Reproduction '' (Peronosporales) on
potato. M:
meiosis, P:
plasmogamy, K:
karyogamy, n:
haploid, 2n:
diploid. Most of the oomycetes produce two distinct types of spores. The main dispersive spores are asexual, self-motile
spores called
zoospores, which are capable of chemotaxis (movement toward or away from a chemical signal, such as those released by potential food sources) in surface water (including precipitation on plant surfaces). A few oomycetes produce aerial asexual spores that are distributed by wind. They also produce sexual spores, called
oospores, that are translucent, double-walled, spherical structures used to survive adverse environmental conditions.
Ecology and pathogenicity of
Achlya sp. (Saprolegniales) isolated from a stream. suffering from
UDN with secondary
Saprolegnia infections. Many oomycetes species are economically important, aggressive algae and
plant pathogens. Some species can
cause disease in fish, and at least one is a pathogen of mammals. The majority of the plant pathogenic species can be classified into four groups, although more exist. • The
Phytophthora group is a
paraphyletic genus that causes diseases such as
dieback,
late blight in
potatoes (the cause of the
Great Famine of the 1840s that ravaged
Ireland and other parts of Europe),
sudden oak death,
rhododendron root rot, and
ink disease in the
European chestnut • The
paraphyletic Pythium group is more prevalent than
Phytophthora and individual species have larger host ranges, although usually causing less damage.
Pythium damping off is a very common problem in greenhouses, where the organism kills newly emerged seedlings. Mycoparasitic members of this group (e.g.
P. oligandrum) parasitize other oomycetes and fungi, and have been employed as biocontrol agents. One
Pythium species,
Pythium insidiosum, also causes
Pythiosis in mammals. • The third group are the
downy mildews, which are easily identifiable by the appearance of white, brownish or olive "mildew" on the leaf undersides (although this group can be confused with the unrelated fungal
powdery mildews). • The fourth group are the
white blister rusts,
Albuginales, which cause white blister disease on a variety of flowering plants. White blister rusts sporulate beneath the
epidermis of their hosts, causing spore-filled blisters on stems, leaves and the
inflorescence. The Albuginales are currently divided into three genera,
Albugo parasitic predominantly to
Brassicales,
Pustula, parasitic predominantly to
Asterales, and
Wilsoniana, predominantly parasitic to
Caryophyllales. Like the
downy mildews, the
white blister rusts are obligate
biotrophs, which means that they are unable to survive without the presence of a living host. == References ==