Phytophthora capsici is one of the most harmful pathogens in agriculture,which threatens the safe production of multiple crops and causes serious economic losses worldwide.Here,we identified a P.capsici expansin-like protein,PcEXLX1,by liquid chromatography-tandem mass spectrometry from Nicotiana benthamiana apoplastic fluid infected with P.capsici.Clustered regularly interspaced short palindromic repeats/crispr associated protein 9 (CRISPR/Cas9)-mediated PcEXLX1 knockout mutants exhibited significantly enhanced virulence,while the overexpression of PcEXLX1 impaired the virulence.Prokaryotically expressed PcEXLX1 acti-vated multiple plant immune responses,which were BRI1-associated kinase 1 (BAK1)-and suppressor of BIR1-1 (SOBIR1)-dependent.Furthermore,over-expression of PcEXLX1 homologs in N.ben-thamiana could also increase plant resistance to P.capsici.A G-type lectin receptor-like kinase from N.benthamiana,expansin-regulating kinase 1(ERK1),was shown to regulate the perception of PcEXLX1 and positively mediate the plant resist-ance to P.capsici.These results reveal that the expansin-like protein,PcEXLX1,is a novel apoplastic effector with plant immunity-inducing activity of oomycetes,perception of which is regu-lated by the receptor-like kinase,ERK1.