ABSTRACT
Just as there is confusion as to the placement of members of the phylum Oomycota in a kingdom, there is also discussion on how the phylum should be divided into classes and orders. Most mycologists and plant pathologists agree that there should be about 10 recognized orders in the phylum. Most of the important plant pathogens fall into two orders: the Peronosporales and the Albuginales (Figure 8.3), and most fall into the families Peronosporaceae, Pythiaceae, and Albuginaceae (Figure 8.3 and Table 8.1). Notorious and damaging organisms such as the downy mildews, the 100+ species of Phytophthora, and the white blister rusts are included in these families. There are also several species of Aphanomyces included in the Saprolegniales (Saprolegniaceae) that can cause root rots of field crops such as alfalfa, peas, and sugar beets. Although this family is primarily composed of saprophytic organisms, some species cause diseases of fish. These organisms will not be discussed in this chapter.
