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Soil borne and
Seed Borne
Diseases
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Soil borne disease
The diseases that are caused by fungal pathogens which
persist (survive) in the soil matrix and in residues on the soil
surface are defined as soil borne diseases.
Thus the soil is a reservoir of inoculums of these
pathogens, the majority of which are widely
distributed in agricultural soils.
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Key features of soil borne plant pathogens
Disease Symptoms
Many soil borne fungal plant pathogens cause disease of the roots or
stem disrupting the uptake and translocation of water and nutrients
from the soil.
Therefore they commonly cause similar symptoms to
drought and nutrient deficiencies; these include
wilting, yellowing, stunting and plant death.
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Host Range
The majority of these pathogens has a wide host range and may
cause different types of diseases on different hosts. A particular
species may cause severe root rot on one host but only cause
superficial and symptomless infection on the roots of another host.
Epidemiology and Survival
In general these fungi can persist for long periods in soil in the
absence of susceptible crops.
Some species persist mainly as resistant hyphae in plant residues,
in large pieces of stubble or small fragments.
The inoculum of such pathogens declines gradually as the residues
decompose. Many species also persist as propagules which are
adapted for long-term survival in soil.
These include thick walled spores such as oospores and
chlamydospores, together with sclerotia and microsclerotia.
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Nature of diseases caused by soil borne plant
pathogens
Root rot
These diseases are caused by a diverse group of fungi and related
organisms.
The most important genera include Pythium and Phytophthora,
Rhizoctonia, Sclerotium, Sclerotinia, Fusarium, Cylindrocladium and
Armillaria. These diseases are characterised by a decay of the true
root system.
Wilt
The main species of fungi that cause these diseases are Fusarium
oxysporum and Verticillium spp. T
he symptoms of these diseases include wilting of the foliage and
internal necrosis of the vascular tissue in the stem of the plant.
Some species of bacteria can also cause similar types of diseases.
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Seedling blights and damping-off
Various common names are used for diseases of seedlings such as seedling
blight and damping-off. The fungi that commonly cause seedling death
include Pythium, Phytophthora, Rhizoctonia, Sclerotium and less commonly
Fusarium spp.
These fungi can infect the seedling during the germination, pre-emergence
or post-emergence phases of seedling establishment.
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Seed borne diseases
Introduction
 Pathology is the science deals with micro-organism infecting seeds.
 Seeds are attached by various fungi, bacteria and virus.
 Seeds are attached by various stages,
 The mother plant get infected by the pathogen, it attack seed
also.
 During processing.
 At the time of transportation.
 During storage
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There are three types of infections
1) Internally seed borne
2) Externally Seed borne
3) Fruity bodies/spores
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Internally Seed borne:
Pathogen attacks seed sod, endosperm & embryo
Externally seed borne:
Pathogens externally carryover on the seeds.
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Effects of seed infection:
 Germination % get reduced.
 Due to changes is morphology, the market level get reduced.
 Due to infection it induces the changes in the content get
reduced.
 Due to infection it induced the secretion of toxic chemicals ex:
Aflatoxn, Rubra toxin, ochre toxin, chitrinin, patulin etc.
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 Methods of testing for the presence are of four main types. The
seeds may be
(1) directly examined ,
(2) examined after incubation,
(3) plated on agar so that the pathogens grow out into
colonies and
(4) subjected to techniques which allow the use of modern
serological developments viz., ELISA, SSEM and IF.
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Methods for Detecting Seed Borne Fungi:
Examination of dry seeds:
 It is applied for detection of seed borne fungal pathogens which
cause discoloration of the seed or change the shape and size of
the seed.
 Also applicable for detecting fungal structures present in, on or
with seed.
Examples:
 Karnal bunt of wheat Neovossia indic
 Ergot of bajra claviceps fusiformis
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Washing test:
 This method is used particularly for smut and bunt fungi in
gramineous hosts except loose smut of wheat and barley.
 It can also be used for downy mildew (peronospora
Manchuria) of soybean and tumor disease (protomyces
macrospores) of coriander.
NAOH seed soak method:
Applied for Karnal bunt of wheat and bunt of rice.
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Blotter method:
 This method is widely used.
 All kinds of cereals, vegetables, crucifiers, legumes,
omamentals and forests seeds are tested by this method.
Seedling symptom test:
 This test is applicable for those fungi which are capable of producing
symptoms on the root and shoot of the young seedlings.
 This test for certain pathogens, provide information perta
 ining to field performance of the seed lot.
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Important Seed- Borne Fungal Diseases of Major Crops
CROPS
Wheat
DISEASES
Loost smut
Karnal smut
Flag smut
Ascochyta blight
Wilt
Chickpea
Crucifers
Grey and black leaf
Spot
Bunt
False Smut
Stackburn
Rice
Cotton
Anthracnose
Wilt
Black kernel rot
Cob rot
Southern leaf blight
Downy mildew
Smut
Anthracnose
Kernel or grain smut
Downy mildew
Maize
Pearl millet
Sorghum
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PATHOGEN
Ustilago segetum var. tritci
Neovossia indica
Urocystis agropyri
Ascochyta rabiei
Fusarium oxysporum f.sp.
