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    Oomycetes in lab tomorrowOomycetes 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 • Disease of the week ‘Unknown’ –

    worksheet to fill in

    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

    EukaryotesEukaryotes

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    University of California Museum of Paleontology

    EukaryotesEukaryotes

    OomycotaOomycota

    Important diseases caused by Oomycetes

    • Pythium damping-off • Pythium blight of turf • Phytophthora root rots Foliar • Late blight of potato/tomato • Sudden oak death • Downy mildews

    Soilborne

    R o o t

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    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 long 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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    Oospores Typically, oospores (long- lived resting spores) play an important role in soil- borne diseases

    With foliar pathogens, oospores are commonly rare and may not be necessary for disease to occur

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    Truck wash in S. Oregon National Forest for the purpose of ………?

    Oospores (and clamydospores) are very long lived in soil but generally are not easily dispersed

    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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    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)

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    Late blight of potato

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    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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    Sporangia of Phytophthora

    infestans

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    Sporangiophores of downy mildew pathogens

    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?