eTopic 20.1 Oomycetes: Lethal Parasites That Resemble Fungi

The Great Irish Potato Famine, "sudden oak death," and "phytophthora dieback": What do all these epidemics have in common? All are caused by oomycetes, or oomycota.


The oomycetes, or water molds, were originally classified as fungi because of the superficial similarity of their infective filaments to fungal hyphae, and the fungus-like appearance of their infections of plants and animals. But genetic analysis has assigned oomycetets to a clade of heterokont protists.

The term “oomycete” means “egg fungi,” referring to the large, round, egg-like form of the female gamete (Fig. 1A). The oomycete genus Phytophthora (“plant destroyer”) includes serious plant pathogens. Destruction of potato crops by Phytophthora infestans was the primary cause of the Great Irish Famine in the 1840s, in which a million people died and many others migrated to the United States. Today, another species of Phytophthora causes “sudden oak death,” the loss of tens of thousands of oaks and other trees on the West Coast of the United States (Fig. 1B). Yet another species, Phytophthora cinnamomi, infects diverse plants around the world, including fruit trees, grasses, and even ferns. P. cinnamomi is listed in the Global Invasive Species Database as one of the 100 world's worst invasive species.

Despite superficial similarities between oomycetes and fungi, their cell forms and DNA sequences show fundamental differences. Oomycete cell walls contain no chitin, being composed primarily of glucans and cellulose. DNA analysis shows that oomycetes are heterokonts, a class of protists with pairs of dissimilar flagella (Heterokonta, a major category in Table 20.1). The heterokont cell form is seen in the primary and secondary zoospores (Fig. 1C). Oomycetes and fungi offer an interesting case of convergent evolution.


Figure 1  Oomycetes are fungus-like pathogens.  A. Phytophthora infestans, cause of potato late blight. B. Oak tree in California dying of Phytophthora ramosum infection, known as “sudden oak death.” C. Life cycle of an oomycete. Primary and secondary zoospores show the pair of differently shaped flagella typical of heterokont protists. Sources: A. Tom Volk. B. ©Joseph O’Brien, USDA Forest Service