The Oomycetes have been classified within the Fungi for a long time, but they are phylogenetically distant from fungi and are related to diatoms and brown algae. Because of their taxonomic position and their biological properties (motile spores, cellulose cell walls, specific metabolism, etc.), the pathogenic capacities of the oomycetes cannot be directly deduced from those of the filamentous fungi, and are poorly understood.
The comparison of sequence data from the oomycetes (Phytophtora) and phytopathogenic fungi (Magnaporthe grisea, Fusarium graminearum, Botrytis cinerea, etc.) have confirmed this great divergence.
Recently, the sequencing of the genomes of three exclusively phytopathogenic oomycetes from the Peronosporales (Phytophtora) has been undertaken in the United States. The genus Aphanomyces, a member of the Saprolegniales, occupies a unique position in the Oomycetes, and it is the only one in which some species can be responsible for diseases of both plants and animals. Aphanomyces euteiches is a major parasite of several legumes including peas, alfalfa and clover, and is a threat to the production of proteins of plant origin, for which there is an ever-increasing demand.

The acquisition of sequence data from Aphanomyces is therefore justified on the basis of its distinct taxonomic position, its host range and its impact on agriculture. Furthermore, the capacity of A. euteiches to infect the model legume Medicago truncatula reinforces the relevance of this microorganism for the study of plant-parasite interactions.