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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 rotsFoliar• Late blight of potato/tomato• Sudden oak death• Downy mildews

Soilborne

Root

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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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OosporesTypically, 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 Forestfor 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 oosporeproductionindividual 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?