Describe the life-cycle of the fungus which causes late blight of potato and their control.
Q. Describe the life-cycle of the fungus which causes late blight of potato and their control.
Ans. The disease is caused by Phytophthora infestans. The mycelium consists of hyaline, branched, coenocytic intercellular hyphae with haustoria which are finger-like protuberances surrounded by thickenings of host wall materials (Fig. a). Sporangiophores emerge through the stomata of leaves (Fig.b) and also through lenticels on tubers. They are slender, hyaline. indeterminate, simple or sparsely branched producing sporangia at their apex. Each sporangiophore after producing a sporangium at its apex, however. continues its growth and produces another sporangium in same manner. The sporangia are multinucleate, thin-walled, hyaline oval or pear-shaped with an apical papilla and measure 22-32 x 16-24 µm. Sporangial germination is direct by germ tube or indirect by formation of biflagellate zoospores (Fig.c). The mode of germination is very much affected by temperature. Low temperature favours zoospore formation, whereas higher temperature the germ tube development. A relative humidity of above 90% is necessary for germination of sporangia. The zoospores, after swimming for sometime, loose their flagella and penetrate the host either through stomata or directly.
Oospores are rarely formed in nature. Whenever sex-organs are formed. antheridia are amphigynous and oogonia are spherical. Probably oospore germinates by germ tube.
Several hypothesis are put forward to explain the annual reoccurrence of late blight. Of these, the two possibilities are generally accepted, (i) survival of the fungus in fruiting stage or as perennating mycelium in the tubers left in the field from preceding crop, and (ii) presence of perennial mycelium in diseased tubers stored and used as seeds in the next season. The high temperature prevailing during summer, rules out the first possibility in India. The only possibility of its survival in the plains is through infected seed tubers. The extensive use of cold storage for seeds and quick means of transportation of seeds from hills to the plains in same season have helped the fungus to survive as mycelium in the tubers.
Fig. Phytophthora infestans. Late blight of potato. A. mycelium growing in stem arising from infected tuber; B. a later stage in which cortical cells have been killed and the mycelium has invaded the pith
The infected tubers are the main source of infection. The survival of fungus in the Indian soils in any form is not possible. The dormant mycelium in the tubers becomes active and grows upward in the stem and sporulates on small shoots. The new leaves become infected from sporangia produced by this mycelium. Under suitable conditions of temperature and humidity this mycelium in the leaves develops sporangiophores which emerge from the stomata. Sporangia are easily detached and disseminated by rain or air. On reaching a suitable host the sporangia germinate.
Depending upon the temperature conditions, sporangia germinate either by germ tubes or by zoospores. Tuber infection occurs through eyes, lenticels or injured surface. The spores from blighted leaves are washed down into the soil, where they infect the healthy tubers. Tubers may come in contact with diseased leaves during harvesting also.
Excessive humidity (above 90% R. H.) coupled with suitable temperature for germination of sporangia are the principal predisposing factors. Epiphytotics of the disease are likely to occur when unusually cool weather coupled with abundant moisture prevails at time of sporangial production. Four important conditions are necessary for the severe development of disease. (1) night temperature below the dew point for at least four hours, (2) minimum temperature 10°C or slightly above, (3) clouds on the next day, and (4) rainfall during the next 24 hours.
Control
Late blight of potatoes may be controlled by the following methods:
(i) Spraying: Some fungicides should be sprayed on leaves which prevent germination of sporangia. Bordeaux mixture, Fytolan, Blitox-50 (0.20.5%), Zineb, Maneb, Dithane Z-78 or Dithane M-45 (2-2.5 kg in 1000 litres of water per hectare).
(ii) Destruction of haulms: Infected leaves should be destroyed and not allowed to come in contact with tubers. This can be done by spraying H2SO4, CuSO4 or sodium chlorate on diseased leaves. High ridging also protects the tubers from infection by diseased foliage.
(iii) Seed selection : Tubers from infected fields should be discarded.
(iv) Storage Tubers should be stored in cool, dry, and well aerated stores. Before storage tubers should be given dip in 1 : 1000 HgCl2 solution.
(v) Disease – resistant varieties: There are several physiological races of late blight fungus, of which races 0, 1, and 4 occur in India. Race 0 is common in Northern India. Wherever possible, resistant varieties should be grown.
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