The genus Albugo (Albus – white) is represented by 25 species distributed in most parts of the world.
All are parasitic on land plants.
They are obligate parasites occurring as endoparasites in higher plants.
They usually infect the members of families Cruciferae, Convolvulaceae, Compositae, Amaranthacae.
It causes a disease called ‘white rust’ or blister rust.
Vegetative Structure
The mycelium is well developed and consists of branched, aseptate, coenocytic hyphae.
The hyphae live and ramify in the intercellular spaces of the host tissue.
The hyphal wall contains cellulose and not chitin.
The hyphal protoplasm is granular and vauolate and contains numerous nuclei.
Reserve food is in form of oil globules and glycogen.
The cytoplasmic membrane forms tubules called lomasomes.
The intercellular hyphae produce intracellular haustoria in the mesophyll cells of the host.
The haustorium is seen as a small spherical structure consisting of two parts:
The haustorial stalk or neck
The terminal haustorial head or body
Reproduction
Reproduction takes place by asexual and sexual methods.
Asexual Reproduction
Takes place by multinucleate structures known as Conidia.
When mycelium reaches a certain stage of maturity it produces pads of hyphae just below the epidermis.
The tips of hyphae constituting the mat grow vertically into short, upright, thick-walled, unbranched club shaped structures.
These are the sproangiophores (= conidiophores).
They are arranged in a closely packed palisade like layer forming a sorus between the epidermis and the mesophyll of the host leaf.
The lower portion of sporangiophore is narrow, thick-walled with a undulating surface.
The upper portion is broader, thin-walled with a smoother surface.
The protoplast of the sporangiophore contains about a dozen nuclei.
Each sporangiophore cuts off sporangia at its tips one below the other in a long chain.
The sporangia develops in basipetal manner i.e., the youngest near the sporangiophore and the oldest at the top of the chain.
The sporangia are small, hyaline, spherical or lemon-shaped.
In between the successive sporangia, there are gelantinous disc like structures called disjunctors.
When sporangia are mature they become detached and free
The pressure from below of the upwardly growing sporangiophores and the sporangia causes the host epidermis to bulge .
And finally the epidermis burst over the sporangial sorus and exposes the white shining pustules consisting of masses of sporangia.
The pustules look like white blisters.
The exposed sporangia are white.
As sporangia mature the disjunctors dry, shrink and finally disintegrate in air.
The sporangia in the chain thus separate.
They are blown away by wind or washed away by rain water.
Germination of sporangia
Landing on the suitable host the sporangia begin to germinate
Germination takes places in two ways depending on temperature conditions
Indirect germination
In presence of moisture and low temperature, the sporangium functions as a zoosporangium.
It absorbs water and swells.
Multinucleate protoplast undergoes division and forms 5-8 uninucleate daughter protoplasts.
Meanwhile an obtuse papilla forms on one side of the sporangium.
Each daughter protoplast converts into a zoospore.
Each zoospore has two flagella, one short and one long.
The former is of tinsel type and the latter whiplash.
As zoospores are formed, the papilla swells and opens
The zoospores emerges out through the papilla.
The released zoospores swim in water for a while.
Finally it settles down on the host, retract flagella and round off.
Each secretes a wall around it.
The encysted zoospore them germinates.
It puts out a germ tube which gains entrance into the host through a stroma.
Once within the host tissue the germ tube grows and forms the mycelium.
Direct Germination
At high temperature and dry conditions the sporangium behaves like a conidium.
It germinates directly to form a germ tube.
The germ tube penetrates the host through stroma, or through an injury in the epidermis.
Within host it develops into a mycelium.
Sexual Reproduction
It it oogamous.
The male sex organ is called an antheridium and the female oogonium.
They are developed near each other in the intercellular spaces of the host tissues.
They develop towards the end of the growing season.
The sex organs arise on separate hyphae called the male and female hyphae.
Oogonium
It arises as a globular enlargement of the tip of the female hypha.
The swelling is multinucleate.
A cross wall appears below the enlargement.
It separates the terminal oogonium from the rest of the female hypha.
The nuclei divide mitotically and increase in number.
Meanwhile the oogonium is differentiated into two distinct regions.
The central, rounded, multinucleate with few cell organelles is called ooplasm.
The ooplasm is surrounded by more vacuolated peripheral cytoplasm called the periplasm.
The ooplasm at maturity has a single centrally located nucleus.
The uninucleate ooplasm functions as the female gamete or oosphere.
Antheridium
The antheridium is developed at the end of a male hyphae lying close to the oogonium.
It is an elongated club-shaped cell.
It is multinucleate.
The end of the male hypha enlarges into club-shaped swelling.
It is cut off by a cross wall from the rest of the male hypha.
This terminal club shaped cell is called an antheridium.
It contains several nuclei, but only one is functional.
The antheridium comes in direct contact with the oogonium at the side.
Fertilisation
At the point of contact of antheridium with the oogonium, the walls become very thin.
A slender tubular outgrowth from the antheridium.
It is the fertilization tube.
The fertilization tube passes through the thin spot in the oogonial wall and enter the multinucleate periplasm.
The tube finally reaches the ooplasm and ruptures and introduce a single male nucleus which fuse with the female nucleus.
Oospore.
After fertilization a oospore is formed.
It develops a thick wall around it.
Oospore is the only diploid structure in the life cycle of Albugo.
It undergoes zygotic meiosis and forms nearly 32 nuclei.
After this it enters the resting stage and tides over the period of unfavourable conditions.
Germination of Oospore.
On the onset of favourable conditions, the oospore germinates.
The diploid nucleus undergoes repeated divisions to form man nuclei.
A small amount of cytoplasm gathers around each daughter nucleus.
Each of these metamorphoses into a biflagellate zoospore.
The oospore hem germinates to release the zoospores.
The released zoospores swim in water for a while.
Finally it settles down on the host, retract flagella and round off.
Each secretes a wall around it.
The encysted zoospore them germinates.
It puts out a germ tube which gains entrance into the host through a stroma.
Once within the host tissue the germ tube grows and forms the mycelium
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