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 • 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 Eukaryotes Eukaryotes 1 Oomycota Eukaryotes University of California Museum of Paleontology Important diseases caused by Oomycetes Soilborne • Pythium damping-off • Pythium blight of turf • Phytophthora root rots Foliar • Late blight of potato/tomato • SuddenR oak death •Downyo mildews o t 2 Damping-off caused by Pythium spp. Favored by cool, wet soils that slow emergence The pathogen is ubiquitous & opportunistic 3 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. 4 Phytophthora root rots (aggressive pathogens) Phytothphora root rot of rhododendron 5 6 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 7 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 8 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’ 9 Foliar Oomycetes Tan Oak with Ramorum Blight (Sudden Oak Death) 10 Late blight of potato 11 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 12 13 Sporangia of Phytophthora infestans 14 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? 15.