Pythium

Pythium is a genus of parasitic oomycete. Because this group of organisms were once classified as fungi, they are sometimes still treated as such.

Morphology

• Hyphae

Pythium, like others in the family Pythiaceae, are usually characterized by their production of coenocytic hyphae, hyphae without septations.

• Oogonia

Generally contain a single oospore

• Antheridia

Contain an elongated and club-shaped antheridium.

Ecological Importance

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 fungi as Phytophthora and Rhizoctonia.

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).

However, the damage Pythium spp. do in field crops is limited to the area affectedthis is because the motile zoospores need ample surface water to travel long distances and the capillaries formed by soil particles act as a natural filter. 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 (Jarvis, 1992; Owen-Going, 2002, Owen-Going et al., 2003). The root rot affects entire operations (tens of thousands of plants, in many instances) within two to four days (Owen-Going, 2002, Owen-Going et al., 2003).

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.