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Perenosporales
- 1. x Agriculture and forestryuniversity Rampur, Chitwan Presentation on PERONOSPORALES Submitted to: Prof. Dr. Sundar Man Shrestha Head of department Department of plant pathology AFU, Rampur Prepared by: Arjun Rayamajhi Msc. Ag. 1st semester Department of plant pathology Exam Roll No: PLP-06M-2018
- 2. Introduction • Most developed/specializedorder of class oomycetes • An order comprising chiefly parasitic lower fungi (class Oomycetes) that have equally biflagellate zoospores and conidia which either germinate directly or act as sporangia and contain the families Albuginaceae, Peronosporaceae, and Pythiaceae, (Merrim-webster dictionary).
- 3. Characteristics • Habitats anddistribution: aquatic, amphibious, and terrestrial species Many species are destructive parasites of economically important plants, frequently causing epiphytotics. The fungi of the order - Peronosporales, cause damping off, white rusts, downy mildews, late blight diseases.
- 4. Characteristics continue… • Foodhabits: • Majority of the members are the obligate parasites (Albuginaceae and Peronsporaceae) of higher plants but some are aquatic or semi-aquatic facultative parasites (Pythiaceae). • Some genera survive also saprophytically on dead or decaying vegetation and few survive easily in soil or in mud.
- 5. Characteristics continue… • Generalbiology: differs in fundamental ways from the organisms in the Kingdom Fungi – cell wall chemistry, sterol biosynthesis Cell wall composition – most fungi contain chitin as the major structural component but oomycote contain cellulose but no chitin Sterol biosynthesis - Ergosterol is characteristics sterol in true fungi whereas peronosporales donot need sterol to grow Griffith et al., 1992).
- 6. Characteristics continue… • SomaticStructures: The mycelium is well developed, consisting of coenocytic, stout hyphae that branch freely. The hyphae of parasitic species are growing intracellularly in most of pythiaceae or intercellularly in the downy mildew parasites (perenosporaceae). Those growing intercellularly produce haustoria within the host cell which may be either spherical in Albugo candida, elongated in Phytophthora infestans and Peronoospora pisi or branched.
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- 8. Zoospores : Asexualreproduction Asexual reproduction is through zoospores produced in the sporangia which are reniform, biflagellate, and monoplanetic. Diplanetism and polyplanetism occur in a few species. The sporangia are borne on somatic hyphae and remain attached even after the zoospores are released but in the more complex types the sporangia are borne on sporangiophores and are deciduous upon maturity. In this case, the whole sporangium acts as a spore and in some species they germinate by a germ tube instead of producing zoospores. Majority of species produces zoospores, upon their release from the sporangium, the zoospores swarm for sometime, come to rest, encysts, and germinates each by a germ tube that develops into the mycelium. • Two types of flagella are found on zoospores • Whiplash flagellum – smooth & tapers • Tinsel flagellum – has lateral filaments perpendicular to the main axis
- 9. Life cycle Sexual cyle: •Gametangia formed & meiosis occurs– • Homothallic & heterothallic species • Male gametangium – antheridium- slightly curved apical portion • Female gametangium – oogonium that contains singleor multiple nuclei – surrounded by periplasm, globose
- 10. • Antheridium growsto oogonium • Forms fertilization tubes to oospheres • Nuclei from antheridia migrate through fertilization tubes • Plasmogamy and karyogamy take place • Oosphere is now diploid • Develops thick wall and becomes an oospore
- 11. Oogonium • Oospore canremain dormant • Germinates to produce zoosporangium or 2n mycelium
- 12. Pythiaceae • Most highlyevolved members of the Oomycota • Include aquatic, amphibious and terrestrial forms • Saprotrophs and parasites • Oogonia produce only one oosphere
- 13. Pythium • Pythium –a large genus that includes saprotrophs (soil and water) and facultative parasites of algae, fungi and plants • One species causes “damping off” of seedlings – grows intercellularly in stems and roots of seedlings and rots tissue • Asexual reproduction by zoosporangia and zoospores (secondary zoospores only)
- 14. Pythium • Contents ofzoosporangium empty into a vesicle, cleave and form zoospores outside zoosporangium
- 15. Phytophthora • Forms lemonshaped sporangia that detach from sporangiophore in asexual reproduction – sporangia can germinate by forming zoospores or forming a germ tube directly
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- 18. Albuginaceae • Albugo –obligate plant parasite, causes white rust • Sporangia detach, dispersed by wind • If moisture available, sporangia produce 4-12 zoospores • Can germinate directly with a germ tube
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- 21. Peronosporaceae • Peronospora andrelated genera – obligate parasites of plants – cause downy mildews • In some species, sporangia always germinate with a germ tube, never form zoospores – called conidia
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