Diseases caused by Downy Mildew fungi

Taxonomic Tree
 Domain: Eukaryota
 Kingdom: Chromista
 Phylum: Oomycota
 Class: Oomycetes
 Order: Peronosporales
 Family: Peronosporaceae
 Genus: Plasmopara
 Plasmopara viticola, Sclerospora
graminicola, Peronospora parasitica, Plasmopara
viticola, Bremia lactucae, Pseudoperonospora
cubensis

Plasmopara viticola
 Kingdom: Chromesta
 Phylum: Oomycota
 Class: Oomycetes
 Order: Peronosporales
 Family: Peronosporaceae
 Genus: Plasmopara
 Plasmopara viticola

Description, Hall (1989)
 Mycelium composed of intercellular, colourless, aseptate hyphae , often
irregularly shaped and swollen, bearing small, rounded vesicular
haustoria, formed predominantly in leaf tissues.
 Sporangiophores hypophyllous, arborescent, branching monopodially
in the upper third at right angles to the main axis, and with a base
tapering to a conical point; branches often with two opposite secondary
branches, all having, diverging at right angles and tapering to a terminal
swelling.
 Sporangia ovoid, colourless, sometimes with a short pedicel, each
producing 1-6 zoospores.
 Zoospores reniform, laterally biflagellate, emerging from opposite the
insertion point of the sporangium, via a papilla, or by direct penetration
of the sporangium wall.
 Oospores formed in leaf tissues, spherical, containing 14-16
chromosomes, covered by two inner oospore membranes and an outer
wrinkled oospore wall, germinating via a tube, to give a pyriform
sporangium, producing 8-20 zoospores.

 Plasmopara viticola, produces asexual, biflagellate
zoospores and sexual oospores.

Pseudoperonospora cubensis (Berkeley
& Curtis) Rostovtsev
 Caused Disease of Downy Mildew of Cucurbits, Members
of Cucurbitaceae, the gourd family, including cucumber
(Cucumis sativus), squash (Cucurbita spp.), melon
(Cucumis melo) and watermelon (Citrullus lanatus).
 Pseudoperonospora cubensis forms large lemon-shaped
sporangia with a conspicuous papilla. Sporangia appear
smooth under the compound microscope, but with the
scanning electron microscope the finely decorated surface
of the sporangium is evident. The sporangia are borne
singly on the pointed tips of sporangiophores that branch
at acute angles.

Intercellular mycelium.
 Mycelium grows within and between host cells and
serves as the body of the pathogen. The intercellular
mycelium is hyaline (colorless, transparent) and
coenocytic (aseptate). The mycelium develops in the
mesophyll, but also penetrates the palisade tissue. The
hyphal diameter is 5.4-7.2 mm. Haustoria are formed
within the host cells and allow for the absorption of
nutrients. Haustoria are varied in shape and appear
stunted, inflated or as branched clusters of hyphae.

Oospores.
 The role of oospores in the life cycle of P. cubensis is
unknown.
 Sexual reproduction and formation of oospores in P.
cubensis are rare.
 Oospores have only been reported from Russia, China,
Japan, India and Italy and their germination was not
observed.
 Oospores could function as survival structures when
living hosts are not present, but this has not been
demonstrated.

Bremia lactucae
Description
 Coarse intercellular mycelium.
 Haustoria are sac-shaped, many times they are present in
each host cell.
 Sporangiophores emerge singly or in small groups through
the stomata and branch dichotomously.
 Tip of each branch expands to form a cup-shaped disc
bearing short cylindrical sterigmata at the margin and
occasionally in the centre, and from these the hyaline
sporangia arise.

 Germination of the sporangia is usually by means of a
germ tube which forms an appressorium to penetrate
epidermal cells or it enters through a stoma.
 Zoospore formation has been reported but not
confirmed.
 Sexual reproduction is usually heterothallic, but
homothallic strains also exist.
 Oospores are formed in leaf tissue and remain viable
for 12 months.

Peronospora parasiticae
Peronospora destructor
 There are about 75 species in the genus, mostly
specific to genera or families of herbaceous
dicotyledonous plants.
 One of the best known Peronospora species
(Peronospora parasitica, on Brassicaceae) has now
been shown to be a species complex in the genus
Hyaloperonospora. The most important plant
pathogen in this complex, on Brassica, is now known
as Hyaloperonospora brassicae.

Peronospora destructor
 Peronospora destructor is a plant pathogen. It causes
downy mildew on leaves of cultivated and wild Allium.
Allium cepa (onion and shallot) is most often affected,
while Allium schoenoprasum (chives) and Allium
porrum (leek) are only occasionally affected.

Hyaloperonospora parasitica
 Hyaloperonospora parasitica
 It has been considered for a long time to cause downy
mildew of a variety of species within the Brassicaceae,
on which the disease can cause economically
important damage by killing seedlings or affecting the
quality of produce intended for freezing.
 The former name for H. parasitica was Peronospora
parasitica until it was reclassified and put in the
genus Hyaloperonospora.

Reproductive structures and infection of
Peronospora ciceris in chickpea leaflets. (A)
Sporangiophore showing dichotomous branching
(rd) and sporangia. (B) Sporangiophore (e) with
sporangia (es). (C) Terminal branching of a
sporangiophore showing a rounded tip (pr) and
acute angle (w). (D) Ovoid (eo) and sub-elliptical (ee)
sporangium. Figura 3. Estructuras de reproducción e
infección de Peronospora ciceris en foliolos de
garbanzo. (A) Esporangióforo mostrando
ramificaciones dicotómicas (rd) y esporangios. (B)
Esporangióforo (e) con esporangios (es). (C)
Ramificación terminal de un esporangióforo
mostrando punta redondeada (pr) y ángulo agudo
(an). (D) Esporangio ovoide (eo) y sub-elíptico (ee)

Diseases caused by Downy Mildew fungi

Diseases caused by Downy Mildew fungi

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Diseases caused by Downy Mildew fungi