Classification
Kingdom- Straminopila
Phylum- Oomycota
Class- Oomycetes
Order- Peronosporales
Family- Albuginaceae
Genus- Albugo
Occurrence: Albugo lives as an obligate parasite causes disease known as white rusts.
Vegetative structure: Albugo mycelium is diploid, aseptate, coenocytic hyphae. The hyphae bear small spherical haustoria, that penetrate the host cell and absorb food material from host.
Reproduction
Albugo reproduces by both sexual and asexual methods.
Asexual reproduction
After maturity dense mat of hyphae is formed in the mesophyll cells of the host. Many of these hyphae produce clusters of short, club shaped and unbranched structures called, sporangiophores. Some protoplasm along with nuclei from the mycelium migrate into these sporangiophores. The tip of the sporangiophore contains 5-11 nuclei together with some cytoplasm, enlarges and is then cut off from the rest of the sporangiophore by a transverse septum resulting in the abstriction of a multinucleate sporangium. The portion of the sporangiophore, just below the sporangium, elongates and enlarges due to the accumulation of more nuclei and cytoplasm and becomes pinched off as a second sporangium. This process is repeated a number of times. In this way, from the tip of each sporangiophore, a chain of sporangia is cut off in basipetal succession. These sporangia is separated from each other by gelatinous discs.
Large number of sporangia that arises in chain push up the overlying host epidermis which firstly bulges out and then bursts, exposing a mass of white sporangia on the surface of the host at this stage the fungus is called white rust. After maturation sporangia separate from each other by gelatinisation of the intercalary discs and disseminated in wind or water.
When sporangia got a suitable host, germinate within few hours to form biflagellate zoospores. Zoospores encysts and from germ tube which penetrate the host epidermis which forms new infection.
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| Asexual reproduction in Albugo |
Sexual reproduction
The sexual reproduction is oogamous type and takes by producing antheridia and oogonia.
The oogonia arises as a globose terminal or intercalary swelling on the hypha deep within the host tissue and protoplasm flows into the developing oogonium. A wall separate it from the hyphae.
The antheridium arises a short lateral, club shaped structure at the end of a hypha lying near the oogonium. It cut off by cross wall from the hypha.
When antheridia come in contact with oogonium, the cytoplasm within the oogonium become differentiated into central dense portion, the ooplasm and a peripheral more vacuolated portion, the periplasm. All nuclei, except the one of the ooplasm, migrate to the periphery and are included in the periplasm. The remainder nucleus functioning as uninucleate egg. A dark granular body called the coenocentrum, appears in the centre of the ooplasm.
During fertilization, at the point of contact of the both gamete, the oogonium develops a papilla like growth which is called the receptive papilla. The receptive papilla is functionless and soon disappears. At that time the antheridium develops a fertilization tube, a single male nucleus passes through it together with some cytoplasm into the oogonium and fuses with the egg nucleus resulting in the formation of oospore. The oospore are liberated by decay and the disintegration of the host tissue at this stage, the oospore undergo a period of rest. On favourable conditions, the nuclear division takes place to form a large number of biflagellate zoospores. The outer thick wall of oospore opens and the endospore comes out to form a vesicle into which the zoospores move out. The zoospores are liberated by the rupturing of the vesicle.
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| Sexual reproduction in Albugo |
