Reproduction in Sclerospora (With Diagram)

In this article we will discuss about the reproduction in sclerospora. This will also help you to draw the structure and diagram of reproduction in sclerospora.

Asexual Reproduction in Sclerospora:

Asexual reproduction occurs by means of sporangia produced on sporangiophores. These are generally formed on leaves when temperature and relative humidity are favourable. Prior to the formation of sporangiophores the hyphae accumulate between the cell of mesophyll of the infected leaves.

When the sporangiophores are to be formed, tufts of vertical hyphae reach the air space beneath the stomata. The development of sporangiophores is associated with accumulation of nutrition by the fungus drawn from the host. Consequently the chloroplasts of the host cells are wholly or partially destroyed giving the pale-yellow colour to the leaves.

The sporangiophores emerge out of the stomata of the infected leaves. Each sporangiophore is a stout broad hypha unbranched in the lower part but giving out a few (2-6) thick short branches, di—or trichotomously, at the upper part. It is 100 µ length and 10-15 µ in width.

The sporangiophores are swollen to about 30 µ at the tip where branching occurs branches or stengmata bear sporangia which are formed due to the swelling of the tip of sterigmata and the cut of by aseptum after a few nuclei and cytoplasm are transferred to the swelling.

When mature, sporangia get detached from the sporangiophores and accumulate on the leaf surface as Downy mildew. Each sporangium is hyaline, round or elliptical slightly papillate at the apex and measure 13-34 x 12-23 µ in size. The sporangium has a thin smooth wall.

Mature sporangia fall off and germinate in. surrounding dew drops by cleavage of their contents into 4-8-13 biflagellate bean shaped zoospores. These swim for some time, come to rest and germinate by germ tubes to form new mycelium. The germ tube shows chemotactic response to roots of Bajra.

Asexual reproduction i.e. sporangial formation and its subsequent behaviour in Sclerospora graminicola has been studied by a large number of plant pathelogists. According to Safeeulla and Thirumalachar (1956), Sporangial formation is favoured by temperatures between 15-25°C, Suryanarayana (1956) concluded that sporangia do not develop at all if the temperature is more than 28°C.

Generally optimum conditions for sporangial development in North India occur during August, September and during this period the disease is more common.

Although Weston (1924) claimed that sporulation in S. graminicola is nocturnal, Safeeulla and Thirumalachar (1956) suggested that if proper and suitable conditions of temperature and relative humidity exist during the day, sporulation can take place. Generally the germination of sporangia occurs within 30 minutes at 20-25°C, these remain viable for six hours at 20-23°C.

Sexual Reproduction in Sclerospora:

Sexual reproduction occurs during unfavourable conditions and is typically oogamous. The sex organs develop within the host tissues in the intercellular spaces, mostly in leaves and malformed spikelets. The oogonia are produced in large numbers in leaf tissues and are usually terminal rarely intercalary in position. Each oogonium is a globose structure containing about 50 nuclei.

Soon the oogonium is cut off by a septum and the nuclei undergo meiosis. All but one nuclei degenerate. The contents of the oogonium become differentiated into a central dense ‘part, the ooplasm and the peripheral periplasm.

The ooplasm containing a single haploid nucleus serves as the egg. The Antheridia are usually clavate structures attached laterally to oogonia and contain 4-6 nuclei. Nuclear division also takes place but only one nucleus remains functional at maturity.

Fertilization takes place when fertilization tube penetrates the oogonial wall and passes rapidly to the egg where it discharges the male nucleus and disintegrates. Plasmogamy occurs, but karyogamy occurs only at maturation of oospore. Fertilized egg secretes a three layered wall and the structure is known as oospore. The mature oospore is globose, dark brown in colour and 34- 52 p in diameter.

The oospores do not germinate readily and usually undergo a prolonged rest period. During harvesting of the crop, the oospores fall on the ground and remain buried in the soil of the field. At the onset of favourable conditions oospores germinate (by 1-4 germtube) and infect the roots of young seedlings of the host from where the mycelium spreads systemically in the entire plant.

Oospores constitute the main source of infection as the disease is soil borne. Suryanarayana (1956, 1962) who studied the germination of oospores and their role in infection of young seedlings, concluded that the oospores generally germinate after a rest period of a year between 20-25°C temperature.

Dry soils are more favourable for infection of the hosts. Infected seeds containing oospores and/or the mycelium also serve as primary source of inoculum.