Generally contain a single oospore
Contain an elongated and club-shaped antheridium.
Pythium root rot is a common crop disease caused by a genus of organisms called "Pythium". These are commonly called water moulds. Pythium damping off is a very common problem in fields and greenhouses, where the organism kills newly emerged seedlings (Jarvis, 1992). This disease complex usually involves other pathogens such as Phytophthora and Rhizoctonia. Pythium wilt is caused by zoospore infection of older plants leading to biotrophic infections that become necrotrophic in response to colonization/reinfection pressures or environmental stress (Jarvis, 1992; Owen-Going, 2005; Owen-Going et al., 2009), leading to minor or severe wilting caused by impeded root functioning (Jarvis, 1992; Bagnall, 2007).
Many Pythium species, along with their close relatives, Phytophthora species are plant pathogens of economic importance in agriculture. Pythium spp. tend to be very generalistic and unspecific in their host range. They infect a large range of hosts (Owen-Going, 2002), while Phytophthora spp. are generally more host-specific.
For this reason, Pythium spp. are more devastating in the root rot they cause in crops, because crop rotation alone will often not eradicate the pathogen (nor will fallowing the field, as Pythium spp. are also good saprotrophs, and will survive for a long time on decaying plant matter).
It has been noted that in field crops, damage by Pythium spp. is often limited to the area affected, as the motile zoospores require ample surface water to travel long distances. Additionally, the capillaries formed by soil particles act as a natural filter and effectively trap many zoospores. However, in hydroponic systems inside greenhouses, where extensive monocultures of plants are maintained in plant nutrient solution (containing nitrogen, potassium, phosphate, and micronutrients) that is continuously recirculated to the crop, Pythium spp. cause extensive and devastating root rot and is often difficult to prevent or control (Jarvis, 1992; Owen-Going, 2002; Owen-Going et al., 2003; Bagnall, 2007). The root rot affects entire operations (tens of thousands of plants, in many instances) within two to four days due to the inherent nature of hydroponic systems where roots are nakedly exposed to the water medium, in which the zoospores can move freely (Owen-Going, 2002; Owen-Going et al., 2003; Bagnall, 2007).
Several Pythium species, including P. oligandrum, P. nunn, P. periplocum, and P. acanthicum are mycoparasites of plant pathogenic fungi and oomycetes, and have received interest as potential biocontrol agents.