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”
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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
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bladmineerders.nl old.botany. upol.cz Plasmopara sporangiophore
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forestryimages.org Peronosclerospora conidiophores
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The picture can’t be displayed. Peronospora conidiophores
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The picture can’t be displayed. Hyaloperonospora conidiophores
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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
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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
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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
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Peronospora manschurica Erisyphe diffusa