Effect of exogenous Salicylic Acid and Azelaic Acid on some biochemical and molecular responses to downy mildew disease in Cucumis sativus L.

Downy mildew is one of the most important diseases of cucumber plants reducing production in the world. Despite the availability of some fungicides to control the disease, demands for new and reliable natural products has been increased. In this study, the effects of salicylic acid (SA) and Azelaic acid (AZA) on some biochemical properties and expression of defense genes in cucumber plants were studied. Gene expression analysis indicated that expression of both β-1,3- glucanase and chitinase genes were increased after inoculation with Pseudoperonospora cubensis, but this increase was much higher for chitinase gene in SA-treated plants and for β-1,3- glucanase in AZA-treated plants. Also, chlorophyll a, b and total flavonoids contents in AZA and SA-treated plants were higher compared to control 6 hours after inoculation with pathogen. Total phenolic content was also increased in SA and AZA treated plants compared to the control 6 and 72 hours after treatment, respectively. These findings indicate that pretreatment of cucumber plants with SA and AZA prompts faster defense responses and increases the efficiency of the defense system against downy mildew.