Late blight of potato

 

 Late Blight of Potato

Late blight of potato is one of the important diseases found in all potato growing areas of the world. It is a native of north Andis in South America. This disease becomes well established in Ireland, Europe and England. In India the late blight was introduced in the Nilgiri hills but now a days it is frequently available in all potato growing region of India.

Causal organism:-

Phytophthora infestans is the causal organism of late blight of potato, Solanum tuberosum. It is also pathogenic to other plants of family Solanaceae including tomato.

Symptoms 

They appear 1^st as water soaked or hydrotic areas along the margin and tips of lower leaflets. The spots enlarge rapidly become necrotic, turned brown and then blackish or blighted. The infected leaves ultimately become limb and rot away producing a characteristic odour.

If dry weather intervenes spreading of the lesions is temporarily checked. But the favourable condition helps the diseased areas to become hard and may break from the leaflet. The leaflets give a withered curled appearance. The progress of the disease is resumed or soon as the weather becomes cool and moist. The pathogen does not go down the stem to the tuber. The disease spreads to tubers from sporangia or zoospores that pass into the soil. Water soaked purplish or brownish blotches appear on the tuber surface. They soon become necrotic.

Disease cycle :

       The major sources of infection are infected tubers. However wind blown spores coming from cool hilly area can also function as primary inoculums. The fungus cannot survive in soil in hot areas. With the growth of seedlings, the spores germinate in hyphae and ultimately become mycelium and spreads into leaves. The overall disease cycle of late blight of potato is as follows-

Control measure :- / Disease management :-

Important measure to control the disease are-

(i) Seed tubers must be free from infection. Thus seed tubers should not be collected from the diseased field. Tubers should be harvested after maturity of the tubers when skin of the tuber come tough.

(ii) Before the plantation tuber treatment is obligatory. Deeping of tubers in dilute fungiside solution kills the pathogen. The tubers are then washed and used as propagule.

(iii) Growing disease resistant varieties is the best method of disease control. However, late blight fungus has a number of races and resistance to all the races is difficult to achieve. Further the tubers of even most resistant varieties are susceptible to pathogen.

(iv) Insusceptible environment and disease proned areas the crop should be spread or dusted with fungisides at regular inte4rvals. Bordeax mixture, copper oxichloride, copper lime, dithane etc. are useful.

(v) Cultivation of tuber on high ridges and covering the tuber with soil will prevent the fungus to grow out from the infected mother tuber, thus reduce the spread of disease.

(vi) The diseased crop should be harvested late. The foliage is lifted 1^st and destroyed before digging the tubers.

(vii) Unmarkable tubers should be properly picked up and burnt. Similarly the shoots of ripe crop should not be allowed to be dumped. Instead it should be destroyed.