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Pathogenesis
Pl. Path. 111 (Cr. Hrs. 3+1)
P.N. Sharma
Department of Plant Pathology,
CSK HPKV, Palampur (H.P.)
Pathogenesis & Parasitism
• Parasite : Parasitism
• Symbiosis, Mycorrhiza -- Beneficial
• Pathogen : Pathogenesis harmful
Levels of Parasitism Symbiotic ---------------------Pathogenic
1. Parasite
a. subsist in whole or in part upon living tissue
b. may or may not be pathogenic
c. Facultative Saprophytes
d. Facultative parasite
e. Obligate parasite
f. Necrotrophs
Level of Parasitism
2. Symbiosis
Two organisms living together for the mutual benefit
of each
Fungi ------------------ algae
Rhizobium ---------------- legume plant nodules
Mycorrhizal -------------- bentgrass (Glomus spp.)
Endophytes ------------- fescue, ryegrass (Acremonium
coenophialum and A. lolii )
3. Pathogenesis
Pathogenicity
Ability of the parasite or pathogen to
interfere with one or more of the essential
functions of the plant.
Host range of the pathogen
Disease development
–Disease triangle
–Disease tetrahedron
Disease Triangle
Disease
Host Pathogen
Environment
Conditions for disease Host should be susceptible
Pathogen should be virulent
Environment should be favourable for the disease
TIME
Environment
Host Pathogen
Human Activity
Time
Disease Tetra-hedron
Disease Cycle Pathogenesis/ Disease Cycle – a series of events
that occur in succession during a pathogenic relationship of a pathogen and host that leads to disease
The study of disease cycle generate information about
– Source of perpetuation of pathogen
– Mode of spread
– Help in formulation of control measures
A pathogen’s life cycle may follow events similar to the disease cycle.
Terminology • Inoculation
• Inoculum – Primary inoculum
– Secondary inoculum
• Inoculum density
• Infection
• Invasion
• Perpetuation/perennation / survival
• Dissemination/ Dispersal
• Host
• Predisposition
• Disease escape
• Invasion
• Colonization
Events in Disease Cycle
Inoculation Penetration Infection Invasion Reproduction Dissemination Survival
Primary
Inoculum
Primary
Infection
Dissemination
Over wintering
Primary Disease Cycle Monocyclic Disease
Secondary Disease Cycle
Primary
Infection
Secondary
Infection
Reproduction and
Dissemination
Over wintering/
summering
Polycyclic Disease
Generic Disease cycle Outside Host
Inside Host
Infection
Incubation
Disease Development
Reproduction
Survival
Deposition
Dissemination OVERWINTERING
REPEATING STAGE ASEXUAL (summer) CYCLE
SEXUAL or ASEXUAL CYCLE
Inoculation
• It is the process by which the pathogen come
in contact with its host
– Inoculum: any part or propagule of pathogen that
can cause disease ( may consist of single unit of the pathogen or
whole of the pathogen)
– Inoculum potential/ density
Types of inoculum
Types of Inoculum
1. Primary inoculum (Sclerotia, mycelium, oospore)
2. Secondary inoculum (conidia, urdospores,
zoospores)
(A) Two groups of zoospores of the grape downy mildew oomycete have gathered over two leaf stomata.
(B) Uredospores of rust. (C) Mitospores (conidia) of a fungus that causes a corn leaf spot disease. (D)
Bacteria of Pseudomonas syringae that causes bacterial spot and canker of stone fruits are seen in and
surrounding a stoma of a cherry leaf.
Types of inoculum
and ways in which
some pathogens
enter a host plant.
A B
C D
• Sources of inoculum:
– Soil: Bacteria, Rhizoctonia, Sclerotinia,
– infected plant parts: Seed, cuttings, bulbs,
corms, tubers etc.
– Diseased debris: Alternaria, Phytophthora
– Alternate hosts: Rusts- wheat rust: Barberis
vulgaris
– collateral hosts: Viruses: BCMV, PVY,
Powdery mildews etc.
Arrival or landing of inoculum: – inoculum is carried by different agencies
like,
• Wind: powdery & Downy mildew, rusts
• Water: bacteria, Colletotrichum spp. Fusarium
• Soil: Rhizoctonia, bacteria, Sclerotinia
• insect vectors: viruses- Potato virus Y by
aphid: Myzus persicae
• planting material,: viruses: BCMV, TMV
• Tools: TMV, bacteria, fungi,
– Some pathogens may also use their own
force to reach host e.g. nematode larvae,
zoospores
Penetration
The process by which the pathogen enter
its host.
