Give an illustrated account of life-cycle of the fungus which causes Black wart disease of potato.
Q. Give an illustrated account of life-cycle of the fungus which causes Black wart disease of potato.
Or, Describe the causal organism, symptoms and methods of control of the Black wart disease of potato.
Ans. Causal Organism : Synchytrium, with some 200 species, is an obligate unicellular parasite chiefly of the angiospermic plants, causing disease of considerable enormous significance. Of all species, S. endobioticum is most significant causing wart disease of potato.
Fig. Endobioticum causing Black wart disease of potato
Symptom infection of S. endobioticum on potato tubers produces large, =yellowish warty outgrowths. In severe conditions, the galls are also formed on aerial stem and leaves and the warts on tubers rot away exuding a putrid smell.
Vegetative Structure: The fungal body is holocarpic monocentric, endobiotic and represented by a naked posteriorly unifiagellae zoospore.
Life-Cycle: The zoospore withdraws its flagellum and penetrates the host cell by a small penetration tube. Once inside the host, it reaches the lower part of the cells, the infected cell enlarge considerably and the surrounding cells divide repeatedly (hyperplasia) and by enlargement (hypertrophy) they form warty outgrowths.
The naked protoplasm, after increasing in size, secretes a double wall around itself. The structure is called prosorus and occupies nearly half of the infected cell. Soon after maturing, the prosours germinates within the host cell. Its outer thick golden brown wall ruptures and repeated mitotic divisions of the nucleus takes place. A number of thin hyaline walls are now laid down in such a way as to divide the prosorus into four to nine multinucleate segments. Each segment thus develops into four to nine multinucleate segments. Each segment thus develops into a zoosporangium or a gametangium, depending on the environment. In favourable conditions they act as zoosporangium, liberating numerous zoospores through an opening, which go to infect other host tissues.
However, in scarcity of water, they behave as gametangia, liberating numerous planogametes, resembling to that of zoospore. Two compatible gametes fuse together to form a diploid zygote. The zygote infects the host cells in the same manner as the zoospores.
The diploid cell swells up considerably within the host cell and finally develops a double wall around itself and now acts as resting or winter sporangium. The resting sporangium (spore) is buried deep within the host due to hyperplastic division of the surrounding and invaded cell. It germinates after a prolonged period of rest when the diploid nucleus undergoes successive divisions; the first of these divisions is being meiotic. The protoplasm within the sporangium then becomes segmented into a number of zoospores which are liberated by the rupture of the resting sporangia. These zoospores are also typically uniflagellate structure and behave in the same manner as the asexual zoospores.
Method of control of Wart Disease of Potato:
1. Breeding of resistant varieties.
2. Killing the resting sporangia in the soil by the application of fungicides.
3. Eradicate diseased tubers.
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