The document discusses the order Peronosporales, which includes primarily parasitic fungi with biflagellate zoospores and conidia. It describes their biological characteristics, reproductive methods, and classifications into four families: Pythiaceae, Peronosporaceae, Peronophythoraceae, and Albuginaceae, each with specific traits and notable species. The order is significant due to its impact on economically important plants, causing diseases such as damping off, downy mildews, and white rusts.

1. PERONOSPORALES
Characteristics and Importance
Course code: PLP 603
Course title: Mycology 4 (3+1)
SUBMITTED BY: SUBMITTED TO:
Arjun Rayamajhi Prof. Dr. Sunder Man Shrestha
PLP-06M-2018 AFU, Rampur
Msc.Ag 1st Semester
DEPARTMENT OF PLANT PATHOLOGY
May 3, 2018

2. Order Perenosporales
Definition:
An order comprising chiefly parasitic lower fungi (class Oomycetes) that have equally
biflagellate zoospores and conidia which either germinate directly or act as sporangia and contain
the families Albuginaceae, Peronosporaceae, and Pythiaceae, (Merrim-webster dictionary).
Habitats and distribution:
The members of perenosporales are the most developed fungi of the class Oomycetes. This
large order of fungi includes aquatic, amphibious, and terrestrial species of highly specialized
obligate parasites. Many species are destructive parasites of economically important plants,
frequently causing epiphytotics. The fungi of the order - Peronosporales, cause damping off, white
rusts, downy mildews, late blight diseases.
Food habits:
Majority of the members are the obligate parasites (Albuginaceae and Peronsporaceae) of
higher plants but some are aquatic or semi-aquatic facultative parasites (Pythiaceae). Some genera
survive also saprophytically on dead or decaying vegetation and few survive easily in soil or in
mud.
General biology:
Cell wall is fibrillar and consists of polysachharides, lipid and protein in phytophthora and
Pythium. About 90% of the wall is constituted by glucans. The cell wall also contains amino acid
hydroxy proline.
Biochemical characters:
Two of the most notable differences between oomycota and fugi relates to the synthesis of
amino acids lysine and to various aspects of sterol mechanism
a) While true fungi make lysine via the alfa aminoadipic pathway, oomycetes synthesize this
amino acid by diaminopimelic acid pathway, same pathway used by plants.
b) Sterol mechanism in perenosporales also significantly differ from that in fungi (Griffith et
al., 1992). Ergosterol is characteristics sterol in true fungi whereas peronosporales donot
need sterol to grow.
Members of peronosporales appears to lack chitin in their walls and insensitive to the chemicals
that targets enzyme chitin synthetase.
Somatic structures:
The mycelium of the Peronosporales is well developed, consisting of coenocytic, stout
hyphae that branch freely. The hyphae of parasitic species are growing intracellularly in most of
pythiaceae or intercellularly in the downy mildew parasites (perenosporaceae). Those growing
intercellularly produce haustoria within the host cell which may be either spherical in Albugo
candida, elongated in Phytophthora infestans and Peronoospora pisi or branched.
Reproduction:
a) Asexual reproduction: Asexual reproduction is through zoospores produced in the
sporangia which are reniform, biflagellate, and monoplanetic. Diplanetism and

