L2 - Epidemiology Flashcards
Define epidemiology
Epidemiology is the study of plant disease development in populations. To prevent epidemics, it is important to understand why they occur.
Whats an epidemic?
An epidemic:
affects a ‘high’ percentage of individuals in the population;
is not necessarily widespread or severe (could be an epidemic in a single field).
what are the 3 requirements for an epidemic?
- Forms of pathogen virulent on (able to attack) host genotypes being grown.
- Adequate inoculum early enough in growing season.
- Favourable environment (especially temperature, moisture) for different stages of pathogen life cycle.
What is an endmeic?
disease constantly present at low level
The logistic equation provides what?
X, RX and R stand for what?
useful (semi-)quantitative description of plant disease epidemics.
X = the fraction of total leaf area infected
RX = this tissue produces enough new spores in 1 day to increase the fraction infected by rx.
R = basic infection rate (usual units: day-1).
Define a lesion
an infected areas originating from a single infection
In logistic growth area infected is proportional to what
number of lesions
In logistic growth, what is the equation for “Increase in fraction of tissue infected in 1 day”
R x X x (1 - X)
Xo represents what
xo is fraction of tissue infected at time zero, which gives rise to initial inoculum.
Logistic equation gives what curve when if plot x against t;
s curve
Logistic equation gives what curve when if plot log10[x/(1-x)] against t;
straight line
r and x0 determine what?
changing r changes…
changing xO changes…
how epidemic develops.
Changing r changes shape of curve (or slope on logistic plot).
Changing x0 changes position, not shape, of curve.
Vanderplank’s equivalence theorem says what?
All influences on disease development can be translated into effects on infection rate, amount of initial inoculum, or both.
Applies to control measures, changes in environment, and changes in host or pathogen.
Disease control methods are divided into what 2 categories
Those that reduce r, rate of build-up of epidemic
Those that reduce x0, source of initial inoculum.
(A reduction in x0 has most effect on disease levels when r is small)
What is an assumption made in the logistic equation?
assumes the infection rate r is constant;
(doesnt predict r)
But r:
- is likely to change over time;
- depends on the underlying processes of the disease cycle.
It is possible to calculate r from knowledge of factors affecting these underlying processes.
Summary
Polycyclic epidemics have many cycles per growing season. The logistic equation often describes epidemic development quite well.
The development of the epidemic can be described by:
the basic infection rate, r;
the amount of initial inoculum, arising from the amount of infected tissue at the beginning of the epidemic, x0.
Vanderplank’s equivalence theorem states that all factors influencing the epidemic will affect the basic infection rate, the amount of initial inoculum, or both.
Diseases without repeating cycles are called ? example
Monocylic
Examples:
Life cycle follows that of theplant, so pathogen repro-duces when plant flowers(notably smut diseases).
In many soil-borne diseases, the pathogen does not spread significantly from plant to plant. T/F
T
Name an example of a monocylic disease? How do they spread?
Spores or other resistant structures remain in soil when plant dies, e.g. sclerotia.
These germinate in favourable conditions
– especially when a root is nearby.
(For such diseases, the plant comes to the pathogen. Thus, the increase of disease over a growing season corresponds to the growth of plant roots)
What is polyetic epidemic?
Disease severity can change in a consistent way from year to year.
Cotton root rot disease is an example of what?
polyetic epidemic - inoculum can build up in soil
