Late Blight of Potato By Dr. Prajna Maitra for Botany (General) Part-I

Brief history of the disease This is one of the most serious of all the diseases of potato when conditions are favorable for its development. It caused a severe epidemic on the European continent. It is probable that the disease was introduced into the United States and Canada at about the same time as the European introduction.

Symptoms The disease attacks the tops causing a blight and may also invade the tubers and cause a dry or wet rot. The symptoms on leaves, stems, and pedicels are similar in character. The first indication of the disease on leaflets, petiole, or on stem consists of brownish-to- purplish-black lesions

Effect of Diseases

How the Disease Expands The lesions are not delimited in size and under favourable conditions enlarge rapidly. If weather conditions are favourable, these infected areas enlarge rapidly so as often to involve the whole surface. Extension to the stem quickly occurs in bad cases, and the entire crown may fall over in a rotten pulp in a few days.

Expansion of Disease (contd.) The influence of the weather is most marked. In dry, clear weather successful infections are limited in number and the resulting spots remains small, brown and dry, while the stems may escape altogether. In warm, humid weather the colour rapidly changes to black, the lesions are wet, the stems are quickly attacked, and a pronounced smell of decaying vegetable matter is given off.

Effect of the Disease as Shown When the blight is advancing rapidly in a potato field, a mildly pungent characteristic odour is given off, The undergo parts, especially the tubers, are also affected. A brown to purple discolouration of the skin followed by a brownish dry rot extends to about ½ inch below the surface of the affected tubers.

Affected Potato

The Disease as Depicted in Parts The dry rot does not soften the tissues but causes rusty-brown markings just below the skin and extends inwards for a variable distance in an irregular fashion. A wet rot is usually set up by the action of secondary organism which follow late blight development. In moist atmosphere, white tufts of my celium and sporangiophores of the fungus appear on the surface of the infected tubers.

The Casual Organism The aseptate myelium is intra- and intercellular. It often produces rudimentary haustoria in foliage cells. Emergence through or between the cells of the epidemis is also some times found. On the tubers they mainly arise from lenticels or abrasions in the rind. The sporangia germinate by the development of biflagellate secondary zoospores.

Casual (contd.) The zoospores when formed, after a swarming period come to rest and germinate by germ tubes which penetrate through the stomata or directly through the epidermal cells of the host tissue.

Geographical Expansion of the Disease Sexual reproduction is oogamous. Oospores are very rarely found in Europe, have recently been discovered in quantity in Mexico, where compatible strains of the fungus coexist.

Disease Cycle Primary inoculum of the disease in the field usually comes from the planning of infected tubers and from oospores in the previous year’s plant debris. It has been conclusively established that the mycelium developed during previous year’s crop infection, frequently survives in the tubers without causing any great damage. This tendency for the infection to start at the base of the plant is due largely to the more favourable conditions of the microclimate, such as abundance of humidity provided by the dense growth in that situation.

Disease Cycle

The susceptibility of the host is governed by factors like The water nitrogen ratio-increased nitrogen supply affords resistance. Water content of the leaves: succulence favours infection. Soil-moisture: dry soils increase resistance. Supply of potash-a shortage reduces infection rate.

Control Late blight of potato disease can be effectively controlled by two principal methods: (a) Direct attack on the pathogen by spraying and dusting of fungicides. (b) Cultivation of resistant varieties.

Control(Measures) Sanitary Measures: Previous year’s plant debris should be thoroughly cleared to cut the source of primary inoculum. Use of disease-free seeds: Since the pathogen perennates as mycelium in the tubers, seed tubers should be used raised from disease-free fields. Improvement of storage of seed potato: Seed tubers should be distinfected with 0.1 per cent, mercuric chloride immediately after harvest.

Control (Soil Management) 1. Frequently earthing-up of growing crop at a four to six inches high ridging diminishes the risk of tuber infection. Spraying of soil with 10 to 20% Sulphuric acid 5% copper sulphate. 12 lb. of copper sulphate with ½ lb of caustic soda in 40 gallons of water at the rate of 100 gallons/acres

Improvement of Harvesting Practice Due to precuations must be observed at harvest time to prevent tuber infection. If the foliage has been blighted, digging should be delayed until the foliage has died and dried out. Care should be taken to prevent the tubers from coming in contact with the diseased foliage.

Use of Resistant Varieties The use of resistant varieties is a very important control. Potato varies in which demissum resistance has been fixed produce promising result.