The Effect of Salicylic Acid to Induce Systemic Resistance in Cucumber Plant to Damping- off Disease Caused by Pythium aphanidermatum

Damping-off disease caused by Pythium aphanidermatum (Edson) Fitzp. is one of the destructive diseases of cucumber in commercial greenhouses which led to considerable loss of yield. In this study, the effect of Salicylic acid on induced resistance in infected cucumber with damping-off disease was investigated. For this propose, expression level of cupi4 and Lipoxygenase genes and the activity of several biochemical enzymes was investigated. A factorial experiment based on randomized complete design with four replications was carried out in the greenhouse condition at 25-27°C with 70% moisture and 14:8 h light:dark photoperiod. The results of disease incidence assay showed that there were a significant difference at the 5% level among treated plants with different concentration of salicylic acid (0, 200 and 400 ppm) and control plants . The assay of antioxidant enzymes activity in treated plant showed that the activity of Proxidase and Polyphenoloxidase enzymes were considerably increased compared to control while the Phenylalanine ammonia-lyase was not significantly increased when compared to non treated plants. Gene expression analysis by qRT-PCR method showed that salicylic acid application is able to increase the expression level of tested genes, but the highest expression level was recorded for Cupi4 gene. The results of this research indicate that application of salicylic acid could play an important role in enhancing the cucumber resistance against damping-off disease.