Pathogenicity and Distribution of Fusarium solani Isolates Associated with Erythrina Decline in Japan

Erythrina spp. trees have been declining since the 2000s worldwide, and fungi belonging to Fusarium solani species complex (FSSC) have been suggested to be a causal factor of decline and mortality of Erythrina variegata trees in Okinawa Island, Japan. In addition to the FSSC isolate grouped as "it-1" based on ITS sequence data (previously called strain A), we conducted an inoculation experiment with two isolates grouped as "it-2" (previously strain B), which is genetically close to it-1. Two it-2 isolates originating from two islands showed pathogenicity to E. variegata with the same symptoms as those caused by it-1 isolate. We also found the isolates of it-1 and it-2 were widely distributed, including on Ishigaki Island, ∼400 km south of Okinawa Island across the ocean. All isolates of it-1 and it-2 belong to the ambrosia Fusarium clade of the FSSC, a group of symbionts of ambrosia beetles, including the pathogens of Fusarium dieback in avocados and teas. The detection of ambrosia beetles Euwallacea spp. from our specimens provided information on the vectors of the pathogens. Our present results suggest the fungi of the FSSC could be responsible for the Erythrina decline in other areas with damage.