BOT 552: PLANT DISEASE
Major Principles of Control
Plant Pathology in Practice
-diagnosis -recognition of symptoms, signs Bot 453/553
-prognosis -disease cycles Bot 350/550
-control -control strategies and tactics
-cropping system management
Major Principles of Plant Disease Management
Strategies before pathogen is present
Exclusion = attempts to prevent introduction
or establishment of pathogen
Strategies after pathogen is present
Therapy = procedures once pathogen becomes
Exclusion of the Pathogen
1. Quarantine = programs and policies to prevent
introduction of pathogens.
2. Pathogen or disease-free planting material
a. Certification of seed and other planting material
b. Treatment of propagation material
“Regulatory Plant Pathology”
Avoidance of the Pathogen
– Avoiding disease by planting:
1. at times when inoculum is inactive or when
inoculum levels are low, or
2. in geographic areas or planting sites in a local
area where inoculum is absent or the environment
Common to grow seed in areas away for commercial
production to avoid pathogens.
Eradication of the Pathogen
– Reducing, removing, eliminating or destroying
inoculum at the source, either from an area or
from an individual plant in which it is already
1. Rotation to crops which cannot sustain pathogen
2. Treatment of source of inoculum with chemicals,
heat, etc. = soil sterilization for potting mix,
3. Eradicant fungicides
4. Biological control of plant pathogen
Protection of the Plant
– Eliminate or reduce the effectiveness of inoculum at
the site of infection by imposing a barrier between
the plant and the pathogen.
1. Chemical spraying or dusting of foliage
2. Seed treatment
3. Protectant fungicides
Resistance of Plant to Pathogen
– Reduces effectiveness of inoculum, thereby
reducing establishment of the pathogen
Therapy applied to a diseased plant
– Cure or minimize losses in diseased plants by using
chemotherapeutants or by altering the severity of
attack by manipulating the environment to reduce
Total of all properties that affect susceptibility
Total of all properties of pathogen Total of all conditions
(virulence, abundance, etc.) that affect disease
Vanderplank’s Equivalence Theorem
“Effects of host, pathogen and environment can be
translated into terms of the rate parameter of an
Changes in any component has an equivalent effect
- More-less susceptible host All affect
- More-less favorable environment amount of
- More-less aggressive pathogen disease
Therefore, disease management principles
and practices are often centered around
the concept of the Disease Triangle so that
management tactics often seek to
manipulate one or more of the components
of the disease triangle.
“All pathogens go
through a cycle with
Knowing how particular pathogens go through
their disease cycle is important in developing
Understanding how disease cycles relate to disease
severity is assisted by the discipline of Epidemiology
Epidemiology is “the study of factors affecting the
outbreak and spread of infectious diseases”
the study of disease in populations, how diseases
increase over space, in severity, or over time.
A central concept to epidemiology is that different
pathogen populations have different disease cycles.
I. Monocyclic = single cycle (simple interest)
Pathogens that complete one or even part of one
disease cycle/year are called monocyclic
In monocyclic pathogens the primary inoculum is
the only inoculum available for the entire season.
There is no secondary inoculum and no secondary
The amount of inoculum produced at the end of the
season, however, is greater than at the start of the
season so the amount of inoculum may increase
steadily from year to year.
This representation of plant disease over time is
referred to as a “Disease Progress Curve”
Graphically, disease caused by monocyclic
pathogens looks like a saturation curve.
xt = Qrt
Rate of increase of disease over time can be
represented by a simple interest function.
Examples of Monocyclic Diseases
Blackleg of potato (Erwinia caratovora)
Cereal Cyst Nematode
II. Polycyclic = multiple cycles/year (compound interest)
Most pathogens go through more than one (2-30) disease cycle
in a growing season and are referred to as polycyclic.
Only a small number of sexual spores or other hardy structures
survive as primary inoculum that cause initial infections.
Once infection takes place, large numbers of asexual spores are
produced as secondary inoculum at each infection site.
