Pythium aphanidermatum

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Pythium aphanidermatum infection on Amaranthus tricolor (click on image to enlarge it)
Author(s): Yuan-Min Shen, Taichung District Agricultural Research and Extension Station
Source: IPM Images
Pythium aphanidermatum sporangium (click on image to enlarge it)
Author(s): Gerald Holmes, Valent USA Corporation
Source: IPM Images

Pythium aphanidermatum (Edson) Fitzp. 1923

The pathogen is a soilborne oomycete with a very wide host range which causes root and stem rots as well as damping-off diseases. It has a worldwide distribution and can be especially found under wet and warm conditions or in greenhouses. It causes economic losses on vegetables and other crops like cotton, maize and beet, as well as "cottony blight" on grasses. In particular, seedlings may be affected and entire plantings can be destroyed.

The pathogen disperses through infected plant debris or through water. It spreads easily in hydroponic cultures. For control, proper drainage, fungicides and the use of resistant cultivars are recommended. Overuse of nitrogen fertilizers should be ovoided. The oospores are durable and persist in the soil for up to 12 years.

The hyphae are up to 10 μm wide and usually lack partitions or septa. They form filamentous or lobed sporangia (about 20 μm wide) which release mobile zoospores. The zoospores encyst when reaching a host plant. These cysts have a diameter of about 12 μm and form a germ tube which penetrates into the host tissue. Oogonia have a diameter of 22-24 μm and form a thick-walled oospore.

See also the related species Pythium ultimum which causes similar diseases, but has spherical sporangia.