Fremont cottonwood is a large and tall tree native to riparian areas in the southwestern United States. It grows along streams, rivers, and wetlands and serves many ecological and socioeconomic functions. In recent years, we observed a severe decline of Fremont cottonwood trees in California. Trees showed branches dying back, with dark-brown internal discoloration and decay of the wood of twigs, branches, or trunks. Eventually, the cambium and the bark were killed, causing a canker. The fungus Cryptosphaeria pullmanensis was isolated consistently from the necrotic wood of branches and twigs. On rare occasion, C. multicontinentalis was also isolated from symptomatic wood. Therefore, we investigated the pathogenicity in Fremont cottonwood of C. pullmanensis and C. multicontinentalis. Koch's postulates were completed in saplings and both species appeared highly virulent, producing internal necrosis and staining of the wood. This study is the first to report Cryptosphaeria dieback of Populus fremontii caused by C. pullmanensis and C. multicontinentalis. Symptoms and signs of this new disease are described and illustrated.