Phytophthora
Q.1. Describe the structure of mycelium and sexual reproduction in phytophthora. (2012)
Ans. The vegetative body of the Phytophthora is the mycelium which consists of irregularly branched, non-septate (coenocytic), and hyaline hyphae. However, the septa are formed in the older parts of the mycelium
Fig. - Phytophthora. A - intercellular mycelium of P. colocasiae, showing haustoria inside the host cells. B - Chlamydospores germinating and forming sporangium.
and at the time of the formation of the reproductive structures. The mycelium develops both intercellularly and intracellularly. The intercellularly mycelium forms short simple or branched outgrowths, the haustorid, by means of which it absorbs the nourishment from the host cells. The hyphae of the intracellular mycelium diractly penetrate and kill the host cells.
Sexual reproduction: -
It is of oogamous type and takes place by means of antheridia and oogonia, the latter developing before the former.
On the basis of the arrangment of antheredium and oogonium, the various species of Phytopthora may be classfied into two distinct groups -
(1) The Amphigynous: - In this the antheredium is first formed at the tip of a hypha them the oogonial incept pentrates, grows entirely through it and swells up to form the spherical oogonium above the antheredium. The mature antheredium thus forms a funnel shaped collar around the base of the mature oogonium.
(II) The peragynous: - In this type the antheredium is applied to the side of the oogonium.
Fertilization: -
Due to the bulging of antheredial 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 nucleus of oogonium. The two nuclei fuse resulting in the formation of oospore.
Fig. Phytophthora. Sexual reproduction A-somatic hypha. B-development of antheridium and oogonium. C-an antheridial initial with two nuclei. D-swelling of oogonial incept just before to its emergence from antheridium. E-swelling oogonium after emergence. F and G-nuclear conditions at various stages of oogonial development. H-degeneration of peripheral nuclei; a central nucleus remains at the centre. I-central nucleus undergoes a division to form two nuclei. Out of which one nucleus survives and the other degenerates. Papillae, which are formed by antheridium are visible. J-plasmogamy. K-formation of oospore karyogamy). L-germinating oospore.
The oospore secretes a wall and undergoes a period of rest. In some species eg, Phytopthora infestans the oospores develop parthenogenetically in the absence of antheridia.
The oospore then germinates by forming a germ tube at the tip of which a sporangium is developed. The sporangium germinates by producing biflagellate zoospores by forming germ tubes.
Late Blight of Potato: -
It is the most serious of all diseases of potato. The disease appears on the leaves only after the blossoming period as small black lesions. If the weather is favourable these brownish black lesions enlarges rapidly, involving the whole surface. Infection soon spreads through stem and branches and the whole plant blights. After the aerial parts of the plant have been affected the underground parts especially the tubers are infected. In the early stages the tubers show brownish or purplish discoulouration, but later they get softened, turn completely brown and decay before the harvest.