Varies in different pathogens.
The penetration process is divided into
• Pre-penetration process
• Penetration
• Post penetration process
Pre-penetration process
• Differs in different host pathogen
combinations
• Attachment of the pathogen to the host
• Spore germination in fungi, multiplication
in bacteria and hatching of nematode
eggs
• Perception of the host surface
Pre Penetration Process
1.Attachment of the pathogen to the host
– Some of the pathogens like Viruses, phytoplasma, viroid, RLOs
etc are directly placed in side their host by different agencies
– Almost all fungi, bacteria, parasitic plants, nematodes, first
come in contact with the host surface and must get attached to
the external surface
• This is done through some kind of adhesive material
presence on the propagules surface consisting of water
soluble polysaccharides, glycoproteins, lipids or fibrillar
material
• Presence of moisture on host surface help
attachment.
• Exact mechanism of adhesions of the
spores is not known, however, – there may be some specific interaction of the spores with
host surface via lectins, ionic interaction or hydrophobic
contact with the plant cuticle.
– Studies have shown that many proteins of the fungal cell
wall play an important role in the adhesion of the fungi, as
well as in the host surface perception by the fungus.
Attachment of the pathogen
Pre-penetration process in
fungal pathogens
• In fungi
– Spore germinations (Resting spores; propagating spores)
– Some spores germinate immediately like conidia, ascospores
– Others need some resting period for their maturation e.g. Sclerotia, teliospore, oospores
Spore germination • Spores of different fungi germinate
according to the nature of spore &
environmental conditions
– Resting spores (Asexual; sclerotia,
chlamydospores,: sexual- teliospores,
Oospore, ascospores)
– Propagative spores (conidia, zoospores)
• Spore may germinate soon after
formation or need dormancy
• Moisture is one of the important factor,
followed by temperature.
Infection structures
• Germtube
• Appresorium
• Infection hyphae
• Haustoria
• Infection cushion
• Rhizomorphs
Spore germination
and perception of the host surface
• It seems that stimulation by the contact with the host
surface, hydration and absorption of low molecular weight
ionic material from the host surface and availability of the
nutrients plays an important role in spore germination.
• stimulations received by the spore, mobilizes their food
reserves (like lipids, polysaccharides and CHO) or spore
contents, and direct them to the cell membrane and cell
wall for the formation of germ tube and its extension.
• The germtube is an specialized structure often
grows short distances before it differentiate into
a appressorium.
• Germ tube also perceives the host surface and
if it does not receive the appropriate external
stimuli, it remains undifferentiated and when the
nutrients are exhausted it stop growing and
dies.
• Appropriate physical and chemical signals (like
host surface hardiness, its topography,
hydrophobicity, and plant signals) leads to germ
tube extension and differentiation.
Spore germination
Spore germination
Once the spore germinate,
then germ tube growth
and movement towards
infection site
- Depends upon physical
factors; host e.g. temp,
moisture, stomatal opening
closing
Factors affecting spore germination – Moisture
– Temperature
– Light: heating effect; UV-harmful; fungi protect by pigments (caratenoids)- Alternaria, Cladosporium in tropics
– pH : generally 5-6.5
(B. cinerea tolerate wide range of pH 1.6- 6.5)
– O2 & CO2
– Biological factors • Host Nutrition • Sugars & amino acids
The amount and duration of particular factor determine the success or failure of spore germination
Spore germination
Multiplication in the
presence of moisture
or host exudates
Pre Penetration Process in bacteria
(A)Agrobacterium, (B) Erwinia,
(C) Pseudomonas, (D) Xanthomonas. Most important genera of plant-
pathogenic bacteria
Pre Penetration Process in nematodes:
hatching of eggs
A B
C D
• In viruses: No activity
• In higher parasitic plants:
– seed germination and
– formation of haustoria
Pre Penetration Process in
Perception means how pathogen
and host recognize each other. It
may take place directly or
indirectly.
It is still unclear how pathogen
recognizes their host and vice-versa.
PERCEPTION
Recognition between host and the pathogen
• it is assumed that when a pathogen comes in contact
with the host cell, an early events takes place that
trigger a fairly rapid response in each organism
which either allows or impedes further growth of the
pathogen and development of the disease.
