WRKY is the genes of a super transcription factors family that related to disease resistance.In order to reveal the role of WRKY40 gene in response to powdery mildew stress in apple.Malus domestica Borkh.cv.Fuji was used for gene cloning by reverse transcription PCR amplification by using specific primers.Quantitative real-time PCR was used in analyzing gene expression in tissues and in response to Podosphaera leucotricha and treatment of 1 mmol·L^-1 SA（salicylic acid）.(1)A 1 240 bp fragment including a 1 002 bp open reading frame to encode 334 amino acids was isolated.The amino acids sequence share 48.9% identity BLASTp to AtWRKY40,HvWRKY1/2/3 and others 4 sequences.It is a homologous gene of Arabidopsis AtWRKY40,and it is named as MdWRKY40b.Its GenBank accession is JX112907.(2)Gene expression indicated that MdWRKY40b expressed in root,stem,leaf and flower,with almost equally expression level in leaf,root and flower but very low level in stem.(3)It was clearly induced by P.leucotricha,and exogenous SA for a long times,with similar expression trends and the highest expression level time.The reason of Fuji apple extremely susceptible to powdery mildew maybe relate to the long-time and high expression of MdWRKY40b.