Phylum Oomycota Pythiales Peronosporales

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Phylum OomycotaPythiales Peronosporales

• General Mycology
• Pl P 421/521
• Lecture 22

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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)

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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

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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

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Asexual reproduction

• Zoosporangia variable in shape and size
• Sporangiophore type delimits families
• Pythiaceaeindeterminate sporangiophores
• Peronosporaceaedeterminate sporangiophores
• Albuginaceaeclub-shaped sporangiophores,
  catenate sporangia

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http//helios.bto.ed.ac.uk/bto/microbes/zoospore.h
tmGermination20of20sporangia
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Zoospore ultrastructure
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http//helios.bto.ed.ac.uk/bto/microbes/zoospore.h
tm
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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/mi
crobes/zoospore.htm
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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
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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

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Pythium
Zoosporangia (above) Oogonium/oospore (right)
Photos by D. J. S. Barr
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vesicles
zoosporangia
From Webster, 1980.
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Zoospore cleavage and release in Pythium
From Webster, 1980.
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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

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Diseases caused by Phytophthora species

• P. infestanslate blight of potato, tomato
• P. cinnamomiroot rot of avocado, eucalyptus and
  gt 1000 hosts
• P. cactorumfruit rots (gt154 host genera)
• P. ramorumSudden Oak Death

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IdentificationWaterhouses 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

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(No Transcript)
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Sporangia, sporangiophores and zoospores in
Phytophthora
From Webster, 1980.
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From Webster, 1980.
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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

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Sexual reproduction

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

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Basidiophora

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

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Sclerospora

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

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Plasmopara

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

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Peronospora

• Elongated, curved, dichotomously branched
  sporangiophores
• Sporangia germinate directly

From Webster, 1980
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Peronospora
Sporangiophores and direct germination of
sporangia
From BioImages
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Pseudoperonospora

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

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Bremia

• Dichotomously branched sporangiophores with
  enlarged tips bearing ring of sterimata
• Sporangia germinate indirectly

From Webster, 1980
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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

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