These spores can produce new (secondary) infections that
produce more asexual spores and so on.
With each cycle the amount of inoculum is multiplied many
Graphically this type of population growth
is represented as a sigmoid curve
Rate of increase of disease over time can be
represented by a compound interest function.
Many of these pathogens are disseminated
primarily by air
Or air-borne vectors and are responsible for most
of the explosive epidemics in most crops
Examples of Polcyclic Diseases
Late blight of potato
Aphid borne viruses
III. Polyetic (multi-year) cycles
Some pathogens take several years before inoculum
they produce can be disseminated and initiate new
May not cause many new infections over a given area
in a year, amount of inoculum does not increase
greatly within a year.
However, because they survive in perennial hosts they
have almost as much inoculum as they had at the end
of the previous year.
Inoculum may increase steadily (exponentially) from
year to year and can cause severe outbreaks when
considered over several years.
Examples of Polyetic Diseases:
Some diseases of trees
Dutch elm disease
Fungal vascular wilts
Implications for Disease Management Strategies
Reduce the amount of primary inoculum, or affect
the efficiency of invasion by the primary inoculum.
Reducing the amount of primary inoculum has less
Reducing the rate of increase of the pathogen more
Other Concepts Related to Disease Cycles
Successful Infections = symptoms
Incubation period = time between inoculation and
penetration and appearance of the disease symptom.
The length of the incubation period of different
pathogens/diseases varies with:
1. the particular pathogen-host combination
2. the stage of development of the host
3. the temperature in the environment.
Can make disease assessments misleading
Latent period = time from infection until
production of new inoculum (reproduction).
Duration can have a large effect on the rate of the
Affected by characteristics of the host (stage of
development, age of tissue, physiological condition),
environment (temperature, moisture).
Distance (m or km)
Dispersal Gradient Curve
Gradients in pathogen densities and disease are
Factors that affect spatial variation in the amount
of incoming inoculum lead to dispersal gradients.
Distance (m or km)
Disease Gradient Curve
Gradients in pathogen propagule density can result in
Disease gradients = change in disease severity along a
straight line away from the source of inoculum.
The percentage of disease and the scale for distance
vary with the type of pathogen or its method of
dispersal, being small for soilborne pathogens or
vectors and larger for airborne pathogens
Disease gradients can also be caused by
environmental gradients such as, variations in soil
type, fertility, or gradual changes in microclimate
Variation in pathogen density as the result of
dispersal gradients or other causes are important
relative to the impact of a Dose Response on disease.
= Crop Loss
Effect of Management on Disease
Purpose of disease management is to prevent disease
from exceeding some level where profit or yield is
Principles of epidemiology indicates that control
measures can do this in only two ways.
1. They may reduce (or delay) disease at the beginning
of the season (x0) or
2. They may decrease the rate of disease development
(r) during the growing period.
b Change planting date
c Partial host resistance
d Eradicant fungicide
e Protectant fungicide
f Adult plant host
Effects on Disease
1. Original progress curve
2. Disease reduced by
reducing x0 (a & d) or by
delay of epidemic (b & e)
Rate same as curve 1
3. Rate changed after
disease has begun (f)
4. Rate changed from
Zadoks and Schein, 1979
Ways to reduce disease (inoculum) at beginning (x0)
Affects monocyclic and polycylic diseases
Fumigation Certified seed
Sanitation Seed treatments
Quarantine Host plant resistance
Ways to decrease the rate of disease development
(infection rate) (r)
Change the environment
Host plant resistance
Ways to change t (see “b” on figure)
Harvest early before disease becomes severe.
Plant early (cereal cyst nematode)
Control of different diseases requires different
Some pathosystems, monocyclic and polyetic diseases
can be effected by use of an x0-reducing practice only.
However, for most diseases more than one control
procedure is used and these are often chosen to
reduce x0 and r.
These integrated control measures use a combination
cultural methods, resistance breeding
regulatory actions, chemical control measures