Pine wilt disease,caused by the pine wood nematode Bursaphelenchus xylophilus,leads to severe damage to pine forests in China.In our previous study,effectors secreted by this pathogen were shown to play roles in the different infection stages of pine wilt disease,and a series of candidate effectors were predicted by transcriptome sequencing.This study identified and characterized a novel effector,BxSapB3,which was among these candidate effectors.Agrobacterium-mediated transient expression was used to identify BxSapB3.BxSapB3 was secreted by B.xylophilus and found to be capable of inducing cell death in Nicotiana benthamiana.Quantitative real-time PCR (qRT-PCR) analysis revealed that BxSapB3 was upregulated in a highly virulent strain of B.xylophilus and expressed at lower levels in a weakly virulent strain at the early stages of infection.When BxSapB3 was silenced in B.xylophilus,the process of infection was delayed.These results indicate that BxSapB3 acts as an effector and contributes to virulence at the early stages of B.xylophilus infection.