The defoliation and mortality of Erythrina spp. are increasing on the southern islands of Japan as well as in Taiwan. Although infestation by the gall wasp Quadrastichus erythrinae has been accepted as the cause of Erythrina decline, its role in the death of hosts has never been investigated. We thus studied the cause of the decline, focusing on physiological changes and any contributions by microorganisms. From declining and defoliated trees, Fusarium sp. within F. solani species complex (FSSC) clade 3 was primarily detected, especially from discolored xylem and necrotic phloem that had an odor. This fungus belongs to the Ambrosia Fusarium clade, a group of symbionts of ambrosia beetles that includes the causal agent of Fusarium dieback on avocado. No specific fungi were detected from twigs and leaves with Q. erythrinae galls. According to histological observations, the internal symptoms of declining E. variegata are similar to those of sudden death syndrome in soybeans, which is caused by F. virguliforme, a member of FSSC clade 2. One of the isolates of the Fusarium sp. isolated from E. variegata induced wilt and necrosis of cortex and phloem in the seedlings after artificial inoculation, and the inoculated strain was reisolated. The present results demonstrated that the isolate was pathogenic to E. variegata and may be a causal agent of Erythrina decline.