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
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
21. Stomata -Rust, downy mildew and powdery mildew
Hydathodes – usually bacteria
Lenticles – Powdery scab of potato – Spongospora subterranea
Root rot- Armillaria melea
Apple stem canker – Nectria sp
Nectaries -Apple fire blight – Erwinia amylovora
Natural openings
22.
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
