经济损害水平与经济阈值

经济损害水平与经济阈值

THE KEYS TO DISEASE MANAGEMENT

From

《

Epidemiology and Plant Disease Management

》

by Jan C. Zadoks, Richard D. Schein. New

York 1979. p350-353

Let us define

disease management

as the total of all actions

，

intentional or not

，

that

serve

to

regulate

disease

1evels

so

that

they

remain

below

the

economic

thresho1d

1evel

；

the

actions

can

be

directed

toward

a

single

disease

or

toward

all

diseases

threatening

a

crop

；

the

actions

may

or

may

not

fin

to

a

system

of

supervised or even integrated contro1

；

disease management and pest management are

parts of crop management

，

but they may demand a specific expertise

．

Development of a disease management program depends on knowledge of three

areas

：

crop

economics,

population

dynamics,

and

disease

control

technology,

With

crop economics, we leave the area of natural sciences and enter an area where value

judgments

pertain

．

Although

the

individual’s

value

judgment

may

be

highly

subjective, the science of economics dealing with large groups of individuals is not

so

．

It is the economist who studies the financial aspects of crop losses

．

For detailed

discussions

，

refer to the appropriate literature (Carlson

，

1971

；

Carlson & Ma in

，

1976

；

Ordish &

Dufour

，

1969)

．

Economic considerations are crucial in the process leading to a decision

for or

against action

；

Value judgments by au thorities

，

consumers

，

merchants

，

and farmers

play an important role

．

The farmer's judgment leads

，

implicitly or explicitly

，

to an

idea about the economically acceptable damage

．

1 The Economic injury level or Damage Threshold

The strategy of

disease management is to to1erate disease

，

but to regulate it at

subeconomic

levels.

It

follows

that

that

the

level

Xt

at

which

disease

begins

to

adversely affect yield and

／

or quality--the

damage threshold

- must be known

．

The

level of this threshold varies with crop, disease

，

and 1ocal economy

．

One farmer or

farming region may be able to tolerate loss of attainable yield better than another and

remain economically viable

．

The

farmer's

judgment

about

the

economically

acceptable

damage

must

be

translated

into

concomitant

levels

of

Xt

.

Such

studies

are

still

rare;

an

excellent

example is given by Calpouzos et al

．

(1976).

The level of Xt at which the highest

，

economically acceptable damage occurs

has

been

called

the

economic

injury

level

by

entomologists

：

it

is

the

lowest

population

density that will anuse economic damage. Economic damage is

the

amount

of

injury

which

will

justify

the

cost

of

artificial

con trol

，

measures

；

consequently, the economic-injury 1evel may vary from area to area, season to season,

or with man's changing scale of economic values'' (Stern et al., 1959)

．

Here

，

We will

call it the

damage threshold

(see Fig. 11.1 where it is shown as an arbitrarily placed

line)

．

Lines

1

and

2

represent

disease

progressing

at

the

same

rate

．

Line

2

either

originates from a lower level of

Xo

(a) or starts later in the

，

season (b)

．

Line 1 crosses

the damage threshold

Xd

earlier than line 2 and reaches a higher harvest level Xh of

disease and

，

proba bly

，

damage

．

lines 3 and 4 are for disease progressing at a much

lower rate

；

they will not reach the damage threshold before harvest. Such a rate would

result

from

horizontal

resistance,

and

the

use

of

cultivars

with

this

quality

would

satisfy the requirements of disease management.

True determination of the damage threshold is part of the fine tuning of a local

management

system

and

depends

not

only

on

biologic

information

but

also

on

accurate knowledge of the costs of inputs to the system

，

the value of outputs

，

and the

required maintenance level. The farmer can afford only that amount of disease control

that will produce yield or profit exceeding the cost of the control measures

．

As intensification increases

，

input costs generally increase

．

Yields plotted against

inputs often show an inflection in the curve after which each unit of input produces a

smaller

increment

of

yield

．

One

of

the

inputs

is

the

cost

of

disease

control

and

diminishing

returns

must

be

taken

into

account

in

judging

how

much

control

is

affordable

．

2 The Economic Threshold or Action Threshold

The

rate

of

disease

development

r

depends

on

host

resistance

，< /p>

pathogen

virulence

，

and

environmental

suitability

．

Cultivars

may

have

different

damage

thresholds

．

If Xo

，

r, and the damage threshold are known

，

it can be predicted at what

time disease will exceed the damage threshold

．

If it is sufficiently before harvest

，

significant damage may result

．

Figure 11.1 illustrates this

．

If the damage threshold

Xd

is known

，

and disease is present

，

the farmer must

know when to act--the action threshold

，

Xa

. At the right time

，

ta

，

the farmer must

apply a control measure that will so reduce r that disease will not reach the damage

threshold before harvest

．

Empirical fungicide studies show what degree of r reduction

will

result

with

different

materials

under

different

conditions

．

Line

3

of

Figure

11.1

illustrates this idea

，

If disease is observed to be progressing at the rate of line 1

and if it is known that a particular treatment will reduce

r

the amount indicated by

f

.

the manager or advisor can calculale the time

，

ta

，