Outlooks on Pest Management – October 2006 217
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Plant diseases result in billions of dollars in damage to
agricultural crops each year. One of the groups of organisms
that cause many serious plant diseases has long been known
as the Oomycota or oomycetes, traditionally classified in the
phycomycetes or “lower fungi.” The phycomycetes are an
informal group that, in addition to the Oomycota, have
historically included such diverse fungi as the slime molds,
chytrids, zygomycetes or bread molds, and arbuscular
mycorrhizae.The “higher fungi” have included the
ascomycetes or sac fungi such as the discomycetes, (e.g.
morels), pyrenomycetes, (including the cause of chestnut
blight), deuteromycetes or imperfect fungi, and yeasts; and
the basidiomycetes with the rust fungi, smut fungi,
mushrooms, gasteromycetes, polypores, and jelly fungi.
Over the past three decades knowledge about relationships
among groups of fungi has increased greatly such that these
traditional groupings based on gross morphology no longer
reflect genetic relationships among them.
The Oomycota or Peronosporomycetes consist of more
than 800 species that may be saprobic or parasitic on
terrestrial or aquatic plants and animals. One member of the
Oomycotahas greatly influenced
Phytophthora infestans, the cause of late blight of potatoes.
As a result of the famine in Ireland caused by this disease,
about 1 million people died and another 1.5 million
emigrated (Alexopoulos, et al. 1997).
diseases are caused by species of Phytophthora including
sudden oak death and ramorum blight caused by P.
ramorum, and cacao black pod caused by P. megakarya.
Other members of the group include species in the genus
Pythium, such as Pythium aphanidermatum, cause of
cottony blight of turf grasses, and downy mildews in the
Peronosporales such as Peronospora tabacina, tobacco blue
mold, Plasmopara viticola, downy mildew of grape, and P.
halstedii, cause of sunflower downy mildew, and many
others. Finally, a group traditionally placed in the oomycetes
is the Saprolegniales or water moulds that cause diseases of
fish and other aquatic vertebrates.
A number of other
Characteristics of the Oomycota
The Oomycota have long been considered fungi because they
obtain their nutrients via absorption and many of them
produce the filamentous threads known as mycelium
characteristic of many fungi.
classified as a distinct group allied with the lower fungi based
on a number of unique characteristics (Table 1).
members of the Oomycota undergo oogamous reproduction,
meaning they produce oospores as a result of fertilization.
These oospores may be large and solitary or smaller and
numerous inside the oogonium (Fig. 1). None of the true
Fungi produce oospores. Another distinction is the cell wall
composition. In the Oomycota, the cell walls are composed
of beta glucans and cellulose rather than chitin as in the true
Fungi. In addition the Oomycota produce motile zoospores
with two kinds of flagellae, one of which is a whiplash
flagellum oriented posteriorly while the other has a fibrous,
ciliated structure and is oriented anteriorly. The occurrence
of two kinds of flagellae is referred to as being heterokont.
Although some true Fungi, namely the Chytridiomycota,
The Oomycota have been
WHY ARE PHYTOPHTHORA AND OTHER OOMYCOTA NOTTRUE
Amy Y. Rossman, USDA Agricultural Research Service and Mary E. Palm, USDA Animal and Plant Health
Inspection Service, Systematic Botany & Mycology Laboratory, Beltsville, Maryland 20705, USA explain why
the pathogens that cause diseases such as downy mildews are not fungi
PHYTOPHTHORA AND OTHER OOMYCOTA
Keywords: Classification, Chromista, oomycetes, Peronosporomycetes,
Table 1. Major distinctions between the Oomycota in the
Chromista and the true Fungi (Chytridiomycota,
Glomeromycota, Zygomycota,Ascomycota, Basidiomycota)
Product of sexual
Nuclear state of
Type of flagellae
on zoospores, if
Heterokont, of two
types, one whiplash
the other fibrous,
With tubular cristae
If flagellae produced,
usually of only one