3. polyplanetism occur in a few species. The sporangia are borne on somatic hyphae and
remain attached even after the zoospores are released but in the more complex types the
sporangia are borne on sporangiophores and are deciduous upon maturity. In this case, the
whole sporangium acts as a spore and in some species they germinate by a germ tube
instead of producing zoospores. Majority of species produces zoospores, Upon their release
from the sporangium, the zoospores swarm for sometime, come to rest, encysts, and
germinates each by a germ tube that develops into the mycelium.
b) Sexual reproduction: Sexual reproduction is oogamous and takes place by gametangial
contact between well-
differentiated oogonia and
antheridia borne on the same or
on different hyphae. The
oogonium is generally globose,
containing uninucleate or
multinucleate oosphere,
surrounded by a layer of
periplasm. The antheridium is
uninucleate or multinucleated
depending upon species and it is
slightly enlarged apical portion
of the mycelium. When the
antheridium come in contact
with the oogonium, a
fertilization tube is formed by
the antheridium, pushes through the oogonial wall, periplasm and reaches to oosphere. The
male nucleus or nucleithen passes through the fertilization tube and are shed into the
oosphere. If the oosphere is uninucleated, a single male nucleus fuses and forms the zygote.
If the oosphere is multinucleated, one or more of its nuclei may be functional, and the
number of male nuclei that will affect fertilization will be regulated accordingly. Thus,
either a single zygote nucleus or a number of nuclei may result from a simple or multiple
fertilization. This type of fertilization is called paragynous. Few peronosporales such as
Phytophthora show amphigynous type of fertilization.
After fertilization, the oosphere develops into thick walled oospore which are
resistant against unfavorable environmental conditions. The periplasm probably serves as
the nourishment for the developing oospore. The oospores germinate producing zoospores
or sometime by a germ tube which soon produce sporangia.
Classification and relationships:
The Order – Peronosporales is classified into four families mainly based on the characters of
sporangia and sporangiophores
1. Pythiaceae, 2. Peronosporaceae, 3. Peronophythoraceae and 4. Albuginaceae
Key to differentiate the families of Order Perenosporales
A. Sporangia borne singly, in clusters, or in whorls.

4. B. Sporangia on somatic hyphae or on sporangiophores of indeterminate growth, periplasm a thin
layer or absent, facultative parasites or saprobes.........Pythiaceae
BB. Sporangia borne on sporangiophores of determinate growth.
C. Periplasm conspicuous, obligate parasites of plants, sporangia wind-
borne..............................................................................................Perenosporaceae
CC. Periplasm inconspicuous, facultative parasites of plants, sporangia not wind-
borne.........................................................................................Perenophythoraceae
AA. Sporangia borne in chain, periplasm conspicuous, obligate parasites of plants
................................................................................................Albuginaceae
Family – Pythiaceae
Saprophytic or parasitic, mycelium intracellular, less often intercellular with haustoria.
It includes aquatic, amphibious and terrestrial fungi. Many of them cause serious diseases of
economically important plants. There are 8 genera, however, the common and economically
important genera are Pythium and Phytophthora. Species of Pythium is aquatic and terrestrial.
P. ultimum, P. aphanidermatum and P. debarianum are the common species causing damping
off diseases of several vegetable crops.
Genus Pythium
The mycelium consists of rather slender, coenocytic hyphae. They live saprophytically
on dead organic matter or parasitically on the young seedlings of various plants. The hyphae are
both intracellular and intercellular and they do not produce haustoria.
Life cycle of Pythium debarianum
Asexual reproduction takes place by producing sporangia either as terminal or intercalary
on the somatic hyphae which are globose to oval in shape. The sporangia may germinate directly
by germ tube or they germinate by producing zoospores in the vesicle. The protoplast of
sporangium moves through a tube into the vesicle and differentiation of the zoospores takes place.
Suddenly the vesicular wall bursts like a soap bubble and the zoospores are scattered in all
directions. The reniform, biflagellate zoospores, after a period of swarming, come to rest, encyst
and germinate by a germ tube developing into mycelium.

