The suppressiveness of compost amendments to pre-emergence damping-off of cucumber incited by isolates of Pythium aphanidermatum, P. myriotylum, and P. irregulare was studied. Growth chamber experiments were designed to examine the effects of temperature (20, 24, 28, and 32°C), compost type (municipal biosolids [MC] and leaves [LC]), and compost dose (40, 80, 160, and 320 mg/cm3) on suppression of damping-off (i.e., increase in seedling stands) caused by different Pythium isolates obtained from different hosts. In dose-response experiments, LC was suppressive at dosage rates ≥80 mg compost/cm3 of sand, whereas MC was suppressive at rates ≥40 mg/cm3. Damping-off severity induced by each of the three Pythium spp. was temperature-dependent. For example, P. aphanidermatum and P. myriotylum caused damping-off at each of the four temperatures tested, whereas P. irregulare caused disease only at 20 and 24°C. MC was suppressive to P. aphanidermatum at 20 and 24°C, whereas LC was suppressive at 28 and 32°C. Both composts significantly suppressed damping-off caused by P. irregulare at 20°C (85% suppression in MC and 60% suppression in LC) and at 24°C (approximately 60% suppression in both composts), and by P. myriotylum at all temperatures tested. In experiments with a variable temperature cycle (32°C for 14 h, day and 22°C for 10 h, night), only P. aphanidermatum and P. myriotylum caused damping-off of cucumber seedlings. Under these conditions, LC significantly suppressed damping-off caused by P. aphanidermatum (20% suppression) or P. myriotylum (37% suppression) but MC was not suppressive. In experiments where the two composts were mixed, a significant negative interaction between the two composts was observed for the suppression of P. myriotylum and P. irregulare at 20°C and of P. irregulare at 24°C, but not for P. aphanidermatum at any of the tested temperatures. There was no difference in aggressiveness among isolates within each of the three Pythium spp., regardless of their host origin. However, a significant variation in suppressiveness of LC was observed among isolates of P. aphanidermatum (11 isolates) derived from the same host, but not for P. irregulare (9 isolates) or P. myriotylum (7 isolates).
