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Phylum : Oomycota Class: Oomycetes

Phylum : Oomycota Class: Oomycetes. Order. Pythiales. Peronosporales. Families. Families. Pythiogetonaceae. Peronosporaceae. Pythiaceae. Albuginaceae. Classification. Order Pythiales Families Pythiaceae (9 genera, 230 species) Pythiogetonaceae (2 genera, 8 species)

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Phylum : Oomycota Class: Oomycetes

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  1. Phylum :OomycotaClass:Oomycetes Order Pythiales Peronosporales Families Families Pythiogetonaceae Peronosporaceae Pythiaceae Albuginaceae

  2. Classification • Order Pythiales • Families • Pythiaceae (9 genera, 230 species) • Pythiogetonaceae (2 genera, 8 species) • Order Peronosporales • Families • Peronosporaceae (8 genera, 600 species) • Albuginaceae (1 genus, 45 species)

  3. General characters • Mostly parasitic with intracellular haustoria • Narrow diameter hyphae • Trend towards terrestrial habitats • Loss of primary zoospore stage • Trend towards loss of secondary zoospore stage. • Inability to synthesize sterols

  4. Sexual reproduction • One oospore/oogonium • Oospores formed in host tissue • Unifactorial mating system • A1 and A2 mating types • Selfing may occur • Relative sexuality • Bisexual strains

  5. Asexual reproduction • Zoosporangia variable in shape and size • Sporangiophore type delimits families: • Pythiaceae: indeterminate sporangiophores • Peronosporaceae: determinate sporangiophores • Albuginaceae: club-shaped sporangiophores, catenate sporangia

  6. Pythium Pringsheim 1858 • Type species P. monospermum • 127 species • First invaders of plant tissue, not good competitors with other organisms • Major components of damping-off disease of seedlings, collar rot of apples, foliar and stem blights, snow rot of winter wheat • Sporangia of various shapes, sizes • Zoospores cleaved out and released from vesicle • Oogonia fertilized by one to many antheridia • Oogonial wall smooth or with spiny or blunt projections • Chlamydospores present in some species

  7. Pythium Zoosporangia (above) Oogonium/oospore (right)

  8. vesicles zoosporangia

  9. Zoospore cleavage and release in Pythium

  10. Phytophthora de Bary 1892 • Type species P. infestans (Montagne) de Bary • 65 species • Wide range of diseases • Sporangia ovoid, obpyriform tolemon-shaped • Sporangia papillate, semipapillate or nonpapillate • Zoospores cleaved out inside zoosporangium • Oogonium fertilized by one antheridium

  11. lemon-shaped infestans Phytophthora • Sporangia ovoid, obpyriform tolemon-shaped

  12. Diseases caused by Phytophthora species • P. infestans— late blight of potato, tomato • P. cinnamomi— root rot of avocado, eucalyptus and > 1000 hosts • P. cactorum— fruit rots (>154 host genera) • P. ramorum— Sudden Oak Death

  13. Family Peronosporaceae • Obligate biotrophs of vascular plants • “downy mildews” • Sporangiophores aerial, determinate, up to 750 microns tall • Sporangia deciduous, released by drying • Sporangia germinate directly or indirectly

  14. Sexual reproduction • Oogonia larger, antheridia paragynous, broader relative to Pythiaceae • Formed inside host tissue • Periplasm forms thick, sculptured wall

  15. Sclerospora Sclerospora • Bulbous sporangiophores with short terminal branches • Most species with indirect germination • Restricted to monocots • Forms digitatehaustoria

  16. Plasmopara • Random branching of sporangiophores, branches at ~ 90 degrees • Sporangiophores cross shape stiff and erect, with short terminal branches • Sporangial germination indirect • Cassed downy mildew of vitis

  17. PeronosporaparasiticaDowny mildew infection on leaf

  18. Peronospora Sporangiophores and direct germination of sporangia

  19. Peronospora • Elongated, curved, dichotomously branched sporangiophoresclaw shape • Sporangia germinate directly From Webster, 1980

  20. Pseudoperonospora • Sporangiophores similar to Peronospora except branching more delicate and not dichotomous • Sporangia germinate indirectly

  21. Bremia • Dichotomously branched sporangiophores with enlarged tips bearing ring of sterimata. Sauccer shape. • Sporangia germinate indirectly

  22. Bremia

  23. Family Albuginaceae • One genus, Albugo • “white rusts” • Sporangia formed in basipetal succession, dispersed by wind • Germinate to form 8 zoospores • Oogonia develop in host, oospores develop thin vesicle similar to Pythium sporangium

  24. Albugo haustoria Release of zoospores Club-shaped sporangiophores and chains of sporangia From Webster, 1980

  25. Albugo candida conidra tjv

  26. White Rust of Crucifers Caused by Albugo candida • Symptoms of White Rust on Shepherd's Purse

  27. Albugo oospores

  28. White rust Host plant : Protulacaolearacea

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