Oomycetes in lab tomorrow
• Quiz (Lab manual pages 1-5 ‘field trip’ and
page 27 ‘Oomycete intro’
• Microscope tutorial
• Look at Oomycete diseases –
signs and symptoms
• ‘Baiting’ isolation for Oomycete pathogens
• Disease of the week ‘Unknown’ –
worksheet to fill in
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Oomycetes (water molds)
Stramenophiles (Kingdom Chromista)
Diploid (2N) – most species non-pathogens
Hyphae are coenocytic (tubes)
Cell wall made of cellulose
Sexual reproduction: oospore
Asexual reproduction: sporangia and
zoospores
Eukaryotes
Eukaryotes
‘Chromista’
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Oomycota
Eukaryotes
University of California Museum of Paleontology
Major species
within the
oomycetes
Highly evolved,
and aggressive
plant parasites
‘Weak’
Opportunistic
pathogens
Soil and
terrestrial
plant associated
‘
Live in water
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Important diseases caused
by Oomycetes
Soilborne (emphasis on survival spores)
• Pythium damping-off
• Pythium blight of turf
• Phytophthora root rots
Foliar (emphasis on dispersal spores)
• Late blight of potato/tomato
• Sudden
oak death
R
o
• Downy
mildews
o
t
Damping-off caused by Pythium spp.
Favored by cool, wet soils that slow emergence
The pathogen is ubiquitous & opportunistic
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Pythium damping off is favored by cool, wet soils
Pythium blight of turf
Pythium spp. are opportunists – they aggressively
colonize dying plant materials: thatch layers, green
manures, etc. – in turf, the disease occurs when the
weather is warm and the grass crowns and thatch layer are
under water for a sufficient period. Plants recently
stressed, or lush from high N show increased susceptibility.
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Phytophthora root rots
(aggressive pathogens)
Phytothphora root rot of rhododendron
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2°symptom of Phytophthora root rot
1°symptom of Phytophthora root rot
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1°symptom of
Phytophthora root rot
Oospores
Typically, oospores (longlived resting spores) play
an important role in soilborne diseases
With some oomycetes,
oospores are rare and
may not be necessary for
disease to occur
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Oospores (and clamydospores) are very long
lived in soil but generally are not easily dispersed
Truck wash in S. Oregon National Forest
for the purpose of ………?
Oospore
• Sexual reproductive spore of an
Oomycete
• Thick, double-walled, survival spore
• Develops from the merging of an
antheridium and an oogonium
• Typically, it germinates to form a
zoosporangium
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After fertilization,
Oogonium matures
into an oospore.
Collar is the
antheridium
Important in plant pathology:
Homothallic:
Self-fertile
individual is ♂ and ♀
Heterothallic:
Different isolates
(i.e., individuals)
required for oospore
production
individual is
♂ or ♀
thus, two (or more)
‘mating types’
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Foliar Oomycetes
Tan Oak with Ramorum Blight
(Sudden Oak Death)
Destruction ofPhytophthora ramorum
infection centers in so. Oregon
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Symptom of infection
by Phytophthora
ramorum under bark
of tan oak
Again, there is a red
brown-colored,
expanding canker
White ‘cottony fluff’ is
sporangia (asexual spores)
Late blight of potato
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Downy Mildew
of grape:
sporangia
Downy Mildew of lettuce
sporangia and zoospores
• Asexual structures (mitosis)
• Zoospores are born in the sporangium
• Foliar oomycetes: sporangia dehisce
(i.e., become windborne spores)
• Zoospores require water for movement
(saturated soil, wet leaf surface)
• Zoospores attracted to plant exudates
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Sporangiophores of
downy mildew pathogens
‘phore’ means tree on which
sporangia are borne
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Oomycete questions:
• What is the basic life cycle of an Oomycete?
• What component(s) of the life cycle concern us
most when attempting to suppress:
soil-borne Oomycetes?
foliar Oomycetes?
• How does the life strategy of the damping off
pathogen, Pythium, differ from the downy
mildew pathogen, Peronospora?
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