The causal agents of damping-off of quinoa seedlings were determined in greenhouse experiments. Ascochyta caulina, Fusarium avenaceum, Fusarium spp., Alternaria spp. and Pythium spp. were isolated from infected parts of quinoa seedlings. The most frequent Pythium sp. was P. aphanidermatum. Pathogenicity tests confirmed that P. aphanidermatum and F. avenaceum were the causal agents of damping-off of quinoa seedlings under greenhouse conditions. A comparison of the reaction of quinoa with that of other susceptible plants (spinach, cabbage, sugar beet) showed that quinoa is most susceptible to the pathogen before emergence, during germination till the end of the stage of the first pair of true leaves. Germinable quinoa seeds seemed to have a lower ability to emerge from the soil. This serious problem is caused not only by pre-emergence damping-off from pathogens but more so by a complex of several adverse factors during germination when quinoa is most sensitive.