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