L2 - Epidemiology

Question 1

Define epidemiology

Answer 1

Epidemiology is the study of plant disease development in populations. To prevent epidemics, it is important to understand why they occur.

Question 2

Whats an epidemic?

Answer 2

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).

Question 3

what are the 3 requirements for an epidemic?

Answer 3

1. Forms of pathogen virulent on (able to attack) host genotypes being grown.
2. Adequate inoculum early enough in growing season.
3. Favourable environment (especially temperature, moisture) for different stages of pathogen life cycle.

Question 4

What is an endmeic?

Answer 4

disease constantly present at low level

Question 5

The logistic equation provides what?

X, RX and R stand for what?

Answer 5

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).

Question 6

Define a lesion

Answer 6

an infected areas originating from a single infection

Question 7

In logistic growth area infected is proportional to what

Answer 7

number of lesions

Question 8

In logistic growth, what is the equation for “Increase in fraction of tissue infected in 1 day”

Answer 8

R x X x (1 - X)

Question 9

Xo represents what

Answer 9

xo is fraction of tissue infected at time zero, which gives rise to initial inoculum.

Question 10

Logistic equation gives what curve when if plot x against t;

Answer 10

s curve

Question 11

Logistic equation gives what curve when if plot log10[x/(1-x)] against t;

Answer 11

straight line

Question 12

r and x0 determine what?
changing r changes…
changing xO changes…

Answer 12

how epidemic develops.
Changing r changes shape of curve (or slope on logistic plot).
Changing x0 changes position, not shape, of curve.

Question 13

Vanderplank’s equivalence theorem says what?

Answer 13

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.

Question 14

Disease control methods are divided into what 2 categories

Answer 14

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)

Question 15

What is an assumption made in the logistic equation?

Answer 15

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.

Question 16

Summary

Answer 16

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.

Question 17

Diseases without repeating cycles are called ? example

Answer 17

Monocylic
Examples:
Life cycle follows that of theplant, so pathogen repro-duces when plant flowers(notably smut diseases).

Question 18

In many soil-borne diseases, the pathogen does not spread significantly from plant to plant. T/F

Answer 18

T

Question 19

Name an example of a monocylic disease? How do they spread?

Answer 19

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)

Question 20

What is polyetic epidemic?

Answer 20

Disease severity can change in a consistent way from year to year.

Question 21

Cotton root rot disease is an example of what?

Answer 21

polyetic epidemic - inoculum can build up in soil