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”