Albugo

CLASSIFICATION

KINGDOM         : MYCOTA

DIVISION          : EUMYCOTA

SUBDIVISION  : MASTIGOMYCOTINA

CLASS              : OOMYCETES

ORDER             : PERENOSPORALES

FAMILY            : ALBUGINACEAE

GENUS            : ALBUGO

DIAGNOSTIC CHARACTERS OF ALBUGO

• The genus Albugo or Cystopus is an obligate parasite on a number of flowering plant. It is represented by 25 species parasitic on plants. Albugo candida a common species responsible for the disease “white rust” or “white blisters” on the members of family Cruciferae.
• Every part of the host plant is infected except the roots. White or cream coloured pustules develop on the surface of leaf.
• The stem become hypertrophied and the floral part become withered.
• Mycelium of Albugo is well developed, endophytic, branched, aseptate, coenocytic, hyaline hyphae, intercellular and with knob like haustoria for the absorption of food material from the host cell. 
• The hyphal wall contains cellulose and not chitin. 
• Reproduction is both Asexual and Sexual
• This organism causes white rust or white blister diseases in above-ground plant tissues. While these organisms affect many types of plants, the destructive aspect of infection is limited to a few agricultural crops, including: beets (garden and sugar), Brussels sprouts, cabbages, Chinese cabbage, cauliflower, collards, garden cress, kale, lettuce, mustards, parsnip, radish, horseradish, rapeseed, salsify (black or white), spinach, sweet potatoes, turnips, watercress, and perhaps water-spinach.
• Albugo is a biotrophic or obligate parasite fungus.
• Mycelium is branched, non-septate and intracellular.
• Buton shaped haustoria are present.
• Sporangiophores are form on lower side of epidermis.
• Sporangiophores bear the sporangia which are in basipetal chain.
• Upper most sporangium is thick walled and not germinate. It put the pressure on epidermis.
• Sporangia are unicellular, multinucelus with smooth and thick walled.
• Size of sporangium is 14× 16 to 16-20 µm and diameter is 15-21µm.
• Sporangia are attached in chain by gelatinous disc like tissue pad which dissolve in
• presence of moisture and sporangia become free from chain.
• When these sporangia germinate directly by forming germ tube than called conidia. But
• it is rare.
• Sporangia detached from mycelium and spread by air or water from one place to other
• place.
• At the time of germination papilla formation take place which later form zoospores.
• Zoospores are unicellular, biflagellate and kidney shaped.
• After release zoospores rest and loose their flagella and encyst.
• Encysted zoospores later germinate and infect to host.
• For formation and germination of sporangia optimum tem is 10ͦ ͦC.
• Sexual reproduction by Oogamy.
• Oogonium (female) is spherical, terminal or intercalary and have about 200 nucleus. 
• Its protoplasm is divided in Periplasm and Ooplasm.
• Antheridium (male) is club shaped and have 6-12 nucleus
• Oogonium and Antheridium come in the contact by a papilla which disappear shortly. At the point of contact of Oogonium cell wall become thin and form papilla.
• Through the contact of both sex organs Antheridium nucleus
• moves towards Oogonium and later Zygote form which is called Oospore.
• Oospores are spherical, yellow to dark brown in colour and diameter is 40-55 µm which cell wall is thick and tuberculate .
• Before germination of these spores need a resting period so called “Resting Spores”.
• After resting period these oospores germinate by forming sessile vesicle which produce zoospores.

Habit and Habitat of Albugo:

Albugo (derived from a Latin word means white), the only genus of family Albuginaceae is represented by more than 25 species. It is an obligate parasite distributed all over the world.

In India about 18 species of Albugo have been reported which attacks mostly crucifers like turnip, mustard, radish, cabbage, cauliflower etc. However, it has also been reported on some members of family Asteraceae (Composite, Convolvulaceae and Chenopodiaceae).

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Symptoms of Albugo:

The disease caused by Albugo is commonly known as white rust because it appears in the form of shiny, white, smooth irregular patches (pustules) or blisters on the leaves, stems and other aerial parts of the plant. The pustules are initially formed on the lower surface of the leaf but in several cases they may be present on both the surfaces (Fig. 1 A).

With this several other effects are also produced. Increase in the size of the cells (hypertrophy) and organs takes place. It results in the formation of large galls on the various parts of the host (Fig. 1 B-D). Severe infection causes proliferation of the lateral buds, discoloration of flowers, malformation of floral parts and sterile gynoecium.

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Vegetative Structure of Albugo:

Thallus is eucarpic and mycelial. Hyphae are intercellular, coenocytic, aseptate and profusely branched(Fig. 2 B). Cell wall is composed of fungal cellulose. The protoplasm contains a large number of nuclei distributed in the cytoplasm.

Reserve food material is in the form of oil drops and glycogen bodies. Some mycelium is intracellular in the form of knob-like haustoria for the absorption of food material from the host cells. The ultrastructure of haustoria is studied by Berlin and Bowen (1964).

It can be differentiated into two parts:

(a) Haustorial head, and

(b) Narrow stalk.

The cytoplasm of the head of haustorium is densely packed with mitochondria, ribosomes, endoplasmic reticulum and lipid inclusions but nuclei are absent.

The base of the haustorium is surrounded by a collar like oeath which is an extension of the host cell wall. Between the haustorium and the host plasma membrane is an encapsulation. Within the plasma membrane of the haustorium lomasomes are more numerous than in the intercellular hyphae.

Asexual Reproduction of Albugo:

1. It takes place by conidia or conidiosporangia. They develop on conidiophore or conidiosporan- giophore (Fig. 71).

2. Mycelium below the epidermis gives off many erect, short, unbranched, club-shaped hyphae called conidiophores.

3. Conidiophores lie parallel to one another and perpendicular to the surface of the host, and form a palisade-like layer.

4. On the thin-walled apical end of the thick-walled conidiophore are present four to six or more spherical, smooth and hyaline bodies called conidia.

5. Conidia are arranged in basipetal succession on the conidiophore, i.e., youngest at the base and oldest at the top (Fig. 72).

6. In between two conidia is present a pad or disc of gelatinous material called mucilaginous disc or disjunctor.

7. Each conidium and conidiophore is a multinucleate structure.

8. In the later stages, the epidermis of the host gets ruptured and conidia disseminate. They germinate either directly by forming a germ tube or form biflagellate zoospores.

Sexual Reproduction of Albugo:

1. Sexual reproduction is oogamous and the two sex organs are antheridium and oogonium.

2. Sex organs generally develop in stem.

3. Two sex organs develop near each other but on different male and female hyphae.

4. Antheridium develops in close contact with the oogonium at the side.

5. Oogonium is globular and multinucleate, and contains a large amount of food material. It bears a septum at the base.

6. Mature oogonium (Fig. 73) in C. candidus remains divided into central uninucleate dense ooplasm and peripheral, multinucleate periplasm.

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7. Antheridium is elongated, club-shaped and multinucleate structure having a septum at its base.

8. The wall between antheridium and oogonium dissolves at the place of their contact, and a tube is formed by antheridium. This is called fertilization tube.

9. Fertilization tube penetrates into periplasm and ooplasm, and through this tube the male nucleus comes in contact with the female nucleus to form the diploid oospore.

10. Oospore (Fig. 74) is a globular body and remains surrounded by outer thick and sometimes spiny exosporium, and inner thin and smooth endosporium. Sometimes, a third middle layer is also present.

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11. Diploid oospore divides first meiotically and then ordinarily into many biflagellate, reniform and haploid zoospores or zoomeiospores.

12. Zoospores, on withdrawing their flagella, germinate and form new mycelial hyphae on the host.