The downy mildew pathogens General taxonomy Workshop on the Identification of Downy Mildews (Oomycetes) in South-east Asia, Chiang Mai University, Chiang Mai, Thailand 29 January – 2 February 2018 Traditional (Old) classification of the Oomycetes • Classified in Phycomycetes or “lower fungi” • Also contained slime moulds, chytrids, zygomycetes and arbuscular mycorrhizae • “Higher fungi” included ascomycetes (discomycetes, pyrenomycetes, yeasts) and basiodiomycetes (rusts, smuts, mushrooms etc) Examples of “lower fungi” The picture can’t be displayed. A slime mould Downy mildew (Plasmopara viticola) on grape Examples of “higher fungi” Sclerotinia apothecia releasing ascospores Ergot (Claviceps africana) on Sorghum Powdery mildew (Podosphaera xanthii) on mungbean More examples of “higher fungi” Boil smut (Mycosarcoma maydis = Ustilago A mushroom maydis) on maize Rust (Puccinia purpurea) on Sorghum Phylogenetic relationships Contains algae and diatoms www.apsnet.org/edcenter/intropp/LabExercises/Pages/Oomycetes.aspx Differences between true fungi and oomycetes Character Oomycota True Fungi Sexual reproduction Oospores Zygospores, ascospores, basidiophores Nuclear state of Diploid; no septa Haploid, dikaryotic; vegetative mycelium (crosswalls) mostly septate Cell wall composition Beta glucans, Chitin; rarely cellulose cellulose Type of flagella on Two types; One type if produced; zoospore whiplash at tail whiplash at tail end end, ciliated at front end Mitochondria With tubular With flattened cristae cristae www.apsnet.org/edcenter/intropp/LabExercises/Pages/Oomycetes.aspx Oogonia of the Oomycetes Oogonium Antheridium Oogonium Antheridium www.apsnet.org/edcenter/intropp/PathogenGroups/Pages/IntroOomycetes.aspx Sporangia of Phytophthora forestphytophthoras.org en. wikipedia.org Modern classification of downy mildew fungi Kingdom Straminipila (Chromista) Phylum Oomycota (Peronosporomycota) Class Peronosporomycetes Order Peronosporales Family Peronosporaceae NOT TRUE FUNGI Phylogeny of the Peronosporomycetes (Oomycetes) Thines & Choi 2016 The Order Peronosporales • Contains Phytophthora, Pythium, F. Salisaplilliaceae (hemibiotrophic /saprophytic) and F. Peronosporaceae (downy mildews; obligate biotrophs on plants) clades • Downy mildews comprise 33% of known oomycete species - 20 genera with 700 species • Peronospora is the largest DM genus with +/- 400 species F. Peronosporaceae (downy mildews) • 14 genera are host specific to a single Family • Basidiophora, Benua, Bremia and Paraperonospora on Asteraceae • Plasmoverna on Ranunculaceae • Baobabopsis, Eraphthora, Graminivora, Paokatesthia, Peronosclerospora, Sclerophthora, Sclerospora, Viennotia on Poaceae Phylogenetic groups in the Peronosporaceae Diagnostic features of the Peronosporaceae – asexual morphs • Form a downy growth on infected plant parts esp. leaf blades, usually at night • “Down” consists of conidiophores/ sporangiophores bearing conidia or sporangia • Conidiophores/sporangiophores originate from hyphae in the sub-stomatal cavities • Conidia germinate by production of germ tubes, sporangia germinate by release of zoospores Soybean downy mildew (Peronospora manschurica) Close-up of soybean downy mildew sporulation Conidiophore/sporangiophore morphology of some DM genera Branching, primary Spore – bearing Genus Persistent branch angle tips On grasses Baobabopsis x unbranched ampulliform Eraphthora x unbranched pedicels Peronosclerospora x dichotamous, upright stout, divide Sclerophthora x unbranched pedicels Sclerospora x dichotamous, upright stout, divided On dicots Bremia a sub dichotamous disc + pedicels Basidiophora a unbranched pedicels Hyaloperonospora a ± monopodial, acute slender, tapering Peronospora a monopodial, acute slender, tapering Plasmopara a monopodial, right angle short, blunt Conidiophore/sporangiophore morphology of some DM genera Alexopolous (1962). Introductory Mycology Conidia/sporangia morphology of some DM genera Genus Colour Papillate Germination On grasses Baobabopsis hyaline x germ tubes Eraphthora hyaline x zoospores, germ tubes Peronosclerospora hyaline x germ tubes Sclerophthora hyaline a zoospores, germ tubes Sclerospora hyaline a zoospores, germ tubes On dicots Bremia hyaline a zoospores, germ tubes Basidiophora hyaline? a zoospores Hyaloperonospora hyaline x germ tubes Peronospora coloured x germ tubes Plasmopara hyaline a zoospores, germ tubes Bremia sporangiophores The picture can’t be displayed. bladmineerders.nl old.botany. upol.cz Plasmopara sporangiophore The picture can’t be displayed. forestryimages.org Peronosclerospora conidiophores The picture can’t be displayed. The picture can’t be displayed. Peronospora conidiophores The picture can’t be displayed. The picture can’t be displayed. Hyaloperonospora conidiophores The picture can’t be displayed. The picture can’t be displayed. today.duke.edu bladmineerders.nl Diagnostic features of the Peronosporaceae – sexual morph • Formation of oogonia in invaded plant parts (often in same site as asexual morph), esp. leaves • Oospheres are fertilised by nuclei from antheridia • At maturity, an oospore is enclosed in an oogonium • Oospores germinate by production of germ tubes or sporangia which release zoospores Oogonia and oospores The picture can’t be displayed. oospore exosporium oogonium Some differences between downy mildew and powdery mildew fungi Powdery Feature Downy mildew mildew Order Peronosporales Erysiphales Family Peronosporaceae Erisyphaceae Sexual state Oogonia Asci/ascospores Hyphae Internal External Usually discrete Later cover Asexual state - sporulation areas entire surfaces Asexual state – Branched/simple Simple conidiophores/sporangiophores Downy mildew (Peronospora manschurica) and powdery mildew (Erisyphe diffusa) on same soybean leaf The picture can’t be displayed. Powdery mildew Downy mildew Downy mildew (L) and powdery mildew (R) on soybean leaves The picture can’t be displayed. Downy mildew sporulation on lower soybean leaf surface Conidiophores of downy mildew (L) and powdery mildew (R) pathogens The picture can’t be displayed. The picture can’t be displayed. Peronospora manschurica Erisyphe diffusa .