10 principles of disease management
DESCRIPTION
Understanding invasive plant pest enabling sound control & management of plant diseases.TRANSCRIPT
BOT 552: PLANT DISEASE MANAGEMENT
Major Principles of Control
Plant Pathology in Practice
Goals/Objective Knowledge/Concept-diagnosis -recognition of symptoms, signs Bot 453/553
-pathogen biology-prognosis -disease cycles Bot 350/550
-principles-control -control strategies and tactics
-quantitative application-cropping system management
Bot 552
Major Principles of Plant Disease Management
Strategies before pathogen is presentExclusion = attempts to prevent introduction
or establishment of pathogen
Strategies after pathogen is presentTherapy = procedures once pathogen becomes
established
Exclusion of the Pathogen
1. Quarantine = programs and policies to preventintroduction of pathogens.
2. Pathogen or disease-free planting materiala. Certification of seed and other planting materialb. Treatment of propagation material
“Regulatory Plant Pathology”
However…………….
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 localarea where inoculum is absent or the environmentis unfavorable.
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 established.
1. Rotation to crops which cannot sustain pathogen2. Treatment of source of inoculum with chemicals, heat, etc. = soil sterilization for potting mix, fumigation3. Eradicant fungicides4. 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 foliage2. Seed treatment3. 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 disease response.
Disease TriangleHost
Total of all properties that affect susceptibility
Pathogen EnvironmentTotal 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 epidemic”
Changes in any component has an equivalent effect on disease
- More-less susceptible host All affect- More-less favorable environment amount of- More-less aggressive pathogen disease
Therefore, disease management principlesand 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.
Disease Cycles
Independent of host
“All pathogens go through a cycle with similar events.”
Knowing how particular pathogens go through their disease cycle is important in developing management strategies.
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”
Or: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 infection.
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.
Dis
ease
Time
Monocyclic Disease
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.
Dis
ease
Time
Monocyclic Disease
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)Verticillium wiltCereal 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 fold.
Graphically this type of population growth is represented as a sigmoid curve
Dis
ease
Time
Polycyclic Disease
Dis
ease
Time
Polycyclic Disease
Rate of increase of disease over time can be represented by a compound interest function.
Many of these pathogens are disseminated primarily by airOr air-borne vectors and are responsible for most of the explosive epidemics in most crops
Examples of Polcyclic DiseasesDowny mildewsPowdery mildewsLate blight of potatoLeaf spotsBlightsGrain rustsAphid borne virusesRoot-knot nematodes
III. Polyetic (multi-year) cyclesSome pathogens take several years before inoculum they produce can be disseminated and initiate new infections.
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 treesDutch elm diseasePear declineCitrus tristeza Fungal vascular wiltsMycoplasmal yellowsViral infections
Implications for Disease Management Strategies
Monocyclic DiseasesReduce the amount of primary inoculum, or affect the efficiency of invasion by the primary inoculum.
Polycyclic DiseasesReducing the amount of primary inoculum has less impact.Reducing the rate of increase of the pathogen more beneficial.
Other Concepts Related to Disease Cycles
Successful Infections = symptomsBefore 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 combination2. 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 epidemic.
Affected by characteristics of the host (stage of development, age of tissue, physiological condition), the pathogen, environment (temperature, moisture).
Prop
agul
e N
umbe
r
Distance (m or km)
Dispersal Gradient Curve
Source ofinoculum
Gradients in pathogen densities and disease are frequently observed.Factors that affect spatial variation in the amount of incoming inoculum lead to dispersal gradients.
Perc
enta
ge D
isea
se
Distance (m or km)
Disease Gradient Curve
Source of inoculum
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
Dis
ease
Inte
nsity
Inoculum Dose
Dose-Reponse Curve
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.
Dis
ease
Time
UnmanagedDisease Level= Crop Loss
ManagedDisease Level
DiseaseManagement
Monocyclic
Polycyclic
Effect of Management on Disease
Purpose of disease management is to prevent disease from exceeding some level where profit or yield is significantly diminished.
Principles of epidemiology indicates that control measures can do this in only two ways.
1. They may reduce (or delay) disease at the beginningof the season (x0) or
2. They may decrease the rate of disease development (r) during the growing period.
Management Strategiesa Sanitationb Change planting datec Partial host resistanced Eradicant fungicidee Protectant fungicidef Adult plant host
resistance
Effects on Disease1. Original progress curve2. Disease reduced by reducing x0 (a & d) or by delay of epidemic (b & e)
Rate same as curve 13. Rate changed after
disease has begun (f)4. Rate changed from
beginning (c)
Zadoks and Schein, 1979
Ways to reduce disease (inoculum) at beginning (x0)Affects monocyclic and polycylic diseases
Fumigation Certified seedSanitation Seed treatmentsQuarantine Host plant resistance
Ways to decrease the rate of disease development (infection rate) (r)
Change the environmentFertilizer applicationHost 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 strategies.
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 breedingregulatory actions, chemical control measures