This document discusses the pathogenesis or disease cycle of pathogens. It defines pathogenesis as the series of events leading to disease development and perpetuation. Pathogens can have monocyclic, polycyclic, or polyetic disease cycles. Monocyclic pathogens complete one cycle per year, while polycyclic pathogens have multiple cycles per growing season, increasing inoculum levels. Polyetic pathogens take over one year to complete a cycle. The stages of pathogenesis discussed are inoculation, pre-penetration, penetration, infection, dissemination, and overwintering/survival. Primary inoculum initiates infections while secondary inoculum spreads disease epidemically. Pathogens penetrate directly or through wounds and natural openings. Infection occurs intracellularly or

1. PATHOGENESIS
(Disease (or) Infection Cycle)

2. The action of the pathogen and whole array of
host reactions and disease development occurring
in host
Definition:
The diseases development may be continuous or
intermittent
The series of more or less distinct events
occurring in sequence and leading to disease
development and perpetuation of the disease
and pathogen

3. Types of Disease Cycle
Pathogens may complete one or more disease
cycles per year or takes more than one year to
complete a disease
1. Monocyclic or single cycle pathogens
2. Polycyclic or multi cycle pathogens
3. Polyetic pathogens

4. 1. Monocyclic or single cycle pathogens
Complete one disease cycle in one year
No secondary inoculum and secondary infection
These pathogens are mostly responsible for
slow epidemics
e.g. Smuts, Root rots and Wilts
Graphically these pathogens represents a
saturation curve

5. More than one disease cycle per
growing season
e.g. Rust, Powdery mildew, Leaf
spot, Leaf Blight, Anthracnose etc.
2. Polycyclic or multi cycle pathogens
Amount of inoculum increased many fold
Primary inoculum initiates the disease and numerous
secondary inocula are produced at each infection site
Graphically these pathogens represents a Sigmoid curve
Mostly cause Rapid epidemics in field

6. 3. Polyetic pathogens
Take more than one year to complete
a disease cycle per growing season
e.g. Dutch elm, Citrus Tristeza and
Pear decline
May cause severe epidemics over several years

7. Various stages in the pathogenesis
I. Inoculation
II. Pre penetration
III. Penetration
IV. Infection
V. Dissemination of the pathogen
VI. Over wintering / over summering / survival
a. Direct penetration
b. Penetration through wounds
c. Penetration through natural openings
Infected host
Dormant structures
Outside the host

8. Inoculum : Any part of the pathogen that can cause
infection or infective propagule coming in contact
with host
Propagule : One unit of inoculum of any pathogen
Primary (1°) Inocuclum : An inoculum that survives,
dormant in the winter / summer and initiates the
infection first time in the season of crop
Secondary (2°) Inocuclum : An inoculum produced
from 1° infections. It helps in disease epidemic
Inocuclum Potential : The amount of inoculum
responsible for successful infection

9. I. Inoculation
Pathogen coming in contact with host

10. Inoculum

11. Inoculum

12. II. Pre penetration
Pathogen is present on the host surface but has
not entered into it
Produce Mucilagenous
substances to adhere to
the plant
Appressorium : The swollen
tip of a germ tube that
facilitates attachment and
penetration of the host by a
fungus

13. III. Penetration
Passive invaders : Pathogen which do not have any
capacity to enter the host cell
e.g. Virus, Viroid, Protozoa, Fastidious bacteria
Actual entry of pathogen into the host plant
Based on their capacity of make enter themselves
into host cells, pathogens are classified as:
Active invaders : Pathogens which make an
aggressive effort to gain entry into intact host cells
and do not require help of any external agency
e.g. Fungi

14. Direct penetration : Pathogen require adhesion to
the plant surface followed by the application of
pressure and enzymatic degradation of the cuticle
and cell wall in order to overcome the physical
barriers of plant surface
Penetration Peg / Infection Peg : A small finger like
growth from underside of appresorial cells
Haustorium: Drinking organ of Fungi
e.g. Erysiphales, Pucciniales

15. III. Penetration

16. Spore germination and Penetration

17. Spore germination and Penetration

19. Penetration -Direct penetration
Pathogen Disease Mode Of Entry
Plasmodiophora brassicae Club root Root tissue
Pythium spp Damping
off
Seed coat and emerging
seedlings
Ustilago tritici Loose smut Stigma and ovary walls
Claviceps spp Ergot Stigma
Erysiphe spp. Powdery
mildew
Epidermis
Phytophtora infestans Late blight Cuticle and cell wall
Magnoporthe grisea Blast Puncturing the cuticle
Penicillium, Botrydiplodia Fruit rot Stalk end

20. Methods of penetration and invasion by fungi

21. Stomata -Rust, downy mildew and powdery mildew
Hydathodes – usually bacteria
Lenticles – Powdery scab of potato – Spongospora subterranea
Root rot- Armillaria mellea
Apple stem canker – Nectria sp
Nectaries -Apple fire blight – Erwinia amylovora
Natural openings

23. IV. Infection
The process of establishment of pathogen
within susceptible cells or tissues of the
host and starts to procure nutrient from
them
Infection period : Time elapsing between
spore germination and established infection
Invasion: Pathogens spread into all the tissues
of the plant organs
Fungi and Bacteria : Inter / Intracellularly
Virus, Viroid and FVB : Intravascularly

24. Systemic Infection : Single infection point to hole
plant or plant organ
e.g. Virus, Viroid, FVB, Candidatus Phytoplasma
and Protozoa
Colonization: Invaded pathogen grows and starts to
multiply
Fructification period: Time gap between infection and
first reproduction
Local Infection : Affecting limited part of a plant
e.g. Fungi (Except smut and few downy mildew
fungi), Bacteria and Phanerogamic Parasites

25. Successful infection results in appearance of
symptoms
Incubation period: Time between Inoculation and
symptom expression
V. Dissemination of the pathogen : transport of
inoculum from the source of production to an area
where a susceptible host is growing
VI. Over wintering / Over summering / Survival of the
pathogen:
Inoculum survives in debris, soil, seed etc. in the
absence of its host
Obligate parasites survives in living plant tissues

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