PATHOGEN VARIABILITYPATHOGEN VARIABILITY

All pathogens exist as populations of individuals, All pathogens exist as populations of individuals, not unlike the people in this room, this city, etc. not unlike the people in this room, this city, etc. They, like people, share many common They, like people, share many common characteristics but vary greatly in many others.characteristics but vary greatly in many others.

Let's look at an example of a single pathogen - Let's look at an example of a single pathogen - Puccinia graminisPuccinia graminis - stem rust of grasses. - stem rust of grasses.

Four Four subspecies,subspecies, depending on species of host depending on species of host plant: These subspecies are host-specific. One will plant: These subspecies are host-specific. One will not infect anothernot infect another

a. a. triticitritici - wheat - wheat b. b. hordeihordei - barley - barleyc. c. secalesecale - rye - rye d. d. avenaeavenae - oat - oat

PATHOGEN VARIABILITYPATHOGEN VARIABILITY

Below this level there are Below this level there are racesraces (> 200 for P. (> 200 for P. graminis graminis triticitritici alone!). Races are defined by ability to develop alone!). Races are defined by ability to develop on specific host genotypes (varieties, cultivars, on specific host genotypes (varieties, cultivars, hybrids, etc.).hybrids, etc.).

Races are identified by ability to cause disease on Races are identified by ability to cause disease on members of a set of 10 differential varieties that members of a set of 10 differential varieties that contain specific resistance genes.contain specific resistance genes.SourceSource V1V1 V2V2 V3V3 V4V4 V5V5 V6V6 V7V7 V8V8 V9V9 V10V10 RaceRace

TexasTexas xx xx xx 1414

ArkansasArkansas xx xx xx 2727

NebraskaNebraska xx xx 3131

Races cont.Races cont.

From this information several things can happen:From this information several things can happen:1. Prevalent races in any given area will be 1. Prevalent races in any given area will be

identified. This will allow identification of varieties identified. This will allow identification of varieties with resistance to these races, and these can be with resistance to these races, and these can be recommended for planting.recommended for planting.

2. USDA Cereal Rust lab in St. Paul, MN keeps tracks 2. USDA Cereal Rust lab in St. Paul, MN keeps tracks of changing race status in US, identifying new of changing race status in US, identifying new races, etc.races, etc.

Within a given race, there are many Within a given race, there are many biotypes.biotypes. These are distinguished by the severity of disease These are distinguished by the severity of disease they cause on any given host genotype. They vary they cause on any given host genotype. They vary in many ways, such as spore longevity and in many ways, such as spore longevity and survival, lesions/leaf, spores/lesion, etc.survival, lesions/leaf, spores/lesion, etc.

"Breakdown" of Resistance:"Breakdown" of Resistance:This term is used when a previously resistant variety This term is used when a previously resistant variety

suddenly develops disease. It implies that the host suddenly develops disease. It implies that the host has changed, that the resistance mechanisms no has changed, that the resistance mechanisms no longer work, etc. New pathogen races have longer work, etc. New pathogen races have developed because of selection pressure that was developed because of selection pressure that was put on population by the host resistance mechanism.put on population by the host resistance mechanism.

Mechanisms of Change in Pathogen Populations;Mechanisms of Change in Pathogen Populations;1. Mutations - occur during mitosis; an accident in 1. Mutations - occur during mitosis; an accident in

duplication of genome results in slight or severe duplication of genome results in slight or severe changes.changes.

2. Sexual recombination - fusion of two In cells > 2n > 2. Sexual recombination - fusion of two In cells > 2n > two In cells with mixed genetic info.two In cells with mixed genetic info.+ +

n n 2n n n

Mechanisms of Change in Mechanisms of Change in Pathogen Populations cont.Pathogen Populations cont.

3. Heterokaryosis - 2 or more nuclei in same somatic 3. Heterokaryosis - 2 or more nuclei in same somatic cell. We've seen this commonly in smut fungi, rust cell. We've seen this commonly in smut fungi, rust fungi, etc. Traits are governed by two nuclei, which fungi, etc. Traits are governed by two nuclei, which makes for much more variability.makes for much more variability.

4. Transformation (in bacteria) - cells rupture and 4. Transformation (in bacteria) - cells rupture and release genetic material. Adjacent cells "absorb" release genetic material. Adjacent cells "absorb" this material and incorporate it. Acceptor cell is this material and incorporate it. Acceptor cell is modified by new material.modified by new material.

5. Conjugation (in bacteria) - cells contact each other 5. Conjugation (in bacteria) - cells contact each other directly and exchange genetic material.directly and exchange genetic material.

Mechanisms of Change in Mechanisms of Change in Pathogen Populations cont.Pathogen Populations cont.

Sectoring - one portion of fungal culture (from a Sectoring - one portion of fungal culture (from a single propagule) is phenotypically different. Can single propagule) is phenotypically different. Can result from mutation, heterokaryosis, etc.result from mutation, heterokaryosis, etc.

Phenotype A

Phenotype BPhenotype C

Monitoring Disease in Time - Monitoring Disease in Time - Disease Progress CurvesDisease Progress Curves

Disease progress curves show the progress of a Disease progress curves show the progress of a disease across time. These are important for disease across time. These are important for evaluating total impact and loss by a disease, evaluating total impact and loss by a disease, as well as determining when to initiate control as well as determining when to initiate control measures.measures.

Two Basic Types of DPCsTwo Basic Types of DPCs1.1. Monocyclic diseases - these are characterized by a Monocyclic diseases - these are characterized by a

large release of inoculum early in the season. large release of inoculum early in the season.

