Abstract:Cucumber seedlings were treated with different concentrations of sodium metasilicate (Si) solutions by supplying Si into nutrient solution, and inoculated with Pseudoperonospora cubensis or not. The effects of Si on disease index and major defense-related enzymes activities (guaiacol-peroxidase, polyphenol oxidase, phenylalanine ammonia-lysae, β-1, 3-glucanase and superoxide dismutase) were investigated. The results showed that the treatment with the lowest disease index (21.3) and the highest efficacy (62.8%) of downy mildew control was at 100mg/L concentration. The supply of Si significantly increased the Si contents in cucumber leaves, and Si showed the largest amount of 2.98mg/g when applied at 200mg/L concentration 7 days later. The supply of Si resulted in significant changes of defense-related enzymes activities in cucumber after P.cubensis infection, and the treatments with the highest enzymes activities at 100-200mg/L concentration. The above results suggested that the concentration of Si played an important role in inhibiting P.cubensis and the changes of major defense-related enzymes activities.