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TYPES AND MECHANISMS
OF SEED PATHOGEN INFECTION
Dr. K. Vanangamudi
Formerly Dean (Agriculture), AC & RI, Coimbatore
Dean, Adhiparashakthi Agricultural College, Kalavai
Professor & Head (Seed Science & Technology)
Tamil Nadu Agricultural University, Coimbatore.
Email: vanangamudi.tnau@gmail.com
Website: https://trinityculturalacademy.com/
YouTube: https://youtube.com/channel/UCWGv08j5jaZ-nkvz46HrBVw
slideshare: /VanangamudiK1/edit_my_uploads?
1. Seed pathology
 Seed pathology is the area of science that studies the relationship between
pathogens and seeds.
2. Types of pathogen
 Diseases of plants are caused primarily by three types of pathogens:
bacteria, fungi, and viruses.
 Although fungi comprise the largest group of pathogens, the bulk of seed-
specific diseases are caused by bacteria or viruses.
 This is because bacteria and viruses are more adopt at entering and then,
traveling through the veins of the plant, a phenomenon known as
‘systemic infection,’ and from the vascular system may make their way
into the developing embryos of seeds.
 Fungi, in contrast, tend to be restricted to the outer layers of the plant,
where they initiate infection by means of air-borne spores and then,
proceed to spread by attacking nearby cells of the outer layers.

 Fungi are much less likely to enter the vascular system of the plant, and
thus, infect seed mostly when they either ‘crawl’ all the way to seed on
the outside of the plant, or else send out spores that land on the seed.
 In either case, the fungal spores are on the outside of the seed, in the
layers of the seed coat, spores on the seed coat are more prone to either
dry up and die, or else to get sloughed off with the seed coat during seed
germination, thereby failing to cause disease on the next generation of
plants.
3. Nature of pathogen infection
3.1. 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.
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.
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 in 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.
Nature of diseases caused by soil-borne plant pathogens
Root rot
 Root rots are caused by Pythium, Phytophthora, Rhizoctonia, Sclerotium
(Plate 1), Sclerotinia, Fusarium, Cylindrocladium, and Armillaria.
Plate 1. Sclerotium rot (S. rolfsii) of potato
 These diseases are characterised by a decay of the true root system.

Wilt
 These diseases are caused by Fusarium oxysporum (Plate 2) and
Verticillium spp.
Plate 2. Fusarium oxysporum wilt in tomato
 The 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.
Seedling blights and damping-off
 The fungi that commonly cause seedling death include Pythium,
Phytophthora, Rhizoctonia, Sclerotium, and Fusarium spp (Plate 4).
Plate 4. Damping off (Pythium aphanidermatum) in tomato
133.3.2. Seed borne diseases
 Seed borne disease refers to the particular plant diseases that transmitted
by seed.

 There are two types of infections.
Internally seed borne
 Pathogen attacks seed, endosperm, and embryo.
Externally seed borne
 Pathogens externally carry over on the seeds (Plate 5).
Plate 5. Karnal bunt (Tilletia inidica) in wheat
4. Mechanism of pathogen infection
4.1. Systemic infection of the seed
 The establishment of a pathogen in any part of the seed is referred as seed
infection (Fig. 1).
 It can be systemic, by the vascular system or plasmodesmata or directly
by natural or artificial wounds.
 The same pathogen can infect the seed using one or more of these
mechanisms.
Fig.1. Systemic infection of the seed
 Example: Xanthomonas campestris pv. phaseoli can infect seed through
the vascular system, by natural openings.

Systemic infection through flowers, fruits, or funiculus
 Most of the systemic seed-borne bacteria and fungi reach and infect the
embryo through the flower or from the peduncle of the fruit, via funiculus
(Fig. 2).
Fig.2. Systemic infection through flowers, fruits, and funiculus
 Viruses go to the embryo from the systemically infected mother plant and
the infected or contaminated pollen.
 They rarely reach the embryo during the formation of the seed or the
embryo itself.
 Examples of some infections that occur through the vascular system.
Pathogens Crops
Fusarium oxysporum Pumpkin, pea, tomato
Plasmopara halstedii Sunflower
Septoria glycines Soybean
Verticillium dahliae Spinach and sugar beet
Xanthomonas campestris Bean, cabbage, rice, sweet pepper
Pseudomonas syringae pv. lachrymans Cucumber
Penetration through the stigma
 During the infection, some pathogens follow the same path as the pollen
grains do.

