IB 371 – GENERAL MYCOLOGY
Lecture 14
Tuesday, October 14, 2003
OOMYCOTA
PYTHIALES, RHIPIDIALES
PYTHIALES
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Occur in aquatic, amphibious, and terrestrial
habitats.
Thallus is of coenocytic mycelium - narrower
and more delicate than in the Saprolegniales.
Septa delimit reproductive structures and may
occur in older hyphae.
Walls generally lack chitin and are mostly
glucan with partially crystalline cellulose-like
material and protein.
PYTHIALES
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Asexual reproduction is by zoospores
formed in sporangia. Sporangia may
produce biflagellate zoospores which
encyst and form a germ tube or germinate
directly to form a germ tube that develops
into a mycelium.
Zoospores are biflagellate and secondary.
PYTHIALES
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Sexual reproduction is oogamous. Each oogonium
contains a single egg (except Pythium multisporem).
Antheridia and oogonia are multinucleate, but only 1
egg and 1 sperm (nucleus) fuse or if more than 1
pair fuse, only 1 oospore is formed.
Meiosis is gametangial and the organism is diploid
throughout its life cycle.
The cytoplasm remaining after oosphere cleavage is
called periplasm. The periplasm may be deposited
around the zoospore to give it a distinctive
appearance.
PYTHIALES
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May be saprophytic or parasitic, but plant
parasitic species do not form haustoria.
+/- ten genera
The two genera with the greatest number of
species and which have the greatest economic
and ecological importance are Pythium and
Phytopthora.
PYTHIUM
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Common in soil, exists as a saprophyte
Causes damping off of seedlings growing in
wet or very poorly drained soils
Sporangia are not well-differentiated and
in some species are hypha-like
Usually sporangia form a vesicle into which
the cytoplasmic contents are discharged
Zoospores are differentiated in the vesicle
and then swim away
PYTHIUM
 Zoospores
swim around for a
time, throw off their flagella,
encyst, and then form a germ
tube.
From
Introductory
Mycology by
John Webster
PYTHIUM
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Most species are homothallic
Oogonia and antheridia are multinucleate
Most of the nuclei in the oogonia degenerate
while a few enlarge & undergo meiosis
Antheridia are multinucleate but all
degenerate except one which undergoes
meiosis
A single oogonial nucleus fuses with a single
antheridial nucleus to form an oospore
PYTHIUM
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The remaining nuclei & cytoplasm forms the
periplasm
Oospores must have a period of rest before
they germinate
Oospores germinate either by forming a germ
tube or a vesicle which cleaves into zoospores
Sporangia of Pythium aphanidermatum (From D.S. Barr)
From Introductory Mycology by John Webster
Oogonia & Antheridia of Pythim aphanidermatum (From D. S. Barr)
Oogonium & Antheridium of Pythium erythroseptica (From D.S.
Barr)
PYTHIUM
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Causes damping off of seedlings growing in wet
or very poorly drained soils.
Symptoms are water soaking of stems and
brown lesions on stems and eventually the
seedling drops over.
The hyphal tips secrete pectolytic and
cellulolytic enzymes that cause plant cells to
separate from one another and this weakens
the stems.
PHYTOPTHORA
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The PLANT DESTROYER
P. infestans - solanaceous hosts
P. megasperma var. sojae - soybeans
P. cactorum - 40 families of flowering plants
P. erythroseptica - pink rot of potato tubers
P. fragariae - red core of strawberries
P. cinnamomi - 1,000 different hosts
P. palmivora - pod-rot & canker of cacao &
many other plant hosts
From:
Lower Fungi
in the
Laboratory
Zoosporangium
of Phytopthora
palmivora
From:
Introductory
Mycology by
John Webster
Slide from B. Kendrick
DISEASE TRIANGLE
 Virulent
pathogen
 Susceptible host
 Favorable environment
PHYTOPTHORA RAMORUM
 Introduced
pathogen
 Susceptable host
Tan oak
killed by
Phytopthora
ramorum
Symptoms of
Sudden Oak
Death on bark
of tan oak
Dr. David
Rizzo –
Investigator
of Sudden
Oak Death