3.4 │ Catalase activity (CAT)
The changes in CAT activity of the six cucumber genotypes after inoculation with P. melonis are shown in Table 2 and 5 and Fig 4. Before inoculation, the CAT activities in resistant and moderately resistant genotypes (Soheil and Baby), were significantly higher than another resistant genotype (Ramezz) (Table 5). After inoculation, the CAT activity in Soheil increased sharply to 20.3 U mg−1 at 14 DAI and then decreased at 21 DAI (Fig 4. A). A similar pattern was also observed in Ramezz. However, the CAT activity in another moderately resistant genotype (Baby) showed a different reaction, with a non-significant decrease after inoculation at 14 DAI and a significant increase at 21 DAI. The maximum amount of CAT activity in roots of inoculated plants was observed in Ramezz and Soheil (Table 5). No significant difference was found in two susceptible genotypes, Extrem and Mini 6-23 (Table 5 and Fig 4.D and E). In another susceptible genotype (Yalda) CAT activity never exceeded the activity before inoculation at 14 and 21 DAI (Fig 4. F). However, no correlation was observed between damping-off susceptibility and CAT activity changes (Supplemental Table 1).