Summary[ edit ] White rust plant diseases caused by Albugo fungal-like pathogens should not be confused with White Pine Blister Rust , Chrysanthemum white rust or any fungal rusts , all of which are also plant diseases but have completely different symptoms and causal pathogens. Symptoms of white rust caused by Albugo typically include yellow lesions on the upper leaf surface and white pustules on the underside of the leaf. The pathogen is spread by wind, water, and insects. Management includes use of resistant cultivars , proper irrigation practices, crop rotation , sanitation, and chemical control. White rust is an important economic disease, causing severe crop losses if not controlled. Hosts and symptoms[ edit ] White rust pathogens create chlorotic yellowed lesions and sometimes galls on the upper leaf surface and there are corresponding white blister-like dispersal pustules of sporangia on the underside of the leaf.
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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. However, it has also been reported on some members of family Asteraceae Composite, Convolvulaceae and Chenopodiaceae. 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. 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. Severe infection causes proliferation of the lateral buds, discoloration of flowers, malformation of floral parts and sterile gynoecium.
Vegetative Structure of Albugo: Thallus is eucarpic and mycelial. Hyphae are intercellular, coenocytic, aseptate and profusely branched Fig. Cell wall is composed of fungal cellulose. The protoplasm contains a large number of nuclei distributed in the cytoplasm.
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 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. Reproduction in Albugo: The fungus reproduces both by asexual and sexual methods Fig.
Asexual Reproduction: The asexual reproduction takes place by conidia, condiosporangia or zoosporangia. They are produced on the sporangiophores. Under suitable conditions the mycelium grows and branches rapidly. These hyphae produce, at right angles to the epidermis are short, thick walled, un-branched and club shaped. These are the sporangiophores or conidiophores.
They form a solid, palisade like layer beneath the epidermis Fig. They are thick walled on lateral sides and thin walled at tip. The sporangiophores contain dense cytoplasm and about a dozen nuclei. After reaching a certain stage of maturity, the apical portion of sporangiophore gets swollen and is ready is cut off a sporangium or conidium Fig. The sporangia are produced at the tip by abstraction method. A Deeping constriction appears below the swollen fig 2.
F end and results in the formation of first sporangium. A second sporangium is similarly formed from the tip just beneath the previous one Fig. This process is repeated several times. The new nuclei migrates from mycelium to cytoplasm and are used in the formation of another sporangium or conidium.
Thus along chain of sporangia or conidia is formed above each sporangiophore in basipetal succession. The walls between them fuse to form a gelatinous disc-like structure called disjunctor or separation disc or intercalary disc. It tends to hold the sporangia together.
The continued growth and production of sporangia exerts a pressure upon the enveloping epidermis. Which is firstly raised up but finally ruptured exposing the underlying sours containing white powdery dust of multinucleate sporangia or conidia Fig. The separation discs are dissolved by water, and the sporangia are set free. They are blown away in the air by wind or washed away by rain water under suitable environmental conditions and falling on a suitable host, sporangia germinates with in 2 or 3 hours.
The sporangia germinate directly or indirectly depending on temperature conditions. It gives rise to a germ tube which in-fact the host tissue through stoma or through an injury in the epidermis Fig.
It absorbs water, swells up, and its contents divide by cleaving into polyhedral parts Fig. Each part later on rounds up and metamorphoses into zoospore Fig. A papilla is developed on one side which later burst and liberates the zoospores. The flagella are attached laterally near the vacuole. Of the two flagella one is of whiplash type and the other tinsel type Fig.
After swimming for some time in water, they settle down on the host. They retract their flagella, secrete a wall and undergo a period of encystment Fig. On germination, they put out a short germ tube which enters the host through stomata Fig. Sexual Reproduction: It takes place when the growing season comes to an end.
The mycelium penetrates into the deeper tissues of the host. The sexual reproduction is highly oogamous type. The antheridium and oogonium develops deeper in the host tissue in close association within the intercellular spaces. Its formation is externally indicated by hypertrophy. The antheridium and oogonium are formed near each other on hyphal branches. They are terminal in position, however, intercalary oogonia also occur, though rarely. Antheridium: It is elongated and club shaped structure.
It is multinucleate nuclei but only one nucleus remains functional at the time of fertilization in C. However, in C. It is paragynous i. It is separated by a cross wall from the rest of the male hyphae. Oogonium: It is spherical and multinucleate containing as many as 65 to nuclei. All nuclei are evenly distributed throughout the cytoplasm Fig. As the oogonium reaches towards the maturity the contents of the oogonium get organised into an outer peripheral region of periplasm and the inner dense central region of ooplasm or oosphere or the egg Fig.
The ooplasm and periplasm are separated by a plasma membrane. The nuclei in the oogonium divides mitotically. The first mitotic division takes place before the organization of the periplasm and oosplasm Fig. After the organization, all the nuclei of the ooplasm, except one, migrate to the periplasm forming a ring and undergo second mitotic division. They divide in such a manner that one pole of each spindle is in ooplasm and the other in the periplasm Fig.
At the end of the division one daughter nucleus of each spindle goes to the oosplasm and other in periplasm Fig. However, at the time of maturity, all nuclei disintegrate, except single functional nucleus Fig. It is represented by C. Group II: The number of functional eggs in ooplasm is many.
It has been observed that in C. The rest of the nuclei disintegrate before fertilization. Fertilization: Before fertilization a deeply staining mass of cytoplasm, Fig.
This is called coenocentrum. It persists only up to the time of fertilization. The functional female nucleus attracted towards it and becomes attached to a point near it. The oogonium develops a papilla like out growth at the point of contact with the antheridium. This is called as receptive papilla Fig. Soon it disappears, and the antheridium develops a fertilization tube.
It penetrates through receptive papilla, oogonial wall and periplasm and finally reaches upto the ooplasm Fig. It carries a single male nucleus. Its tip ruptures to discharge the male nucleus near the female nucleus. Ultimately the male nucleus fuses with the female nucleus karyogamy. Oospore: The oospore alongwith the fusion nucleus is called oospore Fig.
So, the oospore is uninucleate. The same number of functional male nuclei are discharged by the fertilization tube. Both male and female nuclei fuse, and the oospore produced in these species in multinucleate. Such oospore is called a compound oospore.
The oospore on maturity secretes a two to three layered wall Fig. The outer layer is thick, warty or tuberculated and represents the exospore. The inner layer is thin and culled the endospore. Germination of oospore: With the secretion of the wall, the zygotic nucleus divides repeatedly to form about 32 nuclei.
Albugo: Habitat, Symptoms and Reproduction | Mastigomycotina
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