Phylum : Oomycota
Class: Oomycetes
Order
Peronosporales
Families
Pythiales
Families
Peronosporaceae Albuginaceae Pythiogetonaceae
Pythiaceae
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)
General characters
1. Mostly parasitic with intracellular
haustoria
2. Narrow diameter hyphae
3. Trend towards terrestrial habitats
a. Loss of primary zoospore stage
b. Trend towards loss of secondary
zoospore stage.
4. 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
• 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
lemon-shaped
Phytophthora
infestans
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,
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