.png)
SYSTEMATIC POSITION
Division: | Eumycota |
Sub division: | Mastigomycotina |
Class: | Oomycetes |
Order: | Peronosporales |
Family: | Albuginaceae |
Genus: | Albugo |
OCCURRENCE
The genus Albugo includes about 30 species distributed worldwide.
Albugo are usually obligate endoparasites and infect the members of the families Convolvulaceae, Compositae, Amaranthaceae, Cruciferae, etc.,
Albugo commonly found on a wide range of crucifers and causes a disease called “white rust” or “blister rust”.
The fungus infects mainly the aerial parts of the plant, such as petioles, leaves, stems, flowers and fruits.
The infected part becomes thick and malformed and the growth of the entire plant is stunted.
VEGETATIVE STRUCTURE
Thallus is well branched, aseptate and coenocytic (Fig.1).
Cell wall is composed of chitin and the cytoplasm is granular and contains many nuclei.
Reserve food material is present in the form oil globules and glycogen bodies.
This parasitic fungus grows in intercellular spaces of the host tissue (Fig 2).
Some mycelium modifies like haustoria (small knob like) for the absorption of food material from the host tissue (Fig 2).
Ø The haustorium is distinguished into a haustorial head and a slender narrow stalk (Fig 3). The head portion contains dense cytoplasm with large number of mitochondria, ribosomes, endoplasmic reticulum but nuclei are generally absent.
Ø Within the plasma membrane of the haustorium, tubules develop called lomasomes and it is concerned with cell wall synthesis.
Ø At the base of the haustorium there is a collar like sheath which is an extension of the host cell wall. The haustorium is separated from the host cell by an encapsulation made up of extra haustorial matrix (Fig. 3).
REPRODUCTION
The fungus reproduces both by asexual and sexual methods.
Sexual reproduction
Ø Sexual reproduction is oogamous and in A. candida sex organs are generally formed towards the end of the growing season of the host.
Ø Reproductive organs are endogenous. The antheridium and oogonium develops deeper in the host tissue in close association within the intercellular spaces (Fig 5).
Ø The fertilization tube bursts near the coenocentrum, releasing the male nucleus which fuses with the egg and this result in the formation of diploid zygote. The coenocentrum disappears after fertilization (Fig 5E).
Ø The fertilized zygote secretes a thick wall around itself and is now known as oospore (Fig 5F). The wall of the oospore is differentiated into outer layer thick exine, the inner thin intine (Fig 5G-H).
Ø The diploid nucleus of the oospore divides first by meiosis, followed by several mitotic divisions resulting in the formation of about 32 haploid nuclei.
Ø The oospore with 32 nuclei under goes resting period and during this period the host tissue first shows hypertrophy and then decays by releasing oospore.
Ø The oospore germinates on arrival of favourable condition. The nuclei inside oospore divides mitotically and later the protoplast of the oospore is cleaved into many uninucleate segments (Fig 5I-J).
Ø Each segment finally metamorphoses into a uninucleate, reniform and biflagellate zoospore.
Ø Now the exine of the oospore bursts by absorbing water and the intine is protruded as a thin spherical vesicle. The zoospores are then transferred into the vesicle and later released (Fig 5K).
Ø The zoospore germinates on the suitable host by producing a germ tube (Fig 5L-M). The germ tube enters through the stoma and extends into the intercellular spaces of the host and develops into a fresh mycelium.
LIFE CYCLE
The life cycle is diplontic type according to modern mycologists and according to old mycologists the life cycle of Albugo is haplontic type.