Plant material and Pathogenesis assay
We have recently screened thirty-eight cucumber genotypes, including domestic and exotic hybrids, and inbreeding lines from different seed companies, for damping off resistance to P. melonis at two growth stages: seedling (45-dayold seedlings) and maturing (50% flowering) stages (Hashemi et al., 2019). Based on the results of the screening test, Ramezz with high disease resistance, Baby with moderately resistance, Mini 6-23 and Extrem with high susceptibility to P. melonis were used throughout the study (Fig 1). The study was carried out in the laboratory of the Agricultural and Natural Resource Research and Education Centre, Isfahan, Iran, in 2018. Cucumber plants of each genotype were cultivated in a single seedling nursery tray filled with substrate of oven-sterilized mixture of sand-peat moss in equal parts in a growth condition at 26–28°C with 80% relative humidity. 45-day old seedlings were used for experiments and were inoculated with P. melonis isolate, as described by Nasr Esfahani et al. (2014). The pathogen, P. melonis was isolated from naturally infected cucumber plants exhibiting post-emergence damping-off and root rot symptoms and identified as P. melonis (MH924841). Koch’s postulates were conducted by inoculating onto test cucumber genotypes. cucumber seedlings were inoculated by drenching with 5 ml of sporangia suspension (106 sporangia/ml) and incubated for two days under saturated moist condition in the greenhouse and then inoculated and non-inoculated treatments grown at 26 ± 2°C, 16 h photoperiod and 65% relative humidity. Following the inoculation, root collars and leaves of Ramezz, Baby, Mini 6-23 and Extrem were separately collected at four different periods: 0 hour before inoculation, 24, 48 and 72 hours post inoculation.