 Spores of some fungus reach the stigma and germinate, producing an
hypha that reaches the ovary through the style, where they can stay as a
dormant mycelium until seed germination (Fig. 3).
Fig.3. Penetration through the stigma
 Viruses can infect through infected pollen, the male gamete carries the
virus, and when joining the ovule it generates an infected embryo.
 If both the male and female gamete are infected, they can even produce
an infected endosperm.
 Example: Ustilago nuda and U. tritici in barley and wheat, and
Alternaria alternata in sweet pepper.
Penetration through the wall of the ovary or immature seed covers
 Some fungi, like Ustilago nuda and U. tritici penetrate through the wall
of the ovary as a result of the germination of the teliospores on the stigma
or the wall of the ovary.
 The promycelium goes through the wall and other tissues until it reaches
the embryo.
 In some other cases, penetration occurs through breakages on the testa,
establishing itself in the endopleura or the endosperm (Fig. 4).

 In fleshy fruits, like cucumber, melon, eggplant, tomato, sweet pepper,
and others, contamination can occur directly through the funiculus or in
the tegument, during the process of seed formation.
 Examples: Colletotrichum lagenarium in watermelon and Rhizoctonia
solani, when it invades fleshy fruits, like the ones mentioned above, it is
capable of infecting from the placenta and penetrating to the developing
ovule or seeds that are still in its formation process and have not lignified
its cover.
Fig.4. Penetration through the wall of the ovary or immature seed covers
Penetration through wounds and natural openings
 Natural openings like the hilum and the micropyle or wounds generated
during the threshing are spots, where pathogens like Xanthomonas
campestris pv. phaseolina in bean and Pseudomonas syringae pv.
lachrymans in cucumber.
4.2. Seed contamination or infestation
 Contamination or infestation refers to the passive relationship of
pathogens and seeds.
 The pathogen itself or parts of it can stick or can get mixed with the seeds
during seed collection, harvesting, extraction, threshing, selection and
packing.

Pathogens that stick to the surface of the seed
 Pathogens that stick to seeds during harvest or postharvest do so by their
spores (Clamidospores, Oospores, Teliospores, Uredospores), bacterial
cells, and in some cases, virions. Spores of the following fungi can be
carried on seed coat surfaces.
Pathogens Crops
Alternaria brassicae, A.
brassicicola
Crucifers
A. longipes Tobacco
A. radicina Carrot
Ascochyta pinodella Pea
Drechslera sorokiniana, D. oryzae Rice
D. avenae Oats
Fusarium oxysporum f. sp.
callistephi
China aster
Sclerotia of Rhizoctonia solani Eggplant, pepper,
tomato
Tilletia caries, T. foetida, T.
contraversa, and Urocystis agropyri
Wheat
 Several bacteria contaminate seed surfaces.
Pathogens Crops
Corynebacterium flaccumfacjens pv.
flaccumfacjens
Bean
P. syringae pv. lachrymans Cucumber
P. syringae pv. tomato, tobacco mosaic virus ,
tomato mosaic virus
Tomato
C. rnichiganense pv. michiganense Pepper
Xanthomonas campestris pv. campestris Cabbage
Pepper mosaic virus Pepper
Accompanying contamination
 This type of contamination refers to the physical mixing of the seed with
the pathogen's propagation organs like the sclerotium, nematode's galls,
contaminated plant parts, or soil particles containing pathogens.

