Download Phylum: Chytridiomycota

Phylum: Chytridiomycota
The
phylum
Chytridiomycota
contains
the
single
class
Chytridiomycetes.
General characteristics of Chytridiomycota:
1. They produce motile cells at some stage in their life cycle. The
motile cells of these organisms each possess a single, posteriorly
directed whiplash flagellum, exception: few species have poly
flagellate cells.
2. Thallus is coenocytic, whether a globose or a well developed
structure, an elongated single hypha or a well developed
mycelium.
3. The conversion of the zygote into a resting spore or resting
sporangium.
4. Cell wall of these fungi is known to contain chitin and glucan.
Example: Synchytrium endobioticum causes the disease known as
black wart of potato.
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Phylum: Oomycota
General characteristics:
1. The division Oomycota, unlike the Chytridiomycota, is a group of
aquatic fungi that have cell walls composed of cellulose and a diploid
dominant lifecycle (=gametic lifecycle).
2. The mycelium is coenocytic and produces septa only to separate the
reproductive structures from the assimilative portion of the thallus.
3. Asexual reproduction is by zoospores that are produced in
zoosporangia. The zoospores produced are biflagellated with a longer
tinsel flagellum directed forward and a shorter whiplash flagellum
directed backward.
4. Sexual reproduction is heterogamous and occurs by direct injection
of the male nuclei (=sperms) from the antheridium into the eggs
contained in the oogonium . A swimming sperm is absent in the
Oomycota. This type of sexual reproduction is referred to as
gametangial copulation that results in the production of a thick walled
sexual spore termed an oospore.
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5. Their Cell walls composed of primarily of ß- glucans but also
containing the amino acid hydroxyl prolline as well as small amount of
cellulose.
6. The eggs and sperms are products of meiosis and the only parts of
the life cycle that are haploid. Only one class, the Oomycetes, is
recognized in this division.
The combination of cellulose cell wall, biflatellated zoospores, one
flagellum of the tinsel type and the other of the whiplash type, and
gametangial copulation are characteristics that are shared with some
members of the algal divisions Phaeophyta and Chrysophyta. This has
recently led to recognition of yet another kingdom, Stramenopila, which
includes the divisions Chrysophyta, Phaeophyta, Hyphochytridiomycota
and Oomycota. These divisions are now thought to be derived from a
common ancestor
Class: Oomycetes
The characteristics of the class Oomycetes is the same as those of the
division.
We will examine two representatives in the Oomycota: Pythium spp.
and Phytophthora spp. (Peronosporales).
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Phylum Oomycota has one class Oomycetes. Oomycetes class is
grouped into five orders:
1. Leptomitales
2. Saprolegniales
3. Rhipidiales
4. Lagenidiales and
5. Peronosporales
Order peronosporales has three well defined families namely,
i) Pythiaceae ii) Peronosporaceae and iii) Albuginaceae
Family: Pythiaceae
The most important genera of this family are Phytophthora and Pythium.
Both are plant pathogenic causing many severe diseases of plants. e.g.
Phytophthora infestans (Fig. 7) Pythium debaryanum (Fig. 8)
Fig. 7. Sporangia of Pythium Fig. 8. Sporangium of Phytophthora
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Economic importance of Pythium spp.
Members of the genus Pythium are soil inhabiting organisms that
occur all over the world. They are however, most commonly
encountered in cultivated soils where they pose serious problems for
various agronomicaly important crops. Pythium species are the most
important causes of pre and post emergence damping off diseases
of seedlings.
The term post emergence damping off refers to a condition in which
young seedlings are attacked and rotted near the soil line, causing
them to fall over. This condition is particularly troublesome in green
house, where large numbers of seedlings are grown in close
proximity to one another.
Pythium species also attack and rot seeds in the field and may
destroy seedlings before they emerge from the soil, this condition
referred to as pre emergence damping off.
Soft fleshy plants and plant parts such as squash, cabbage, beans
and potatoes also may be rotted by certain species of Pythium.
Economic importance of Phytophthora spp.
Phytophthora species cause a wide variety of diseases on
tremendous number of host including both herbaceous and woody
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plants. Most species cause rots, damping off of seedling and rots of
lower stems, tubers and corms. Others cause blight of the foliage,
young twigs and fruits.
Some plant disease caused by Pythium and Phytophthora species.
Diseases caused by Pythium spp.
Diseases
caused
by
Phytophthora sp.
Disease
with Caysal fungus
Disease with host
host
Caysal
fungus
Damping off of P. debaryanum
Late
tobacco,
potato and tomato
Chilli
Rhizome rot of P. myriotylum
ginger
blight
Leaf
blight
of P. infestans
of P.
Colocasia
Damping off of P.
Damping off, root P. cactorum
aphanidermatum rot
potato
colocasiae
fruit
rot
of
many plants
Soft
rot
of P.
aphanidermatum tobacco
papaya
Rhizome
root
Black
rot
and P. graminium
of
shank
of P.
nicotianae
Bud rot of coconut P. palmivora
turmeric
Damping off of P. ultimum
Leaf
vegetables
sesame
seedlings
blight
of P. parasitca
var.sesami
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Family: Peronosporaceae: This is the highly specialized family in
Peromosporales. All species are in nature biotrophic parasites of higher
plants, causing diseases known collectively as downy mildews. The
important diseases caused by Peronosporaceae are:
Dowry mildew Plasmopora viticola
Dowry
of grape vine
mildew
Peronospora
of destructor
onion
Dowry mildew Bremia lactucae
Dowry
of lettuce
mildew
Pseudoprenospora
of cubensis
cucurbits
Family Albuginace:
Obligate parasites of higher plants, the diseases caused by the fungus
known as white blister or white rast. There are about 30 species under
the genus. Only one genus Albugo. Some examples of some plant
pathogenic species are shown below:
Albugo candida attacks crucifers
A. occidentalis on spinach
A. bliti on various members of Amaranthaceae
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