• The nature of early events is not known with
certainty in any host-pathogen combination;
– however, it may be one of the many biochemical
substances structures and pathways.
– These may include specific host signal compound or
structures or either of them may induce specific action or
formation of specific products by the other organism.
Pathogen components that act as elicitors of recognition
by the host plant and subsequent mobilization of plant
defenses are still being investigated fully.
• Elicitors may be released by attacking pathogen before
or during entry into host and they may have narrow
host range
• Some elicitors may be components of the cell surface of
the pathogen (b-glucans, chitin or chitosan) that are
released by the action of host enzymes (gluconases,
chitinase) and have broad host range.
• Some may be synthesized and released by the
pathogen after it enters the host in response to host
signals.
Interaction between host and pathogen
Penetration Process
• Direct penetration
• Indirect penetration
–Fungi may penetrate in either way
–Bacteria mostly enter through wounds and some time by natural openings
–Viruses, viroids, phytoplasma, RLOs etc. by mechanical means (wounds) and by vectors
Direct penetration • Through Appressorium (e.g. Involved in direct
penetration (eg. Colletotrichum spp. and M. grisea)
• Germtube e.g. (Apple scab- V. inequalis)
• Appressorium give rise to penetration peg
• Haustoria e.g. powdery mildews
• Infection cushion e.g. Fusarium, Sclerotinia – Includes fungi imperfecti & ascomyctes
Leaf structure
Methods for fungal penetration
Penetration of rice leaf by Magnaporthe grisea – Rice blast
penetration peg
Direct penetration by appressoria
Zoospores of Phytophthora sojae
germinating and penetrating the root
Direct Penetration by
• Nematodes : larvae
• Higher parasitic plants: haustoria
Penetration by nematodes
Infective second-stage juveniles of
soybean cyst nematode penetrating
roots (stained red)
Second-stage juveniles of soybean
cyst nematode penetration the root
of soybean (stained red) Heterodera
glycines
Root knot nematodes
Meloidogyne incognita, M. arenaria, and M. hapla
Penetration by parasitic plant
microscopic section shows the dodder haustorium
(left) penetrating the vascular bundle of the host plant
(right).
Cuscuta campestris
Indirect Penetration
Penetration through natural openings
a. Stomata
b. Hydathodes
c. Nectorthodes
d. Lenticels
Penetration through natural opening
Stomatal penetration
Appressorium
Penetration peg
Sub stomatal cavity
Infection hyphe (penetrate diff cells)
Directly haustoria (Biotrophs)
Through Stomata
• Some fungal and bacterial pathogens
• Penetration depends upon
– Size of stomata
– No. of stomata
– Location
– Time of opening & closing of stomata
• Majority of the fungi produces appressorium
• Stomatal invaders includes: ustilaginales,
uredinales & Peronosporales
A. Uredinosorus
B. Germinated urediniospores (U) with germ tubes
(T) and appressorium (A) over leaf stomata C. Developed
appr.
D. An empty appressorium (A) over leaf stomata, indicating
that the fungus already penetrated
E. Hyphae in the intercellular spaces
F. Haustorium (h) inside a mesophyll cell Coffee Rust
Penetration of fungi
1. Entry through Stomata
e.g. Bean Rust – Uromyces appendiculatus
Stomata
uredospore
germtube
Stomatal entry
Bacteria
Stomata
• Entry through hydathodes :
Bactera e.g. Xanthomonas campestris
pv. Oryzae
• Nectorthodes:
Erwinia amylovora causing fire blight of
apple & pear
• Lenticels:
E. amylovora
Hydathodes Special glands or pores at the end of
vascular tissue on leaves through which
water exudes and are a natural opening
for black rot bacteria
Lenticels:
small pores or openings in the outer skin of plants
that provide a pathway for gas exchange between
the atmosphere and plant cells.
