The walnut twig beetle (WTB), Pityophthorus juglandis Blackman (Coleoptera: Curculionidae), vectors a fungus, Geosmithia morbida Kolařík, Freeland, Utley, and Tisserat (Ascomycota: Hypocreales), which colonises and kills the phloem of walnut and butternut trees, Juglans Linnaeus (Juglandaceae). Over the past two decades, this condition, known as thousand cankers disease (TCD), has led to the widespread mortality of Juglans species in the United States of America. Recently the beetle and pathogen were discovered on several Juglans species in northern Italy. Little is known about the extra-generic extent of host acceptability and suitability for the WTB. We report the occurrence of both the WTB and G. morbida in three species of wingnut, Pterocarya fraxinifolia Spach, Pterocarya rhoifolia Siebold and Zuccarini, and Pterocarya stenoptera de Candolle (Juglandaceae) growing in the United States Department of Agriculture-Agricultural Research Service, National Clonal Germplasm Repository collection in northern California (NCGR) and in the Los Angeles County Arboretum and Botanic Garden in southern California, United States of America. In two instances (once in P. stenoptera and once in P. fraxinifolia) teneral (i.e., brood) adult WTB emerged and were collected more than four months after infested branch sections had been collected in the field. Koch’s postulates were satisfied with an isolate of G. morbida from P. stenoptera, confirming this fungus as the causal agent of TCD in this host. A survey of the 37 Pterocarya Kunth accessions at the NCGR revealed that 46% of the trees had WTB attacks and/or symptoms of G. morbida infection. The occurrence of other subcortical Coleoptera associated with Pterocarya and the first occurrence of the polyphagous shot hole borer, a species near Euwallacea fornicatus Eichhoff (Coleoptera: Curculionidae), in Juglans are also documented.