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-topurplish-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.