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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

• 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

Introduction
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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 DISEASES PATHOGEN
Wheat Loost smut Karnal smut Flag smut Ustilago segetum var. tritci Neovossia indica Urocystis agropyri
Chickpea Ascochyta blight Wilt Ascochyta rabiei Fusarium oxysporum f.sp. Ciceri
Crucifers Grey and black leaf Spot Alternia brassicae Brassicicola
Rice Bunt False Smut Stackburn Neovossia horrid Ustilaginoidea virens Pyricularia oryzae Trichoconiella padwickii
Cotton Anthracnose Wilt Colletotrichum indicum F.oxysporum f.sp.vasinfectum
Maize Black kernel rot Cob rot Southern leaf blight Botryodiplodia theobromae Fusarium Moniliformae Drechlera maydis
Pearl millet Downy mildew Smut Sclerospora graminicola Tolyposporium penicillsriae
Sorghum Anthracnose Kernel or grain smut Downy mildew Colletotrichum graminicola Sphacelotheca sorghi Peronosclerospora sorghi
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Soybean Anthracnose Pod stem blight Purple seed stain Colletotrichum dematium Phomopsis sojae Cercospora kikuchii
Cucumis spp. Anthracnose Colletotrichum lagenarium
Brinjal Fruit rot Phomopsis vexans
Carrot Black root rot or Seedling blight Alternia radicina A.dauci
Onion Damping off Downy mildew Purple blotch Stemphylium Blight Botrytis allii Peronospora destructor Alternia porri Stemphylium vesicarium
Pepper chilies Anthracnose Or ripe fruit rot Colletrotrichum capsici
Radish Grey leaf spot Leaf spot Alternia brassicae Raphani
Tomato Buck eye rot Damping off Early Blight Late blight or Fruit rot 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 pseudomonads.

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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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