Laboratory experiments were established to study the influence of silicon on improvements in the defense response of cucumber seedlings to Pythium-induced damping-off disease. Supplementing cucumber seedlings with different concentrations of silicon (0, 100, 200, 500, 1000 and 2500 ppm) for 7 days prior to inoculation with P. aphanidermatum isolates did not result in a significant reduction in seedling mortality at 20 and 25°C. Extending the period of supplementation to 14 days also did not result in any improvement of defense responses. In addition, mortality levels in the presence of silicon were not reduced when cucumber seedlings were inoculated with three isolates of P. aphanidermatum differing in their aggressiveness levels (weak, moderate and very aggressive). This could be related to a rapid infection by Pythium of cucumber seedlings in relation to a slower rate of induction of defense responses by silicon.