Agriculture Reference
In-Depth Information
disease. The definition of sustainability used here would embrace the
integrated crop
management (ICM) systems, which have become the key elements of recent attempts
in western Europe to produce a more environmentally benign agriculture, and would
also include 'organic' systems of production. In this chapter we use the term 'organic'
to describe crops grown “without artificial chemicals or genetic modification” (HRH
Prince of Wales, Soil Association literature).
Disease control in both ICM and organic systems involves the sound application
of epidemiological principles and may be seen as the practical application of the
concepts expressed by Van der Plank (1960) as:
∆
t =(230/r)log(I
0
/I'
0
)
(11.1)
where the expected delay (
t
) in the development of an epidemic is dependent upon
the reduction of the initial inoculum from
1
0
to
1'
0
and the rate of disease increase
(
r
). This chapter considers how such epidemiological information can be used in
conjunction with cultural and crop management to reduce the initial inoculum (
I'
0
)
and the rate of disease development (
r
)
.
∆
11.2 INOCULUM
The first phase of a disease epidemic depends on the level of inoculum present when the
crop is first exposed to infection and on the ability of the pathogen to take advantage of
this initial vulnerability of the host to become established in the crop. This ability is
usually dependent upon climatic (or micro-climatic)
conditions at the time. The
importance of these factors relative to those influencing the later stages of epidemic
development varies widely from disease to disease, as does the feasibility of reducing
inoculum potential as a method of disease control in a sustainable arable system. This
may be illustrated by reference to some of the more common diseases of arable crops.
11.2.1 Seedborne inoculum
There are many examples of seedborne inoculum initiating epidemics and this is the
primary infection source for a number of major crop diseases (see Chapter 13). Bunt
of wheat (caused by
Tilletia tritici
), may be considered as an
example. The control
of bunt has long depended primarily on the control of inoculum. The disease is
usually introduced into a crop on the seed sown, although infection from soilborne
inoculum may occur. A low level of infection in a crop may go unnoticed but, if
seed is saved from such a crop, a much higher level of disease is likely to occur in
the following year. The millions of spores released from bunted ears during
threshing become dusted onto the surrounding healthy seeds and thus act as a potent
source of inoculum for the following season's epidemic. Dillon Weston and
Engledow (1933) calculated the potential rate of increase in infection levels that
could occur in a stock of wheat from which seed was saved, but not treated, over a
three-year period. If one ear in 8500 is affected by bunt in year 1, one in 450 could
be infected in year 2 and one in four in year 3. The proportion of ears developing the
