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

2. Classification Order Pythiales Order Peronosporales Families
Pythiaceae (9 genera, 230 species)
Pythiogetonaceae (2 genera, 8 species)
Order Peronosporales
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

7. 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

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

9. vesicles
zoosporangia

10. Zoospore cleavage and release in Pythium

11. Phytophthora de Bary 1892 Type species P. infestans (Montagne) de Bary
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

12. lemon-shaped infestans Phytophthora
Sporangia ovoid, obpyriform to lemon-shaped

13. 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

15. 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

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

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

18. 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

19. Peronospora parasitica Downy mildew infection on leaf

20. Peronospora
Sporangiophores and direct germination of sporangia

21. Peronospora
Elongated, curved, dichotomously branched sporangiophores claw shape
Sporangia germinate directly
From Webster, 1980

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

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

24. Bremia

25. 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

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

27. Albugo candida conidra tjv

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

29. Albugo oospores

30. White rust
Host plant :
Protulaca olearacea