Phylum Oomycota Pythiales Peronosporales

1. Phylum OomycotaPythiales Peronosporales General Mycology Pl P 421/521 Lecture 22

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. Advanced 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. http://helios.bto.ed.ac.uk/bto/microbes/zoospore.htm#Germination%20of%20sporangiahttp://helios.bto.ed.ac.uk/bto/microbes/zoospore.htm#Germination%20of%20sporangia

7. Zoospore ultrastructure

8. http://helios.bto.ed.ac.uk/bto/microbes/zoospore.htm

9. Zoospore taxis Zoospore settles and orients on host surface Adhesion and encystment with release of adhesins and cell wall formation Germination with a fixed orientation from predetermined point adhesins Modified from: http://helios.bto.ed.ac.uk/bto/microbes/zoospore.htm

10. Left: zoospores of Phytophthora palmivora accumulating at the mouth of a capillary filled with a fluorescently tagged amino acid (dansyl-asparagine) [supplied by MJ Carlile, from JN Cameron & MJ Carlile, unpubl.]. Right: zoospores of Pythium aphanidermatum accumulated and encysted at the mouth of a capillary containing malt-extract agar. http://helios.bto.ed.ac.uk/bto/microbes/zoospore.htm

11. 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

12. Pythium Zoosporangia (above) Oogonium/oospore (right) Photos by D. J. S. Barr

13. vesicles zoosporangia From Webster, 1980.

14. Zoospore cleavage and release in Pythium From Webster, 1980.

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

16. 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

17. Identification—Waterhouse’s groups • Papillate, narrow exit pore, not proliferating, deciduous, homothallic, paragynous • Zoosporangia as in I; heterothallic, amphigynous • Semipapillate, narrow exit pore, not proliferating; deciduous or not, homothallic, mostly paragynous • Zoosporangia as in III, usually deciduous; heterothallic, amphigynous • Nonpapillate, broad exit pore, not deciduous, homothallic, paragynous • Zoosporangia as in V; heterothallic, amphigynous

19. Sporangia, sporangiophores and zoospores in Phytophthora From Webster, 1980.

20. From Webster, 1980.

21. 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

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

23. Basidiophora • Short, stout sporangiophores • No branching • Sterigmata at apex • Sporangial germination indirect

24. Sclerospora • Bulbous sporangiophores with short terminal branches • Most species with indirect germination • Restricted to monocots • Forms digitate haustoria

25. Plasmopara • Random branching of sporangiophores, branches at ~ 90 degrees • Sporangiophores stiff and erect, with short terminal branches • Sporangial germination indirect

26. Peronospora • Elongated, curved, dichotomously branched sporangiophores • Sporangia germinate directly From Webster, 1980

27. Peronospora Sporangiophores and direct germination of sporangia From BioImages

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

29. Bremia • Dichotomously branched sporangiophores with enlarged tips bearing ring of sterimata • Sporangia germinate indirectly From Webster, 1980

30. 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

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

32. Albugo oospores From Webster, 1980

33. White rust