Pythium, a genus of destructive root-parasitic fungi of the family Pythiaceae (phylum Oomycota, kingdom Chromista), having filamentous sporangia, smooth-walled, spherical oogonia, and stalked antheridia. Species of Pythium are often responsible for serious diseases in plants, such as damping-off and root rot. Generally plants are stunted, root tips are brown and/or dead, plants wilt at mid-day and may recover at night, plants yellow and die, brown tissue on the outer portion of the root easily pulls off leaving a strand of vascular tissue exposed.