Corn yield reduction following a cereal rye cover crop has been attributed to, among other factors, allelochemicals released from decomposing cereal rye residue. The allelopathic effect of 6-methoxy-2-benzoxazolinone (MBOA) was evaluated on corn seedling growth, mycelial growth of seven pathogenic species of Pythium, and root rot of corn seedlings caused by Pythium spp. at 13, 16, and 22 to 23°C (room temperature) using a plate assay. Mycelial growth of all Pythium spp. tested was slower with MBOA at 0.25 mg/ml compared with MBOA at 0.125 and 0.0625 mg/ml and the check (4% V8 juice medium containing neomycin sulfate and chloramphenicol with 0.5% dimethyl sulfoxide). Therefore, no further tests were done with MBOA at 0.25 mg/ml. In general, MBOA reduced corn radicle length and did not cause root rot across all temperatures. However, greater root rot severity in corn was observed on corn seedlings grown in the presence of Pythium lutarium and P. oopapillum on media amended with MBOA compared with the check at all temperatures. Similarly, more root rot caused by P. torulosum and P. spinosum was observed when MBOA was present at 16°C compared with the check with no MBOA. These data suggest that corn seedling disease caused by Pythium spp. could be more severe when corn is planted following a cover crop of winter cereal rye due to the presence of allelochemicals that are released from the cover crop.