5. Sexual reproduction takes place by gametangial contact. Oogonia and antheridia are
developed in close proximity, often on the same hyphae. The oogonium is globose with a
multinucleated oosphere surrounded by a layer of periplasm. The antheridia are much smaller, and
somewhat elongated or clubshaped. A fertilization tube is developed at the point of contact

6. penetrating the oogonial wall and periplasm. In the mean time, meiosis takes place in both the
gametangia. The other nuclei disintegrate but only one remains functional in each of them. The
mail nucleus now passes into the oosphere and unites with female nucleus resulting into zygote.
The oosphere germinates after a rest period either by germ tube or by forming vesicle in which the
zoospores are developed which are released and germinate by germ tube growing into a mycelium.
Family – Peronosporaceae
This is the most highly specialized family in the Order – Peronosporales. All the species
are obligate parasites of vascular plants, causing downy mildew diseases such as Plasmopara
viticola (downy mildew of grape), Peronospora destructer (downy mildew of onion), Bremia
lactucae (downy mildew of lettuce), Pseudoperonospora cubensis (downy mildew of cucurbits),
Sclerospora
graminicola
(downy mildew of
sorghum) etc.
The genera
included under
Peronosporaceae
can be identified
mainly by their
branching habit of
the
sporangiophores.
The
sporangiophores
are of determinate
growth, bearing
sporangia at their
tips.
Genus –
Basidiophora: The
sporangiophore is
club-shaped with a
swollen head over
which the sporangia
are borne on minute
sterigmata.
Genus –
Sclerospora: The
sporangiophore is
long, stout hyphae,
with many upright
branches near the
end, bearing
sporangia at the
tips.

7. Genus – Plasmopara: The sporangiophore is branched at right angles and is irregularly spaced,
bearing sporangia at the tips.
Genus – Peronospora and Pseudoperonospora: The sporangiophores are dichotomously branched
at acute angles and taper to gracefully curved pointed tips on which sporangia are borne. In case
of Pseudoperonospora the sporangia are of Phytophthora type.
Genus – Bremia: The sporangiophore is similar to Peronospora but the tips are expanded into cup-
shaped structure with four sterigmata each bearing the sporangia.
The life cycle of these genera is similar to that of the Pythium.
Family – Albuginaceae
Members are strictly obligate parasites causing white rust diseases of vascular plants. The
mycelium of the fungus is intercellular with knob like haustoria. The genus Albugo has several
species. attacking different crops. Albugo candida (Crucifers), Albugo ipomoeae-panduranae
(sweet potato), Albugo occidentalis (spinach), and Albugo bliti (Amaranths).
Life cycle of Albugo candida:
When the mycelium matures, it produces short club-shaped sporangiophores from the
tips of hyphal branches at one locality below the epidermis of the host. The sporangiophores
produce number of sporangia in chain being the oldest one at the tip. When the sporangia mature
they will be detached and accumulated in between the spaces of the host epidermis and the
sporangiophores. They will exert a pressure, to cause bulge and eventually bursts the epidermis
exposing the sporangia which will form white crust like on the surfaces of the host. The sporangia
are globose. Wind, water, and other agencies disseminate them. The sporangia may germinate
by means of germ tube or by producing zoospores.
Sexual reproduction is followed by producing the oogonium and antheridium within the
tissues of the host. The gamentangia are produced terminally and when they contact a fertilization
tube is formed and the male nucleus along with cytoplasm is passed. Plasmogamy and karyogamy
take place resulting into zygote. Nuclear division takes place several times while the fertilized
oosphere is being transformed into oospore by the development of thick wall and that enters the
resting stage. It germinates in two ways, either it extrudes zoospores into a sessile vesicle or it
forms a short exit tube terminating into a vesicle. They never germinate directly. When the vesicle
wall bursts, the zoospores are liberated, swim for some time and encyst and germinate by germ
tube which infect the host.

9. References:
Class notes of Mycology by Prof. Dr. Sundar Man Shrestha
Alexopoulos, C.J., C.W. Mims, and M. Blackwell. 2015. Introductory Mycology. 4th
ed. Wiley
and Sons. Singapore. Pp:683-738
Sharma, O.P. 1988, Textbook of Fungi. Tata MC-Graw hill education. Pp 72-78
http://www.biologydiscussion.com/fungi/peronosporales-characteristics-and-classification/63289
https://www.merriam-webster.com/dictionary/Peronosporales