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.