Agriculture Reference
In-Depth Information
crop variety (Boudreau, 1993; Boudreau & Mundt, 1994; Bulson
et al.,
1997) and there
can be effects on yield. Thus, under-sowing leeks with clover reduced the incidence
of rust (
Puccinia allii
)
and although quality was improved, crop growth was reduced
(Theunissen & Schelling, 1996). Similarly, intercropping tomato with soybean or ses-
ame, in combination with sanitation, limited late blight (
Phytophthora infestans
) devel-
opment, but taller intercrops reduced tomato growth and production (Tumwine
et al.,
2002). Interestingly, Kinane & Lyngkjaer (2002) found that in barley-legume intercrops,
disease incidence was reduced, irrespective of location, although not always signifi -
cantly. For example, net blotch (
Pyrenophora teres
) on barley was reduced whenever it
was intercropped with grain legumes, while on pea, ascochyta blight (
Ascochyta pisi
)
was reduced (Kinane & Lyngkjaer, 2002). In this work, although brown rust (
P. hordei
)
on barley was reduced when intercropped with legumes, these reductions were not sig-
nifi cant. In some recent work on barley-grain legume intercropping, disease reductions
were observed in all intercrop combinations, compared to the sole crop (Hauggaard-
Nielsen
et al.,
2008).
Mechanisms proposed to account for reductions in disease in intercropped systems
include alterations in microclimate, competition and induced resistance. Fininsa & Yuen
(2002) examined the effects of intercropping bean (
Phaseolus vulgaris
) with maize and/
or sorghum in four cropping systems (sole cropping, row, mixed and broadcast intercrop-
ping) on common bacterial blight caused by
Xanthomonas campestris
pv.
phaseoli.
They
found that intercropping delayed epidemic onset, lowered disease incidence and severity
and reduced the disease progress rate, although the magnitude of these effects varied
depending on the cropping system. Fininsa & Yuen (2002) suggested that in mixed and
broadcast intercropping, where the plants are under the associate crop and not in separate
rows, there are likely to be competition and dispersal interference effects. In contrast,
in row intercropping competition, microclimate changes and interference effects would
be less important (Fininsa & Yuen, 2002). According to these workers, the fi nal effect
on disease would depend on the canopy and root structure and tillering capacity of the
associate crop.
Intercropping has been shown to enhance and stabilise yields, reduce weeds and plant
diseases and improve resource use. However, there is a need for increased understand-
ing of the ecological mechanisms associated with planned spatial diversity, in order to
enhance the benefi ts achieved from intercropping.
2.7
Conclusions
Cultural control can be an effective and sustainable approach to the management of plant
disease. Indeed, continuing problems with fungicide resistance and breakdown of host
plant resistance, together with increasing concern for the environment means that there is
renewed interest in cultural practices for the management of crop diseases. However, the
choice and use of cultural practices will depend on the crop and the pathogen, although
it might be possible to integrate the management of more than one disease by combining
several appropriate cultural practices. In order to maximise the potential of cultural prac-
tices in disease control, a sound understanding of the mechanisms by which they exert
their effects is required.
