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

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)

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

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

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

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

Pythium Zoosporangia (above) Oogonium/oospore (right)

vesicles zoosporangia

Zoospore cleavage and release in Pythium

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

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

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

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

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

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

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

Peronospora parasitica Downy mildew infection on leaf

Peronospora Sporangiophores and direct germination of sporangia

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

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

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

Bremia

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

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

Albugo candida conidra tjv

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

Albugo oospores

White rust Host plant : Protulaca olearacea