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(of plants) epidemic among plants of a single kind especially over a wide area. disease increase in a population over time


study of disease increases in populations

Models for plant disease epidemics

-monocyclic pathogens
-polycyclic pathogens

Monocyclic diseases

-one generation of inoculum per season
-primary inoculum only
-often soilborne pathogens

Polycyclic diseases

-several generations of inoculum per year
-primary and secondary inoculum produced
-often airborne or vectorborne

Disease progress curve

-tracks the amount of disease in an epidemic over time
-time plotted on x-axis
-disease plotted on y-axis

Parameters used to measure disease

-# of plants infected
-the percent of plants infected
-the percent leaf area with symptoms
-percent defoliation
-amount of inoculum produced

Disease incidence

-fraction of total plant units that are infected

Disease severity

-fraction of total plant tissue area that is infected

Disease rating scale

disease severity can range from 0% to 100%

Weber-Fechner law of visual discrimination

ability to see differences decreases by the logarithm of the intensity of the stimulus

Horsfall-Barratt rating scale

Assigns 12 levels, each of which include disease severity ranges that decrease as disease approaches 0 or 100%

Disease gradient

At a single point in time, severity on individual plants is expected to decrease as distance increases from the original infection

"Focal disease"

typically diseases caused by nematodes and soilborne organisms because spread through soil is limited

Important determinants of monocyclic diseases

-amount of primary inoculum (from the reservoir)
-how much susceptible tissue or how man healthy plants remain to be infected

Important determinants of polycyclic diseases

-amount of primary inoculum
-number of generations of secondary inoculum

Logistic growth model

-describes polycyclic diseases
-3 phases: exponential, logistic, and terminal

"Infection chain"

new spores create new infections, which produce even more new spores and infections..

Polycyclic processes: exponential phase

-rate of disease increase in not constant but dependent (directly proportional) to the amount of disease present.
-occurs from start of epidemic until onset (disease first visible)

Polycyclic processes: logistic phase

-rate of disease increase is dependent on the amount of disease and health tissue present
-occurs from onset to mid-time (y=.5)

Correction factor

-accounts for a limited food supply (susceptible host tissues)
- 1-y, portrays the amount of healthy plant tissue still available for infection by the pathogen

Polycyclic processes: terminal phase

-occurs from mid-time until the end of the epidemic (y=1.0)

Monomolecular growth model

-describes monocyclic diseases
-2 phases: linear and terminal

Monocyclic processes: linear phase

during early phase of disease, the increase in disease is fairly constant

Monocyclic processes: terminal phase

Later on, the amount of susceptible tissue left decreases, so rate of disease decreases also

Polycyclic Epidemics

-contain many infection cycles
-are described by the logistic growth model
-have exponential, logistic, and terminal phases

Monocyclic Epidemics

-contain a single infection cycle per season
-are described by the monomolecular growth model
-have a linear and terminal phase

Delay the epidemic (polycyclic)

-management practices that reduce primary inoculum
-shifts the disease progress curve to the right

Reduce the slope of the epidemic

-management practices that reduce the rate of infection and reduce the ability of inoculum to cause infections
-Practices include: applications of fungicides, minimizing leaf wetness, and using cultivars with general resistance


-to measure the area under the disease progress curve
-measure of the total amount of disease

Trapezoidal method

-an AUDPC calculation
-calculate the average disease intensity between each pair of adjacent time points

Factors that affect how quickly the disease curve reaches 100% (all plants dead)

-number of generations of secondary inoculum
-susceptibility of the plants
-environmental conditions


predicting whether and when preventative measures are necessary

Prediction of monocyclic epidemics

-bioassays (such as looking for propagules in soil at the beginning of the season
-monitoring environmental conditions

Prediction of polycyclic epidemics

-Some diseases require multiple pesticide applications throughout the season
-Knowledge of environmental conditions that favor infection
-Time required for infection to occur (usually hours of leaf wetness at a particular temperature) : infection period
-computer models can be used to forecast infection periods

Prediction periods

For some diseases, warns growers each time that an infection period occurs or is anticipated

Severity values

-When infection periods accumulate
-a designated severity value is used as an action threshold for the appropriate management strategy

Human influence on susceptibility of plants

-Introduced Plants
-Genetic uniformity


-In agriculture, large expanses of land are planted with reduced spacing of the same crop
-easier for pathogen to spread
-example: coffee rust

Introduced Plants

-People move plant species from their center of origin to other parts of the world
-Introduced plants may be susceptible to native pathogens
-example: fire blight of pear

Genetic uniformity

-Plants in agriculture are selected for desirable characteristics and are propagated so all plants have similar traits
-all plants will be susceptible or resistant to the same pathogens
-example: late blight of potato

Human activity that influences the environment in ways that increase disease

-Water management
-Various cultural practices


-In perennial (reoccurring) plantings, populations of pathogens can build yearly
-Some annual crops are planted in the same location every year so pathogens such as the soybean cyst nematode build in population over the years

Water management

Irrigation can increase leaf wetness and disseminate (spread) propagules (seed pods)


-many pathogens of foliar disease invade plants because they are "low-nitrogen" or "high-nitrogen" pathogens
-fertilizer applied in relatively large, infrequent doses causes plants to rocket between under and over fertilized conditions, increasing susceptibility to disease

Various cultural practices

-various tools create wounds and distribute pathogens to wound sites
-mowers carry spores to cut grass
-no-till practices (leaving stubble in field to decrease soil erosion and conserve water) increases survival of primary inoculum

Human practices that affect the ability of pathogens to cause disease

-Introduced pathogens
-Introduced vectors
-Seedborne pathogens
-Vegetative propagation

Introduced pathogens

humans move pathogens when they move host plants

Seedborne pathogens

-infested seed (pathogen on the surface)
-infected seed (cotyledons infected)

How plant disease epidemics affect people

-economic impact
-aesthetic losses
-crop production changes
-famine and emigration
-food security and safety

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