The oomycetes are a class of heterotrophic filamentous organisms that belong to the kingdom Protista, of the heterokont (or stramenopile) phyla which includes brown algae and diatoms. Until recently, oomycetes were thought to be fungi due to their mycelia and apical growth. Oomycetes are no longer considered to be fungi due to the presence of cellulose in the cell wall (instead of chitin), and due to their aseptate and dikaryotic condition in the vegetative state, unlike fungi which are haploid.
The oomycetes have made a considerable impact on human lives. A good example of this is illustrated by the Irish potato famine: in the 1840's, Ireland was devastated by a late blight of potatoes caused by Phytophthora infestans. As a result over a million people starved, and another million were forced to leave Ireland (at the time, Ireland's population was six million). Another agriculturally important oomycete is Pythium, which causes root rot and pre-emergent death of seedlings in many different crops. Pythium can survive for long periods on decaying matter, as a saprophyte, which makes it difficult to eradicate by traditional methods of crop rotation. Pythium also has a wide host range which makes it a more pernicious plant pathogen than the more host specific Phytophthora.
Phytophthora infestans (x 100)
Image showing the asexual lemon-shaped zoosporangium produced by Phytophthora infestans, the causative agent of the 1845-1849 Irish potato famine.
Phytophthora infestans (x 400)
Lemon-shaped zoosporangium produced by Phytophthora infestans.
Saprolegnia (x 100)
The asexual zoosporangium of the fresh-water oomycete Saprolegnia. Saprolegnia was first
identified as a pathogen of fish by the 18th century polymath William Arderon. The asexual motile zoospores develop inside the zoosporangium and, once fully mature, they hatch from the tip and come swimming out looking for fresh substrate.
Saprolegnia (x 100)
Images of Saprolegnia zoosporangium containing motile zoospores.