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Oomycota

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Pythium damping off is favored by cool, wet soilsPythium blight of turfPythium spp. are opportunists – they aggressivelycolonize dying plant materials: thatch layers, greenmanures, etc. – in turf, the disease occurs when theweather is warm and the grass crowns and thatch layer areunder water for a sufficient period. Plants recentlystressed, or lush from high N show increased susceptibility.4


Phytophthora root rots(aggressive pathogens)Phytothphora root rot of rhododendron5


2°symptom of Phytophthora root rot1°symptom of Phytophthora root rot6


1°symptom ofPhytophthora root rotOosporesTypically, oospores (longlivedresting spores) playan important role in soilbornediseasesWith some oomycetes,oospores are rare andmay not be necessary fordisease to occur7


Oospores (and clamydospores) are very longlived in soil but generally are not easily dispersedTruck wash in S. Oregon National Forestfor the purpose of ………?Oospore• Sexual reproductive spore of anOomycete• Thick, double-walled, survival spore• Develops from the merging of anantheridium and an oogonium• Typically, it germinates to form azoosporangium8


After fertilization,Oogonium maturesinto an oospore.Collar is theantheridiumImportant in plant pathology:Homothallic:Self-fertileindividual is ♂ and ♀Heterothallic:Different isolates(i.e., individuals)required for oosporeproductionindividual is ♂ or ♀thus, two (or more)‘mating types’9


Foliar OomycetesTan Oak with Ramorum Blight(Sudden Oak Death)Destruction ofPhytophthora ramoruminfection centers in so. Oregon10


Symptom of infectionby Phytophthoraramorum under barkof tan oakAgain, there is a redbrown-colored,expanding cankerWhite ‘cottony fluff’ issporangia (asexual spores)Late blight of potato11


Downy Mildewof grape:sporangiaDowny Mildew of lettucesporangia 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 exudates12


Sporangiophores ofdowny mildew pathogens‘phore’ means tree on whichsporangia are borne14


Oomycete questions:• What is the basic life cycle of an Oomycete?• What component(s) of the life cycle concern usmost when attempting to suppress:soil-borne Oomycetes?foliar Oomycetes?• How does the life strategy of the damping offpathogen, Pythium, differ from the downymildew pathogen, Peronospora?15

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