5. Seed transmission
 In general terms, infection can be classified into systemic and
nonsystemic.
 It is systemic when the pathogen introduces itself to the plant when the
seed germinates, and develops with it.
 Nonsystemic infection occurs when there is a localized infection caused
by the pathogen in the seedling at the stage of pre or post-emergency, in
this situation there are no systemic symptoms.
5.1. Systemic transmission
 This type of infection can be produced by pathogens that are carried with
the seed in various parts, like the embryo (Plate 6), endosperm or
episperm, or by contamination of the outer portion of this one.
Plate 6. Systemic transmission through seed
Infection of the embryo
 When the seed germinates, if the embryo is infected, the pathogen
initiates its growth together with the plant (Plate 7).
Plate 7. Systemic infection from roots or seed

 Symptoms can show up during different stages of development.
 In the case of Ustilago triticci, the mycelium grows together with the
plant and expresses symptoms only at the flowering stage.
 Some pathovars of Xanthomonas campestris that infect cabbage, bean or
sweet pepper move between the cells of the host until they reach the
vascular system and produce symptoms in leaves or stems.
 Most seed-borne viruses persist inside the embryo.
 Its multiplication and movement accompany the plant during its
development, and there can be symptoms at any stage, from the formation
of first leaves to flowering or fructification.
Nonembryo infection
 Infection of the episperm (testa and endopleura) occasionally leads to a
systemic infection.
 Some bacteria, like Corynebacterium michiganense pv. michiganense in
tomato and Xanthomonas campestris pv. campestris in cabbage, penetrate
through the stomata of cotyledons, and from there, reach the vascular
system, initiating the systemic infection.
Episperm contamination
 In a few cases, this type of contamination conducts to a systemic
infection.
 Most of these exceptions are fungi that are highly specialized in their
pathogenesis and produce in cereals the so called smuts, rusts or mildews.
 In these cases, generally, spores are carried outside the seed, they
germinate, penetrate the coleoptile, and start a systemic infection.
 This can occur directly o by a more complex system of haploid hypha
fusion in genera like Tilletia, Ustilago, Uromyces, Sclerospora,
Pernospora, and Puccinia.

5.2. Nonsystemic transmission
 Nonsystemic infection (Plate 8) is very common, and in the same way as
systemic infection, it can come from an infection, outside contamination,
or by pathogens mixed with the seeds.
Plate 8. Non systemic transmition through seed
Infection of the embryo
 This case is restricted to some pathogenic fungi that maintain themselves
in the embryo or the episperm as hypha inside the seed.
 The primary infection starts as injuries in the cotyledons or primary
leaves, stems, or petiole. Fungi fructifications (Pycnidium, Acervulus)
can develop on these organs, under certain favorable conditions
(temperature and humidity).
 These fructifications produce spores that, with the action of water and
wind, disperse the disease to other parts of the plant and other plants.
 This occurs in Ascochyta pisi in peas, Colletotrichum lindemuthianum in
beans (Plate 9), and C. truncatum in soybeans.
Plate 9. Colletotrichum lindemuthianum in bean

Infection of the episperm
 Generally, seeds in which the episperm is infected, do not germinate, or
germinate and contaminate the soil.
 Rarely produce a systemic infection, but can infect the seedling from the
outside. Over the injuries, new inoculum is produced, this one infects
other parts of the plant or other plants.
 For example, Septoria nodorum in wheat under high humidity conditions,
forms pycnidia in the coleoptile, whose spores disseminate the disease to
other plants.
Contamination of the episperm
 Contamination outside the seed testa can produce healthy seedlings, but
the inoculum infects the soil and from there it can cause infections at
more developed stages of the plant.
 Wheat and rice seeds that are contaminated with teliospores of Neovossia
indica in wheat and N. torrida in rice, produce esporidispores in the soil,
that can be carried by the wind and infect flowers, originating the smuts.
Accompanying contamination
 The mixture of seeds with sclerotiums of the fungi or contaminated soil
particles produces non systemic infections, at any stage of the plant
growth and development.
 It is common for Sclerotinia sclerotiorum to produce mycelium from the
sclerotiums.
 This mycelium can directly infect the plant or produce fructifications that
produce spores that can be carried by the wind and infect flowers of
different species, like sunflower, soy, peanut, etc.

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