Virulence of Fungi Isolated from Ambrosia Beetles to Acer amoenum Branches

In Japan, no association between the ambrosia beetle and their fungal symbionts causing branch dieback or tree mortality on maple, , has been reported. However, we identified dieback of several branches and numerous holes created by three species of ambrosia beetles, , , and , on trees at the University of Tokyo Tanashi Forest, Tokyo Metropolis, Japan, in 2016. The high attack density of the beetles was observed on the weakened trees; however, the contribution of the associated fungi to the branch dieback was still unknown. We isolated fungi carried by these three beetles and inoculated them to cut main trunks and sapling branches to determine whether the associated fungi caused the branch dieback. was isolated from all and , whereas , , and were isolated from , with 35, 15, and 5% isolation frequencies, respectively. Inoculation with and induced statistically significantly wider sapwood discoloration (six and four times wider for and , respectively) than the controls, and larger water-conductance loss (2 and 1.7 times larger for and , respectively) than the controls. However, the observed lesions were not large enough to cause discoloration, and symptoms of dieback were not observed, even 13 months after the inoculation. Therefore, we concluded that the virulence of the four investigated fungi to was very low and that these fungi were likely not the primary cause of the branch dieback.