Ex: Charcoal rot of most dicots, caused by Ex: Charcoal rot of most dicots, caused by MacrophominaMacrophomina phaseolinaphaseolina

Ex: Cabbage club root, caused by Ex: Cabbage club root, caused by Plasmodiophora Plasmodiophora brassicaebrassicae

Many soilborne root pathogens tend to be monocyclic.Many soilborne root pathogens tend to be monocyclic.

Two Basic Types of DPCsTwo Basic Types of DPCs1. Monocyclic diseases 1. Monocyclic diseases

2. Polycyclic diseases - these are characterized by 2. Polycyclic diseases - these are characterized by a small amount of primary inoculum but several a small amount of primary inoculum but several secondary cycles to increase inoculum. secondary cycles to increase inoculum.

Ex: Potato late blight, caused by Ex: Potato late blight, caused by PhytophthoraPhytophthora infestansinfestans

Ex: Apple scab, caused by Ex: Apple scab, caused by Venturia inaequalisVenturia inaequalis

Most diseases are polycyclic, including nearly all Most diseases are polycyclic, including nearly all we discussed for Ascomycetes.we discussed for Ascomycetes.

Disease Progress CurvesDisease Progress CurvesThe curve appearance also depends on both pathogen The curve appearance also depends on both pathogen

and host factors. For example:and host factors. For example:

1. Initial inoculum dose can determine rate of disease 1. Initial inoculum dose can determine rate of disease increase, to a point. Extremely high doses do not increase, to a point. Extremely high doses do not necessarily result in very high disease levels.necessarily result in very high disease levels.

Remember - disease is limited by number of available Remember - disease is limited by number of available infection courts.infection courts.

Disease Progress CurvesDisease Progress CurvesThe curve appearance also dependsThe curve appearance also depends

1.1. Initial inoculum doseInitial inoculum dose

2.2. Host resistance can modify rate of disease Host resistance can modify rate of disease increase, through any of the mechanisms we increase, through any of the mechanisms we discussed earlierdiscussed earlier

Monitoring Disease in Space - Monitoring Disease in Space - Disease Gradient CurvesDisease Gradient Curves

These measure the amount of disease as you go away from These measure the amount of disease as you go away from a source. These do not work for diseases such as wheat a source. These do not work for diseases such as wheat leaf or stem rust, where inoculum tends to settle in a leaf or stem rust, where inoculum tends to settle in a field like a cloud and foci are uniformly distributed field like a cloud and foci are uniformly distributed everywhere. They work best with discrete sources of everywhere. They work best with discrete sources of inoculum.inoculum.

Area Under Disease Progress Curve Area Under Disease Progress Curve AUDPC AUDPC

Sometimes it is hard to distinguish Sometimes it is hard to distinguish Resistant from Susceptible varieties Resistant from Susceptible varieties if the curves aren't as 'perfect' as the if the curves aren't as 'perfect' as the previous diagrams.previous diagrams.

Therefore we determine, the AUDPCTherefore we determine, the AUDPCAUDPC = Σ ( [yi + yi+1] / 2) (ti+1-ti)

Disease Progress CurveDisease Progress CurveD

isea

se s

ever

ity

(%)

Time (days)

0

25

50

7

5

100

1/2 of crop diseased

Exp

onen

tial

gro

wth

Lag phase

Note: Slide is the property of B.M. Pryor, U. Arizona

Thresholds for actionThresholds for action Area under the disease progress Area under the disease progress

curve (AUDPC) is an important curve (AUDPC) is an important determinant for management determinant for management actionaction

Damage threshold: the point at Damage threshold: the point at which an increase in disease will which an increase in disease will result in an economic lossresult in an economic loss

Economic threshold: the point at Economic threshold: the point at which the cost of management which the cost of management equals the increase in yield due equals the increase in yield due to managementto management

Dis

ease

sev

erit

y (%

)

Time (days)

0

25

50

75

1

00

1/2 of crop diseased

Exp

onen

tial

gro

wth

Lag phase

harvest

Note: Slide is the property of B.M. Pryor, U. Arizona

Slope = r2

Slope = r1

time

disease

Slope equals disease units/time (usually expressed in days)

Comparison of disease progress curves

Condition B

Condition A

Note: Slide is the property of B.M. Pryor, U. Arizona

Disease ForecastingDisease ForecastingThese are efforts to predict the amount of disease These are efforts to predict the amount of disease

that you will have sometime in the future.that you will have sometime in the future.

There are several important factors necessary for There are several important factors necessary for accurate disease forecasting:accurate disease forecasting:

I. Pathogen factors - we've seen these beforeI. Pathogen factors - we've seen these before1. Virulence1. Virulence2. Quantity of primary inoculum2. Quantity of primary inoculum

a. This is the amount available at the beginning a. This is the amount available at the beginning of the season.of the season.b. More important for mono- than for polycyclic b. More important for mono- than for polycyclic diseases.diseases.c. It's very hard to determine the amount of c. It's very hard to determine the amount of secondary inoculum.secondary inoculum.

Disease ForecastingDisease ForecastingThese are efforts to predict the amount of These are efforts to predict the amount of

disease that you will have sometime in the disease that you will have sometime in the future.future.

3. Length of reproductive cycle3. Length of reproductive cycle

4. Location of reproductive 4. Location of reproductive structuresstructures

5. Mode of spread5. Mode of spread

Early Work in Disease Early Work in Disease ForecastingForecasting

One of the first diseases targeted was One of the first diseases targeted was late blight of potato.late blight of potato.

In 1926, the first predictors were In 1926, the first predictors were published:published:

1. > 4 hr dew at night .1. > 4 hr dew at night .

2. Min temperature > 10 C 2. Min temperature > 10 C

3. > 80% cloud cover the next day 3. > 80% cloud cover the next day

4. > 1 mm rainfall in next 24 h4. > 1 mm rainfall in next 24 h

..