The effect of oxalate,MB,EGTA,chlorpromazine (CPZ) and Li⁺ on the POD activity and the disease index of Pseudoperonospora cubensis leaves was investigated,with which treated the leaves,and the role of nitric oxide and calcium signaling in the resistance induced by oxalate in cucumber leaves was studied.The results showed that oxalate could increase POD activity in different degree with the concentration from 10 mmol/L to 70 mmol/L,and improve the resistance to P.cubensis,and reduce the disease index.The treatment with 30 mmol/L was the best.Four inhibitors could inhibit POD activity induced by oxalate and increase the disease index when they were applied at the same time as oxalate or before oxalate treatment,or after the oxalate treatment of 30 mmol/L.These results suggested that NO,Ca²⁺,calmodulin (CaM) and inositol-1,4,5-trisphosphate (IP₃) might be involved in the signal transduction by which oxalate induced the systemic resistance to P.cubensis.