Systematic position
Kingdom-Straminopila
Phylum-Oomycota
Class-Oomycetes
Order-Pythiales
Family-Pythiaceae
Genus-Phytophthora
Vegetative structure
The vegetative body of the Phytophthora is the mycelium which has irregularly branched, non-septate, coenocytic hyphae.
However, the septa is formed in the older parts of the mycelium and at a time of the formation of the reproductive structures. The mycelium develops intercellularly forming short simple or branched outgrowths, the haustoria, by means of which it absorbs the nourishment from the host cells.
Reproduction
There are two types of reproduction occur in Phytophthora
1) Asexual reproduction
It takes place by means of sporangia which are borne on special reproductive hyphae called sporangiophores.
Some of the hyphae from the internal mycelium of the fungus, emerge out singly or in groups of 1-5 either through the stomata or, sometimes, through the epidermal cells of the leaves of the host plant. These aerial hyphae which are differentiated from vegetative hyphae are called sporangiophores. They may be branched or unbranched sympodially.
Each sporangiophore bears an ovoid or lemon shaped sporangium at its tip. After the formation of a sporangium, the branch continues its growth with the result that the first formed sporangium, becomes lateral and new sporangium is developed at the tip. A succession of sporangia may be produced sympodially at the tips of branches by the continued growth of the sporangiophores.
The sporangia are deciduous. They get detached from the sporangiophores by the rain splashes or air currents and are carried through air current to the leaves and stem of other potato plants.
Germination of Sporangium
Sporangia of Phytophthora are greatly affected by temperature and relative humidity. If the relative humidity falls much below 100 percent, the sporangia die in a few hours. The germination of the sporangium is largely affected by temperature. When the temperature is high, the sporangia germinate directly by putting forth germ tubes, and if the temperature is low, the sporangia germinate indirectly by producing the zoospores.
When the sporangium falls in moist substratum and the temperature is low, it behaves as zoosporangium. The protoplasmic contents of the sporangium undergo cleavage into uninucleate pieces. Each uninucleate piece rounds off and metamorphosis itself as a kidney shaped biflagellate zoospore.
The zoospore are liberated by the bursting of papilla. Each zoospore swims about for some time, then comes to rest. It drops its flagella, secretes a thick wall, and then germinates by a germ tube which penetrate the host leaf, the germ tube develops into a new mycelium.
When the temperature is high, the sporangium behaves as a conidium and germinates directly by putting out a germ tube at the apex which causes new infection.
Sexual reproduction
Sexual reproduction in Phytophthora infestans takes place by means of antheridia and oogonia.
When two mating types grow adjacently, the female hyphae grows through the young antheridium and develops into a globose oogonium above the antheridium. The mature antheridia forms funnel shaped collar around the base of the mature oogonium. This type of arrangment of antheridia and oogonium is called amphigynous.
Some species of Phytophthora are homothallic where as some are heterothallic.
Antheridium
It arises earlier than the oogonium, as it terminal clavate swelling of a hypha. It is then separated by a septum from its stalk. When young, each antheridium contains one to two nuclei.
Oogonium
It arises from the lateral branches of the hypha. The tip of the oogonial branch comes in contact with the antheridium. The oogonium is cut off from the stalk of an irregular plug like septum. The antheridium forms a funnel shaped collar around the base of the oogonium. In young condition, each oogonium is a multinucleate structure. The protoplast within the oogonium becomes differentiated into a central ooplasm, with one nucleus and a peripheral vacuolated and multinucleate, periplasm. The nucleus in the center divides into two.
At maturity, all the nuclei of the oogonium, except one in center, degenerate. The nucleus that survives functions as an uninucleate egg or oosphere.
In the meantime, all the nuclei within the antheridium except one degenerate. The surviving nucleus functions as a male nucleus.
Fertilization
Due to the bulging of antheridial wall, a number of receptive papillae are developed. One receptive papilla grows towards the oogonium and becomes closely applied to the side of the oogonium. The common wall is dissolved through which a male nucleus with little cytoplasm is delivered to the female oosphere of oogonium. The two nuclei fuse resulting in the formation of an oospore.
The oospore secretes a wall and undergoes a period of rest. After a period of rest, the oospore germinates by forming a germ tube at the tip of which a sporangium is developed. The sporangium germinates by producing biflagellate zoospores or by forming germ tubes.
