Agriculture Reference
In-Depth Information
options differ greatly. At the simplest level, we consider cryptic diversification of
resistance genes within monoculture systems. At the most complex, there are
developments in new or improved systems of polyculture, for example in
agroforestry (Nair, 1993) or perennial polyculture (Jackson, 1980; Soule and Piper,
1992). A major objective of these latter options is to develop systems that are able to
deliver many ecological benefits and services simultaneously, for example, control
of erosion, weeds, pests and diseases (Altieri, 1995; Altieri
et al.
, 1996), while
maintaining or enhancing the scale and efficiency of production. Although we
concentrate here on crop diseases, we comment also on pests, weeds and other
problems since, in the real world, our concern is for simultaneous restraint of all
major unwanted organisms and their side effects. The examples given show how
system modifications can be introduced to reduce costs, increase production and
reduce stress on the environment.
10.2 DEFINITIONS
Possibilities for host diversity at the three levels of species, variety and resistance
genotype, both for monoculture and for polyculture, are listed in Table 10.1. Based
on the ranges of diversity indicated in this table, we use the following definitions:
Monoculture
refers usually to the continuous use of a single crop species over a
large area. For the pathologist, however, the term monoculture alone is inadequate
since it can be applied at the level of
species, variety
or
gene
. This additional
specification is important; for example, within a species monoculture, attempts have
been made to encourage farmers to diversify among different varieties with different
disease resistances. But, if the varieties available within the species all possess the
same gene for resistance to a particular pathogen, then the system is effectively a
monoculture with respect to that resistance
(variety diversification with resistance
gene monoculture)
. Conversely, the multiline approach is an attempt to combine
the benefits of monoculture and diversity by widespread use, for example, of a
population of plant genotypes that are uniform for desired quality and other
characteristics but diversified for a range of resistance genes against a major
pathogen of the crop
(variety monoculture with resistance gene diversification)
.
Polyculture
refers usually to the use of a range of different species over time on
a single management unit, which may be a farm or a field. Given that it may be
difficult to define a field, we follow a modification of the scheme adopted by
Vandermeer (1989).
Sole (single) cropping
:
different crops are used but are managed separately. This
includes:
-
rotational cropping
, involving production of different crops among fields and
over years or,
sequential cropping
, involving two or more crops in sequence in
the same field in one year.
Inter-cropping
:
two or more crops or varieties are managed simultaneously in the
same field. The degree of their interaction depends on their arrangement as follows:
