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Title: WHY AREN’T PHYTOPHTHORA AND OTHER OOMYCOTA TRUE FUNGI?

Author
item Rossman, Amy
item PALM, MARY - APHIS, BELTSVILLE, MD

Submitted to: Review Article
Publication Type: Review Article
Publication Acceptance Date: 6/27/2006
Publication Date: 9/15/2006
Citation: Rossman, A.Y., Palm, M.E. 2006. Why aren’t Phytophthora and other Oomycota true Fungi? Outlooks in Pest Management. 17:217-219.

Interpretive Summary:

Technical Abstract: Plant diseases result in billions of dollars in damage to agricultural crops each year. One of the groups of organisms that cause serious plant diseases has long been known as the oomycetes or Oomycota. The Oomycota or oomycetes are considered fungi because they obtain their nutrients via absorption and many of them produce the filamentous threads known as mycelium produced by most fungi. However, unlike other fungi, all members of the Oomycota undergo oogamous reproduction, meaning they produce oospores as a result of fertilization. Over the past three decades knowledge about relationships among groups of fungi has increased greatly such that traditional groupings based on gross morphology no longer reflect genetic relationships among them. As new tools for determining phylogenetic relationships are developed, especially those using molecular sequence data, they have been applied to questions such as whether the Oomycota are more closely related to the heterokont algae or the true Fungi. Results from a number of studies using molecular sequence data combined with ultrastructural similarities confirm unequivocally that the Oomycota share a common ancestor with and are most closely related to the heterokont algae or Chromista. The heterokont algae are distinctive among the algae in having the same two kinds of flagellae as occur in the Oomycota. This explains why many fungicides are not effective against diseases caused by members of the Oomycota. The true Fungi or Eumycota are now restricted to five major groups, each of which is regarded as a phylum: Chytridiomycota, Glomeromycota, Zygomycota, Ascomycota and Basidiomycota. Most mycologists have not abandoned the study of the Oomycota and still define the organisms they study “as eukaryotic heterotrophic osmotrophs in which assimilation takes place through a cell wall.” Understanding the evolutionary relationships among these groups of organisms contributes greatly to our ability to develop strategies to control the diseases these organisms cause.