Phylum : Oomycota Class: Oomycetes

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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

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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

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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