oomycota - narajole raj college

OOMYCOTA

BYBANGAMOTI HANSDA

ASSISTANT PROFESSORDEPARTMENT OF BOTANY NARAJOLE RAJ COLLEGE

COMPILED AND CIRCULATED BY BANGAMOTI HANSDA, ASSISTANT PROFESSOR,

DEPARTMENT OF BOTANY, NARAJOLE RAJ COLLEGE

BOATNY: SEM-II, PAPER-C 3T: MYCOLOGY AND PHYTOPATHOLOGY, UNIT 6: OOMYCOTA

General characteristics

Oomycota or oomycetes form a distinct phylogenetic lineage of fungus like eukaryotic microorganisms.

The cell wall is composed mainly of cellulose-glucan and devoid of chitin.

The plant body ranges from unicellular, holocarpic, endobiotic parasites to well developed coenocytic and branched mycelium.

Some important parasitic members of this group are Pythium, AlbugoPhytophthora, Saprolegnia, Plasmophora etc.

The vegetative thallus is diploid and meiosis takes place in gametangiumrather than in zygote.

They can reproduce both sexually and asexually.




Sexual reproduction is of oogamous type. The non-motile gametes are

produced in male (antheridium) and female (oogonium) gametangium.

The male gametes are transferred to the egg through fertilisation tube,

results in the formation of oospore.

Asexual reproduction takes place by biflagellate zoospore without cell

wall. The zoospores may be either pear-shaped with anteriorly placed

flagella or reniform with laterally placed flagella. The flagella are

unequal in length, the shorter one tinsel type and longer one whiplash

type. Some members produce conidia as asexual spore.




Ecology

Members of this class are generally aquatic but some are terrestrial,

grows saprophytically on soil or parasitically on shoots of higher

plants.

Oomycota is responsible for the decomposition and recycling of waste

in their ecosystem, which tends to be moist, cool areas such as damp

soil and freshwater habitats.

Parasitic species cause the destruction of crops in many parts of the

world, and have wreaked havoc on agriculture throughout history.

The most famous example of this destruction is Ireland's Great Potato

Famine in 1845-48, when the oomycete Phytophthora infestans,

accompanied its vegetable host to the British Isles.




Phytophthora

Phytophthora is a genus of plant-damaging oomycetes (water molds),

whose member species are capable of causing enormous economic losses

on crops worldwide, as well as environmental damage in

natural ecosystems. The genus was first described by Heinrich Anton de

Bary in 1875. Approximately 170 species have been described, although

100–500 undiscovered Phytophthora species are estimated to exist.

Systematic Position: Ainsworth (1973)

Kingdom- Mycota

Division- Eumycota

Sub-division- Mastigomycotina

Class- Oomycetes

Order- Saprolegniales

Family- Pythiaceae

Genus- Phytophthora




P. infestance visualised under

microscope

Life cycle of Phytophthora

In the dormant phase, the fungus could penetrate in the tuber as mycelium andin plant debris as oospore. During favourable condition, the perennatingmycelium becomes active and develops active mycelium.

The active mycelium develops branched sporangiophore bearing sporangia onit. On the other hand, the oospore germinates by producing germ tube bearingsporangia at their tip.

In both the cases, the developed sporangia disperse by wind or rain, germinateon host surface by producing zoospores and cause infection to the host tissue.

The infected host again develops sporangia on zoosporangiophore. Thesporangia develop zoospore which cause further infection. This processrepeats several times in the growing season.

During favourable condition, the tuber infection may also takes place oncontact with infected foliage during harvest.




The perennating mycelium inside the tuber remains active, if the infected

tubers are kept in the storage for seed potato. In the next season, after showing

the seeds, the perennating mycelium becomes active and causes further

infection.

Towards the end of the season, inside the aerial parts, both the sex organs –

antheridium (male) and oogonium (female) – develop and undergo sexual

reproduction of amphigynous type.

The product of sexual reproduction is oospore remains dormant inside the host

tissue during unfavourable season.

During favourable condition in the next season they germinate and produce

next crop of zoospore.




Life cycle




AlbugoAlbugo is a genus of plant-parasitic oomycetes. Those are not true fungi

(Eumycota), although many discussions of this organism still treat it as a

fungus. The taxonomy of this genus is incomplete, but several

specie are plant pathogens. Albugo is one of three genera currently described

in the family Albuginaceae, the taxonomy of many species is still in flux.

This organism causes white rust or white blister diseases to a few agricultural

crops.

Systematic Position: Ainsworth (1971)

Kingdom- Fungi

Division- Eumycota

Sub-division- Mastigomycotina

Class- Oomycetes

Order- Perenosporales

Family- Albuginaceae

Genus- Albugo




A. candida visualised under microscope

Life cycle of Albugo




The mycelium grows intercellularly and forms knob-like haustoria in hostcells to draw nutrition. From this underlying hyphae numerous shortsporangiophores are developed under the epidermis.

With further growth blister-like areas developed on the epidermis are calledsori and sporangia are developed on sporangiophore.

With further growth pressure exerted inside the epidermis, caused theepidermis to rupture and sporangia are exposed. The sporangia usuallygerminate by zoospores.

After a period of swimming, they are encysted and then germinate to formgerm tube, on contact with stomata, penetrates the host tissue cause infection.

At the end of growing season, oogonia and antheridia are formed from theinternal mycelium in the intercellular space and by union they developoospore. After a long period of rest, they germinate by the formation ofzoospores

Life cycle




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oomycota - narajole raj college

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