Ciceri
Alternia brassicae
A. Brassicicola
Neovossia horrid
Ustilaginoidea virens
Pyricularia oryzae
Trichoconiella padwickii
Colletotrichum indicum
F.oxysporum f.sp.vasinfectum
Botryodiplodia theobromae
Fusarium Moniliformae
Drechlera maydis
Sclerospora graminicola
Tolyposporium penicillsriae
Colletotrichum graminicola
Sphacelotheca sorghi
Peronosclerospora sorghi
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Soybean
Cucumis spp.
Brinjal
Carrot
Onion
Pepper chilies
Radish
Tomato
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Anthracnose
Pod & stem blight
Purple seed stain
Anthracnose
Fruit rot
Black root rot or
Seedling blight
Damping off
Downy mildew
Purple blotch
Stemphylium Blight
Anthracnose
Or ripe fruit rot
Grey leaf spot
Leaf spot
Buck eye rot
Damping off
Early Blight
Late blight or
Fruit rot
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Colletotrichum dematium
Phomopsis sojae
Cercospora kikuchii
Colletotrichum lagenarium
Phomopsis vexans
Alternia radicina
A.dauci
Botrytis allii
Peronospora destructor
Alternia porri
Stemphylium vesicarium
Colletrotrichum capsici
Alternia brassicae
A. Raphani
Phytophthora parasitica
Phythium
aphanidermatum
Alternia solani
Phytopthora infestans
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Detection Method For Seed Borne Bacteria:
 Several methods have been developed worldwide to detect
various phytopathogenic bacteria in seed and planting materials.
 The detection techniques generally employed are:
Examination of Dry Seeds:
As in Fungi
‘Growing on ’test:
 The ‘growing out’ bioassay of a working seed sample involves the
sowing of test seeds into seedlings under conditions optimal for the
disease development in glass house or closed environmental chambers.
 ‘Growing –on’ test has been successfully used for a large number of
Xanthomonads and pseudomonad’s.
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Isolation on agar medium:
As in fungi
Indicator test:
 Working seed sample is sterilized with (2.6%) sodium-hypochlorite
for 15 min. and rinsed with sterile water.
 The seed sample is incubated for 18-24 h in sterile water.
 The water suspension is inoculated by infiltration into the primary
leaf node of 10 day old bean seedlings.
 The appearance of lesions followed systemic necrosis is positive
reaction.
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Serological Technique:
 Serological tests are based on ‘In vitro’ reactions between antigens
and antibodies.
 This specific recognition of antigens by antibody has offered the
basic principle for the development of various serological methods
for detection and identification of phytobacteria.
 The washing of the working seed samples are cultured for 36 h
using sterile distilled water.
 The supernatant is tested with antiserum of the suspected
pathogen.
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Detection of Seed- Borne Viruses:
Viruses are obligate parasi tes and they differ
in their genetic make up and require special
technique for detection.
1.Dry seed examination
Visual observation of seeds may reveal abnormalities
like discoloration, shriveling, reduced seed rice size,
staining and seed coat necrosis.
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2.Growing on test:
Growing on test is used to determine virus
seed transmission.
Seeds are planted in blotter paper, sand, soil,
vermiculite or in other growth medium.
Seedlings are examined at regular
intervals for the viruses
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3.Indicator test:
Viruses can be detected in seeds by assaying the
extracts of different parts of seeds and seedlings
raised from infected seeds on suitable indicator
plants.
Susceptible hosts which produce local
lesions or systemic symptoms are used as
indicator plants.
This test has been used to detect BCMV in
bean and urd bean, LMV in lettuce, TMV in
tomato and tobacco ring spot virus in
soybean.
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4. Serological methods:
Serological methods are used for characterization
and determining relationship between viruses.
The tests are based on the reaction between an
antiserum, a blood serum containing specific
antibodies produced by injecting laboratory animals
with a pure virus preparation, and an antigen-virus
protein.
The tests are specific since an antibody
combines only with the antigen which contain
which contains similar grouping of amino- acid
sequences.
The union of antigen and antibody can be
detected in the form of precipitation and
agglutination
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To sum up
The diseases that are caused by fungal pathogens survives in the soil
matrix and in residues on the soil surface are defined as soil borne
diseases.
Many soil borne fungal plant pathogens cause disease of the roots or stem
disrupting the uptake and translocation of water and nutrients from the soil.
Therefore they commonly cause similar symptoms to drought and nutrient
deficiencies; these include wilting, yellowing, stunting and plant death.
Seed borne diseases are caused by micro-organisms infecting seeds.
Seeds are attacked by various fungi, bacteria and viruses at various
stages viz., in the field ,during processing, at the time of transportation,
and during storage.
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