Among the Oomycota (pronounced oh-oh-mycota), or oomycetes, are a number of very destructive plant pathogens, that have traditionally been studied by mycologists and were classified with the fungi, as "lower fungi."
The most notorious member of this group is Phytophthora infestans, the causal agent of late blight of potatoes, which was responsible for about 1 million deaths in the Irish Potato famine in the 1840s, and continues to cause losses in potato crops. Other examples are Plasmopara spp., and Peronospora spp. which cause downy mildew diseases, and Pythium spp., responsible for damping-off and a number of root rots on different crops.
These organisms were grouped with fungi for a long time, because they have a life style similar to that of the true fungi, getting their nutrients through absorption from their host or substrate. They also produce mycelium (the stringy, fluffy stuff often associated with fungi), which looks a lot like that of fungi, and they produce spores that look superficially like those of fungi. Sporangia are asexual spores from Oomycota and are formed on sporangiophores (see picture below). Under cool, wet conditions, short-lived zoospores with two different flagella can form within the sporangia. In the case of Phytophthora infestans, 20-50 zoospores can form within one sporangium, spreading the disease much faster than under warm, dry conditions, when each sporangium can only result in one infection.
Coenocytic sporangiophores with (asexual) sporangia from Peronospora sp.
Despite the similarities listed above, there are numerous differences between the Oomycota and the true fungi, and only fairly recently has DNA sequencing confirmed that Oomycota are close relatives of algae.
1. Oomycota are diploid (have 2 copies of each chromosome in one nucleus per cell), while fungi are haploid (have one copy of each chromosome in one nucleus per cell) or dikaryotic (two nuclei per cell, each with a haploid set of chromosomes).
2. The cell wall of Oomycota consists of beta glucans and cellulose, while fungal cell walls are predominantly made of chitin.
3. The mitochondria (organelles in cells responsible for energy production) of Oomycota have tubular cristae (internal compartments of mictochondria), while those of fungi are flattened.
4. Oomycota have distinctive sexual reproduction with oospores (that is where the name "Oomycota is derived from) that are fertilized by antheridia, while fungi don't have oospores, instead their sexual reproduction involves zygospores, ascospores, or basidiospores.
5. Zoospores (a different kind of spore also produced by Oomycota) have flagella that are distinct from those produced by true fungi if they do produce flagella.
6. Oomycota usually do not have cell walls between adjacent cells (they are coenocytic, see picture).
Fairly recent molecular evidence supports microscopic observations that indicate Oomycota are closely related to algae. However, mycologists continue to study these organisms in addition to the true fungi, mainly because their life styles are so similar. For plant pathologists, the newly revealed phylogenetic information has implications for population studies and control measures.
Rossmann, A.Y., and Palm, M.E. 2006. Why are Phytophthora and other Oomycota not true Fungi?
Feature Story May 2007 on APSnet
Kendrick, B. 2000. The Fifth Kingdom. Mycologue Publications. Online version.