Develop on any part of the plant such as roots,
twigs, stems, or trunks
2. Penetration through wounds
(trauma infection
Examples:
Hail, Frost, Insect feeding, Cultivation
heat scorching, topdressing, mowing traffic damage
Pathogen 1st multiply on wounds in the presence of
moisture and then penetrate host cell by
• Directly
• Haustoria
• Enzymes or toxins
Penetration through wounds
Infection process
Establishment of organic relationship of the
pathogen with susceptible cells of the host
TIME-LINE OF INFECTION
Fungal pathogen:
Host:
Preinfection: Germination Germ tube search Appressorium formation Penetration peg
Postinfection Haustorium formation (biotroph) Toxin formation (necrotroph) Detoxification of phytoalexins Reproduction
General induced Papillae Cork & lignin Systemic defenses: formation layers acquired resistance Specific recognition: Hypersensitivity Phytoalexins
structural CONSTITUTIVE DEFENSES chemical
Outside of host Inside of host
• Successful infection evident as: symtpoms
• Infection is of different types
– Local infection
– Systemic infection
– Latent infection
• Invasion and reproduction are the two
concurrent stages that occur during
infection as pathogen invade different
cells, grow & multiply
Infection
• Infection process is affected by various
factors
– Resistance & susceptibility of the host
– Aggressiveness & virulence of the pathogen
– Environmental factors
– Host nutrition & pH
– Incubation period ( depends upon host-
pathogen combination, stage of host and
environment etc.
Invasion
pathogens enters to variable extents inside
the host
• Different pathogens invade their host as:
– Ectoparasite e.g. powdery mildew
– Endoparasites e.g. wilts, viruses etc.
• Sub-cuticular pathogens (Apple scab- Venturia inequalis)
• Sub-epidermal pathogens (wheat rust )
• Vascular pathogens (Pseudomonas solanacearum)
– Ecto-endo parasites e.g. potato canker
(Corticium solani)
Invasion behaviour Ectoparasites: powdery mildew
Endoparasites: bacteria in xylem vessels
Sub cuticular: Apple scab
Sub epidermal: Wheat rust
Growth & Reproduction • Different pathogens grow intercellulary &
intracellularly inside their host
• Growth & reproduction rate depends upon
invasion behaviour of the pathogen
• Method of reproduction – Asexual
– Sexual
– Replication in case of majority of sub-microscopic
pathogens
Growth and reproduction of the pathogen is affected by
Pathogen aggressiveness
Susceptibility of the host tissue
And environmental factors
Two general forms of reproduction
1. Asexual spores
Conidium (pl. conidia) :an asexual, non-motile fungal
spore that develops externally or is liberated from the
cell that formed it
Conidia of powdery mildew Conidia of Helminthosporium (Dreschlera)
Conidia
An acervulus
Conidia
Asexual spores: Sporangium & Zoospores
Sporangia of downy mildew Sporangia & Zoospores of Phytpphthora
Formed by specialized spore bearing branches
called Sporangiphores- sporangiospores-
zoospores
Sporangiphores
zoosporangium
Asexual spores b. Direct transformation of certain
hyphal cells Chlamydospores
The three round
structures are
chlamydospores. One
has germinated to form
two sporangia.
Chlamydospores formation by Phytophthora
chlamydospores
2. Sexual spores
a. May occur at random on surface of mycelium
b. Enclosed in a fruiting body
c. Sexual stage - Usually once a year
Cleistothecia in powdery mildew
Oospores formation in Phytophthora
result from sexual recombination.
Sexual spores
An apothecium produced by Sclerotinia
Perithecium In Venturia
a flask-shaped stroma of seudoparenchyma
is formed inside the leaf tissue.
Dissemination of the pathogen
1. Transfer of inoculum
2. Passive - wind, water, insects,
man, animals, machinery
3. Active - move on own power
bacteria (some)
Pythium (Zoospores)
Fungi- spores expelled forcibly
Insect transmission: Viruses, viroids,
Phytoplasma, Rlos etc.
Movement of Plant Viruses
Short distance
movement Long distance
movement
Cell to Cell movement
Movement of plant viruses as nucleoprotein e.g. cucumo, bromo, tobamo, alfalfa viruses group
Movement of plant viruses as virion e.g. tospo, nepo, como, caulimo viruses
Pathogen Survival
1. In infected crop debris
2. In seed
3. In soil
4. On growing plants
5. Infected material on host plants
6. In propagating material
7. Alternate host
8. Collateral host
9. As dormant structures e.g. sclerotia,
chlamydospores
Acknowledgements
• I gratefully acknowledge the use of some
very important photographs given in text
book “Plant Pathology” by G N Agrios.
• I also acknowledge the scientists who spent
valuable time in generating information on
various aspects of plant pathology and
displayed the same on internet for use by
students, teachers and researchers
• Presentation dedicated to respected
“G N Agrios”