. Sundgaard also wrote nonfiction articles for publications like The New Yorker and Atlantic Monthly, and children's books with collaborators like illustrator Eric Carle.
Mr. Sundgaard's first marriage, to Margaret Christensen, ended in divorce. His second wife, Marge Kane, died in 1998.
He is survived by his son, Steve Sundgaard; two daughters, Joy Kaiser of Palo Alto, California, and Jill Jones of Belmont, Massachusetts; a sister, Marguerite Bowers of Vero Beach, Florida; nine grandchildren and 12 great-grandchildren.