Perenosporales

this slide contains general idea on the order peronosporales and its families pythiaceae, peronosporaceae and albuginaceae.

1. x Agriculture and forestry university 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. 1 st semester Department of plant pathology Exam Roll No: PLP-06M-2018

2. Introduction Most developed/specialized order 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 and distribution: 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… Food habits: 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 s aprophytically on dead or decaying vegetation and few survive easily in soil or in mud.

5. Characteristics continue… General biology: 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… Somatic Structures: 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 .

7. Reproduction

8. Zoospores : Asexual reproduction 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. Life cycle Antheridium grows to 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 can remain dormant Germinates to produce zoosporangium or 2n mycelium

12. Pythiaceae Most highly evolved 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 of zoosporangium empty into a vesicle, cleave and form zoospores outside zoosporangium

15. Phytophthora Forms lemon shaped sporangia that detach from sporangiophore in asexual reproduction – sporangia can germinate by forming zoospores or forming a germ tube directly

16. Phytophthora

17. Phytophthora

18. A lbuginaceae 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

19. Albugo

20. Albugo

21. Peronosporaceae Peronospora and related genera – obligate parasites of plants – cause downy mildews In some species, sporangia always germinate with a germ tube, never form zoospores – called conidia

22. Peronosporaceae

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