RENO, Nev. (AP) — A federal judge has thrown out a defamation lawsuit a Western art collector filed against a prestigious auction house and the owner of a Reno gallery who claimed an early 20th century cowboy painting he sold for $750,000 was a fake.

The judge in Reno dismissed the lawsuit last week against Peter Stremmel Galleries, the Coeur d'Alene Art Auction of Nevada and its partner Mark Overby of Hayden, Idaho.

Gerald Peters, who owns galleries in New York City and Santa Fe, New Mexico, said in the lawsuit filed last year that the defendants told his longtime friend and business partner...